Astronomy

Are there any planned future observations of TRAPPIST-1 and its planets?

Are there any planned future observations of TRAPPIST-1 and its planets?


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The recent discovery of new planets around the star TRAPPIST-1 happened by observations from 7 ground-based telescopes, as well as the Spitzer Space Telescope. The team of astronomers responsible has noted that there are many relevant parameters which still have large uncertainties, including orbital information and planetary mass. There's also some excitement about possible atmospheric composition.

Are there any definite planned future observations of the system? I assume that at some point it will be targeted by the James Webb Space Telescope, but that won't launch for another year and a half, and it has plenty of other targets.

I'm not looking for detailed information - I know that cannot be easily gathered - but if there are any general mentions, like this one about the Subaru Telescope and Planet Nine, that would be fantastic.


A website trappist.one has been created to detail information about this discovery. In the "Future" section, they specifically state:

In the short term, photometric follow-up using the repurposed Kepler satellite (named K2) along with with newer observations using Spitzer ought to reveal the period of planet 1h. We will also search for additional planets. Theses lightcurves, combined with ground-based measurements will increase the number of transit timing measurements for each of the planets. This will give us more accurate masses and orbital eccentricities. This will confirm whether the planets are mostly rocky or whether they contain a certain amount of volatiles, like water.

In the medium term, we can expect the first attempts at detecting the atmospheres of the TRAPPIST-1 planets, using Hubble, followed by deeper investigations thanks to the James Webb. The James Webb could in principle measure the temperature of the planets, and detect the chemical composition of their atmospheres. It will do so by collecting dozens of eclipses of the TRAPPIST-1 planets. The advantage of having seven planets in one system is that we will be able to compare them to one another.

It seems they're currently using Kepler and Spitzer and plan to use Hubble in the near future as well as JWST after it launches.


The video shared by NASA specifically mentions that JWST will be used to observe the TRAPPIST 1 system. Infact that's going to be one of the prime targets.


TRAPPIST-1: A Dark Star With a Bright Future

Brett Morris is a Ph.D. candidate at the University of Washington studying astronomy and astrobiology. Before that, Brett studied astronomy and physics at the University of Maryland and grew up on Long Island in New York. Brett has always wanted to be an astronomer, and is almost there! *[email protected]

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Abstract

Of the thousands of stars known to host exoplanets, which are planets outside of our solar system, a particularly fascinating star stands out. It is known as TRAPPIST-1: a tiny star about the size of Jupiter, which is home to not one, not two, but seven Earth-sized planets! These planets are probably rocky worlds, like earth, and some of them might have the correct surface temperature for liquid water to exist, but that depends on whether or not these planets have atmospheres, and what those atmospheres are made of. Astronomers are currently working on figuring out whether TRAPPIST-1 has bright or dark spots on it, which may affect the way we see its planets.


Do the TRAPPIST-1 Planets Have Atmospheres?

In February of 2017, the scientific community rejoiced as NASA announced that a nearby star (TRAPPIST-1) had a system of no less than seven rocky planets! Since that time, astronomers have conducted all kinds of follow-up observations and studies in the hopes of learning more about these exoplanets. In particular, they have been attempting to learn if any of the planets located in the stars Habitable Zone (HZ) could actually be habitable.

Many of these studies have been concerned with whether or not the TRAPPIST-1 planets have sufficient water on their surfaces. But just as important is the question of whether or not any have viable atmospheres. In a recent study that provides an overview of all observations to date on TRAPPIST-1 planets, a team found that depending on the planet in question, they are likely to have good atmospheres, if any at all.

The study, which recently appeared in the journal Space Science Reviews, was conducted by an international team of researchers from the Geneva Astronomical Observatory (GAO), the University of Bern, the Laboratoire d’astrophysique de Bordeaux (LAB), the Astrophysics Research Group at Imperial College London, and the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado.

Initially, it was a team of astronomers from the University of Liege, Belgium, who detected three of the system’s exoplanets using Transit Spectroscopy (aka. the Transit Method). For this method, astronomers monitor stars for dips in their luminosity, which are the result of planets passing in front of the star (aka. transiting) relative to the observer.

The system was named TRAPPIST-1 in honor of the instrument used to detect them, which was the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) located at the ESO’s La Silla Observatory in Chile and the Observatoire de l’Oukaïmeden in Morocco. In February of 2017, the existence of four more exoplanets were confirmed, as well as the fact that three were orbiting with the star’s HZ.

Ever since then, the TRAPPIST-1 system has been considered an excellent candidate by astronomers for exoplanet research. There are a number of reasons for this, which Martin Turbet (a postdoctoral researcher at the GAO and the lead author on the study) explained to Universe Today via email:

“The TRAPPIST-1 system is very well-suited for habitability studies because it is the planetary system made of potentially habitable exoplanets that is easiest to observe and thus characterize with telescopes. This is mainly due to the fact that (1) the TRAPPIST-1 system is very nearby (39 light years from us), (ii) the planets are transiting (frequently) in front of their star, and (iii) the host star TRAPPIST-1 is an ultra-cool dwarf with an extremely small radius.”

In short, having seven exoplanets around a star means that there will be plenty of opportunities to spot them making transits in front of the star. On these occasions, astronomers are able to gather spectra from the planet as light from the star passes around the planet and through its atmosphere (a process known as transmission spectroscopy). Scientists are then able to examine this data to determine what chemical elements are present.

Because TRAPPIST-1 is an M-type (red dwarf) star – which are low in mass, cool, and relatively dim compared to other types of stars – the dips in luminosity that are caused by planetary transits are comparatively large. This property makes the transmission spectroscopy technique much easier to perform for any rocky planets in orbit, which applies to all seven planets in the TRAPPIST-1 system.

However, not all of the research conducted thus far has been very encouraging. In fact, multiple studies have been conducted that indicated that for some of the TRAPPIST-1 planets, water might make up a large part of their mass (making them “water worlds“). On top of that, there’s the nature of red dwarf stars, which are prone to flare-ups that could wreak havoc on their planets’ atmospheres.

However, other studies have found that exoplanets orbiting red dwarfs could still be habitable as long as they had sufficient atmospheres and cloud cover to deal with the radiation. To assess the likelihood that the TRAPPIST-1 planets had such atmospheres, Turbet and his colleagues considered all of the data that has been obtained on the TRAPPIST-1 system to date.

Artist’s impression shows several of the planets orbiting the ultra-cool red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser

This includes transit observations made of the planets, as well as density measurements, transmission spectroscopy, the system’s irradiation environment, theories on planetary formation and migration, the planets’ orbital stability, climate modeling, and models that consider how much gas the planets lose to space (aka. escape models).

“We reviewed all existing works on the topic, ranging from observations with the best telescopes available (Hubble Space Telescope, Spitzer Space Telescope, Very Large Telescope, etc.) to the most sophisticated theoretical models such as three-dimensional numerical climate models,” said Turbet.

What they found was rather encouraging. For starters, they were able to determine that most of the TRAPPIST-1 planets didn’t have cloud-free, low molecular weight atmospheres – similar to what Earth’s primordial atmosphere was like. Second, they found compelling evidence that those planets that did have atmospheres were likely composed of elements that have higher atomic weights. Or as Turbet summarized:

“We determined that the seven TRAPPIST-1 planets are unlikely to have hydrogen-dominated atmospheres. We also suggested that the atmospheres (if present) of the TRAPPIST-1 planets are most likely to be carbon dioxide-dominated, oxygen-dominated, or water-dominated.”

Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. Credits: NASA/JPL-Caltech

In other words, of the seven TRAPPIST-1 planets, those that have atmospheres are likely to have that kind that are favourable to life (at least, as we know it). That means carbon dioxide, an essential climate-stabilizer and necessary for photosynthetic organisms, oxygen gas, nitrogen, and volatile elements like water. It also includes cloud cover, which is not only an indication of water, but provides protection against stellar radiation.

Unfortunately, Turbet and his colleagues cannot say with confidence that the TRAPPIST-1 planets have atmospheres with all of these elements. This study does, however, place constraints on that possibility based on what we know about the system so far. In the end, knowing if any of the exoplanets in this system are habitable will have to wait on next-generation telescopes. As Turbet said:

“Next-generation missions – in particular the James Webb Space Telescope and the near-infrared ground-based spectrographs – will have the power to detect ‘heavy’ molecules such as carbon dioxide, oxygen, methane, etc. and thus they may have the potential to determine whether or not the TRAPPIST-1 planets have atmospheres, and if so, what they are made of.”

The JWST is scheduled to launch next year, whereas ground-based telescopes equipped next-generation spectrographs are expected to come online throughout this decade. With these and even more powerful instruments planned for the future, astronomers expect to finally know for certain if their is life beyond Earth in our corner of the galaxy!


TRAPPIST-1 System Ideal for Planet-Hopping Life?

Exoplanet hunters struck gold earlier this year with the discovery of seven rocky bodies orbiting around dwarf star TRAPPIST-1, a find that both raised hopes, and provided a new target, for understanding if life exists elsewhere in the cosmos. Now, a new Harvard paper suggests this populous planetary system could also test the ease with which life can hop between planets, and perhaps even end uncertainty over our own status as true Earth-lings.

This artist’s impression displays TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle.

For several decades, proponents of interplanetary exchange of life have imagined a broadly similar scenario — a meteorite or asteroid impacts a life-supporting planet throwing up rocks containing living or almost living stowaways.

Traveling across interplanetary space some of this material eventually impacts a neighbor planet where life, or its seeds, are successfully introduced.

Known as panspermia some suggest this contamination mechanism may be the most common method for life’s distribution across the Universe, and even raises the possibility that all life on Earth might have originated elsewhere.

However, the panspermia model is plagued by uncertainties. The intense heat generated by impacts could be a roadblock, as could the high concentration of cosmic and UV rays found across interplanetary space. Anything hitching a ride would need to survive a long time — several millions years between Earth and Mars.

All these uncertainties remain today despite advanced simulations and modeling.

Perhaps there is an easier way? Could we prove panspermia’s validity by finding evidence of it happening elsewhere?

“If the same biosignature gases are detected on planets in a single system, or if the spectral feature of vegetation occurs at the same wavelength, this could be a ‘smoking gun’,” says Manasvi Lingam from the Harvard-Smithsonian Center for Astrophysics, who believes such observations may fall within the capabilities of future planned telescopes like the Large UV/Optical/Infrared Surveyor (LUVOIR).

So where to look? Lingam thinks TRAPPIST-1 might not be a bad place to start.

The distance between the three planets confirmed to orbit within the system’s narrow habitable zone is 50 times less than that from Earth to Mars.

Lingam and his colleague Avi Loeb reasoned this should benefit panspermia’s chances both by increasing the amount of material exchanged, and reducing the travel time through dangerous interplanetary space.

“Even if one believes that the probability for life, as we know it, is small, the dice was rolled three times in the TRAPPIST-1 system leading to a higher chance of success,” says Loeb.

To get firmer answers, the two ‘did the Math’ – or at least some of it.

In a new paper published on the arXiv.org site, Lingam and Loeb use a simple model of the mechanics of the TRAPPIST-1 habitable zone to answer two questions: if debris is ejected from one planet, what is the probability it will be captured by a neighbor, and what would be the average travel time for this journey?

“These are two quantifiable mechanical factors that have significant biological implications,” says Lingam.

Their model suggests panspermia is several orders of magnitude more likely to occur in the TRAPPIST-1 system than the Earth-Mars system. In fact, they conclude that the more congested planetary orbits of planetary systems around most of these M-dwarf stars (the most common stars in our Galaxy), means the fraction of rock leaving one planet and hitting another could be as much as 1,000 times higher than between Earth and Mars.

For Loeb and Lingam, the close proximity of the TRAPPIST-1 planets was not just common to other M-dwarf star systems, but also reminiscent of an environment on the Earth, namely islands, which are subject to their own ‘immigration.’

Drawing on models of island biogeography and theoretical ecology, they suggest it might not be just the likelihood of panspermia that increases in around M-dwarfs but also the number of species potentially transferred, increasing biodiversity.

There are, however, limitations of this simple model.

Caleb Scharf, Director of Astrobiology at Colombia University, cautions against assuming the scale of impacts would be the same as we see in our solar system.

“In a system like TRAPPIST-1 where planets are so close-packed, there may not be a population of long-term asteroids or short period comets to provide the impacts needed to eject material and allow transfer between planets.”

The model also says nothing about the chances of life started in the first place, and is only able to quantify the fraction of rocks that would impact a particular planet, not the total number. Finally, it says nothing of the complex chemical biology that would underpin whether life would survive the impact of the travel journey.

Despite this, a confirmation of TRAPPIST-1’s relatively optimal conditions for panspermia could have significant implications in the future.

“If life is confirmed within this system but we find no evidence that panspermia has transferred it to another planet, it would be hard to envisage it happening in a far less suitable system like our own,” says Lingam, dealing a blow to theories of our own Martian origin.

Whilst any future discovery of panspermia around TRAPPIST-1, or another M-dwarf system, might seem like coming ‘after the Lord Mayor’s show,’ considering it would follow the realization that we are not alone in the Universe, it would still be revolutionary in its own right.

Vindicating this life spreading mechanism would fundamentally change our understanding of how life is distributed around the cosmos and change completely any debate around our own extra-terrestrial origins.

Manasvi Lingam & Abraham Loeb. 2017. Enhanced interplanetary panspermia in the TRAPPIST-1 system. PNAS, submitted for publication arXiv: 1703.00878


Do the TRAPPIST-1 planets have atmospheres?

Artist’s impression shows several of the planets orbiting the ultra-cool red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser

In February of 2017, the scientific community rejoiced as NASA announced that a nearby star (TRAPPIST-1) had a system of no less than seven rocky planets. Since that time, astronomers have conducted all kinds of follow-up observations and studies in the hopes of learning more about these exoplanets. In particular, they have been attempting to learn if any of the planets located in the stars' habitable zones (HZ) could actually be habitable.

Many of these studies have been concerned with whether or not the TRAPPIST-1 planets have sufficient water on their surfaces. But just as important is the question of whether or not any have viable atmospheres. In a recent study that provides an overview of all observations to date on TRAPPIST-1 planets, a team found that depending on the planet in question, they are likely to have good atmospheres, if any at all.

The study, which recently appeared in the journal Astrobiology, was conducted by an international team of researchers from the Geneva Astronomical Observatory (GAO), the University of Bern, the Laboratoire d'astrophysique de Bordeaux (LAB), the Astrophysics Research Group at Imperial College London, and the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado.

Initially, it was a team of astronomers from the University of Liege, Belgium, who detected three of the system's exoplanets using transit spectroscopy. For this method, astronomers monitor stars for dips in their luminosity, which are the result of planets passing in front of the star relative to the observer.

The system was named TRAPPIST-1 in honor of the instrument used to detect them, the Transiting Planets and Planetesimals Small Telescope (TRAPPIST), located at the ESO's La Silla Observatory in Chile and the Observatoire de l'Oukaïmeden in Morocco. In February of 2017, the existence of four more exoplanets were confirmed, as well as the fact that three were orbiting with the star's HZ.

Ever since then, the TRAPPIST-1 system has been considered an excellent candidate by astronomers for exoplanet research. There are a number of reasons for this, which Martin Turbet (a postdoctoral researcher at the GAO and the lead author on the study) explained to Universe Today via email:

"The TRAPPIST-1 system is very well suited for habitability studies because it is the planetary system made of potentially habitable exoplanets that is easiest to observe and thus characterize with telescopes. This is mainly due to the fact that (1) the TRAPPIST-1 system is very nearby (39 light years from us), (ii) the planets are transiting (frequently) in front of their star, and (iii) the host star TRAPPIST-1 is an ultra-cool dwarf with an extremely small radius."

In short, having seven exoplanets around a star means that there will be plenty of opportunities to spot them making transits in front of the star. On these occasions, astronomers are able to gather spectra from the planet as light from the star passes around the planet and through its atmosphere (a process known as transmission spectroscopy). Scientists are then able to examine this data to determine what chemical elements are present.

Because TRAPPIST-1 is an M-type (red dwarf) star—which are low in mass, cool, and relatively dim compared to others types of stars—transmission spectroscopy obtained from its planets is less likely to be subject to the transit light source effect (TLSE, or "stellar contamination"). This is where spectra readings obtained from the planets are thrown off by spectra from the star itself.

However, not all of the research conducted thus far has been very encouraging. In fact, multiple studies have been conducted that indicated that for some of the TRAPPIST-1 planets, water might make up a large part of their mass (making them "water worlds"). On top of that, there's the nature of red dwarf stars, which are prone to flare-ups that could wreak havoc on their planets' atmospheres.

However, other studies have found that exoplanets orbiting red dwarfs could still be habitable as long as they had sufficient atmospheres and cloud cover to deal with the radiation. To assess the likelihood that the TRAPPIST-1 planets had such atmospheres, Turbet and his colleagues considered all of the data that has been obtained on the TRAPPIST-1 system to date.

This includes transit observations made of the planets, as well as density measurements, transmission spectroscopy, the system's irradiation environment, theories on planetary formation and migration, the planets' orbital stability, climate modeling, and models that consider how much gas the planets lose to space (aka. escape models).

"We reviewed all existing works on the topic, ranging from observations with the best telescopes available (Hubble Space Telescope, Spitzer Space Telescope, Very Large Telescope, etc.) to the most sophisticated theoretical models such as three-dimensional numerical climate models," said Turbet.

What they found was rather encouraging. For starters, they were able to determine that most of the TRAPPIST-1 planets had cloud-free, low-molecular-weight atmospheres, similar to what Earth's primordial atmosphere was like. Second, they found compelling evidence that those planets that did have atmospheres were likely composed of elements that have higher atomic weights. Turbet said, "We determined that the seven TRAPPIST-1 planets are unlikely to have hydrogen-dominated atmospheres. We also suggested that the atmospheres (if present) of the TRAPPIST-1 planets are most likely to be carbon dioxide-dominated, oxygen-dominated or water-dominated."

In other words, of the seven TRAPPIST-1 planets, those that have atmospheres are likely to have the kind that are favorable to life (at least as we know it). That means carbon dioxide, an essential climate stabilizer necessary for photosynthetic organisms, oxygen gas, nitrogen, and volatile elements like water. It also includes cloud cover, which is not only an indication of water, but provides protection against stellar radiation.

Unfortunately, Turbet and his colleagues cannot say with confidence that the TRAPPIST-1 planets have atmospheres with all of these elements. This study does, however, place constraints on that possibility based on what we know about the system so far. In the end, knowing if any of the exoplanets in this system are habitable will have to wait on next-generation telescopes. Turbet said, "Next-generation missions—in particular the James Webb Space Telescope and the near-infrared ground-based spectrographs—will have the power to detect 'heavy' molecules such as carbon dioxide, oxygen, methane, etc. and thus they may have the potential to determine whether or not the TRAPPIST-1 planets have atmospheres, and if so, what they are made of."

The JWST is scheduled to launch next year, whereas ground-based telescopes equipped with next-generation spectrographs are expected to come online throughout this decade. With these and even more powerful instruments planned for the future, astronomers expect to finally know for certain if their is life beyond Earth in our corner of the galaxy.


New Clues to Compositions of TRAPPIST-1 Planets

The seven Earth-size planets of TRAPPIST-1 are all mostly made of rock, with some having the potential to hold more water than Earth, according to a new study.

The seven Earth-size planets of TRAPPIST-1 are all mostly made of rock, with some having the potential to hold more water than Earth, according to a new study published in the journal Astronomy and Astrophysics. The planets' densities, now known much more precisely than before, suggest that some planets could have up to 5 percent of their mass in water -- which is 250 times more than the oceans on Earth.

The form that water would take on TRAPPIST-1 planets would depend on the amount of heat they receive from their star, which is a mere 9 percent as massive as our Sun. Planets closest to the star are more likely to host water in the form of atmospheric vapor, while those farther away may have water frozen on their surfaces as ice. TRAPPIST-1e is the rockiest planet of them all, but still is believed to have the potential to host some liquid water.

Astronomers using the Hubble Space Telescope have conducted the first spectroscopic survey of Earth-sized planets in the TRAPPIST-1 system's habitable zone. Hubble reveals that at least the inner five planets do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune. This means the atmospheres may be more shallow and rich in heavier gases like carbon dioxide, methane, and oxygen.

"We now know more about TRAPPIST-1 than any other planetary system apart from our own," said Sean Carey, manager of the Spitzer Science Center at Caltech/IPAC in Pasadena, California, and co-author of the new study. "The improved densities in our study dramatically refine our understanding of the nature of these mysterious worlds."

Since the extent of the system was revealed in February 2017, researchers have been working hard to better characterize these planets and collect more information about them. The new study offers better estimates than ever for the planets' densities.

TRAPPIST-1 is named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, which discovered two of the seven planets we know of today -- announced in 2016. NASA's Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed these planets and uncovered the other five in the system.

Since then, NASA's Kepler space telescope has also observed the TRAPPIST-1 system, and Spitzer began a program of 500 additional hours of TRAPPIST-1 observations, which will conclude in March. This new body of data helped study authors paint a clearer picture of the system than ever before -- although there is still much more to learn about TRAPPIST-1.

The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of these worlds would have a spectacular view of the neighboring planets in the sky. Those planets would sometimes appear larger than the Moon looks to an observer on Earth. They may also be tidally locked, meaning the same side of the planet is always facing the star, with each side in perpetual day or night. Although the planets are all closer to their star than Mercury is to the Sun, TRAPPIST-1 is such a cool star, some of its planets could still, in theory, hold liquid water.

In the new study, scientists led by Simon Grimm at the University of Bern in Switzerland created computer models to better simulate the planets based on all available information. For each planet, researchers had to come up with a model based on the newly measured masses, the orbital periods and a variety of other factors -- making it an extremely difficult, "35-dimensional problem," Grimm said. It took most of 2017 to invent new techniques and run simulations to characterize the planets' compositions.

This chart shows, on the top row, artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity as compared to those of Earth. Credit: NASA/JPL-Caltech
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What might these planets look like?

It is impossible to know exactly how each planet looks, because they are so far away. In our own solar system, the Moon and Mars have nearly the same density, yet their surfaces appear entirely different.

"Densities, while important clues to the planets' compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life," said Brice-Olivier Demory, co-author at the University of Bern.

Based on available data, here are scientists' best guesses about the appearances of the planets:

TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than Earth's. TRAPPIST-1c also likely has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets -- about 30 percent the mass of Earth. Scientists are uncertain whether it has a large atmosphere, an ocean or an ice layer -- all three of these would give the planet an "envelope" of volatile substances, which would make sense for a planet of its density.

Scientists were surprised that TRAPPIST-1e is the only planet in the system slightly denser than Earth, suggesting it may have a denser iron core than our home planet. Like TRAPPIST-1c, it does not necessarily have a thick atmosphere, ocean or ice layer -- making these two planets distinct in the system. It is mysterious why TRAPPIST-1e has a much rockier composition than the rest of the planets. In terms of size, density and the amount of radiation it receives from its star, this is the most similar planet to Earth.

TRAPPIST-1f, g and h are far enough from the host star that water could be frozen as ice across these surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules of Earth, such as carbon dioxide.

"It is interesting that the densest planets are not the ones that are the closest to the star, and that the colder planets cannot harbor thick atmospheres," said Caroline Dorn, study co-author based at the University of Zurich, Switzerland.

This graph presents known properties of the seven TRAPPIST-1 exoplanets (labeled b thorugh h), showing how they stack up to the inner rocky worlds in our own solar system. Credit: NASA/JPL-Caltech
› Full image and caption

Scientists are able to calculate the densities of the planets because they happen to be lined up such that when they pass in front of their star, our Earth- and space-based telescopes can detect a dimming of its light. This is called a transit. The amount by which the starlight dims is related to the radius of the planet.

To get the density, scientists take advantage of what are called "transit timing variations." If there were no other gravitational forces on a transiting planet, it would always cross in front of its host star in the same amount of time -- for example, Earth orbits the Sun every 365 days, which is how we define one year. But because the TRAPPIST-1 planets are packed so close together, they change the timing of each other's "years" ever so slightly. Those variations in orbital timing are used to estimate the planets' masses. Then, mass and radius are used to calculate density.

This illustration shows the seven Earth-size planets of TRAPPIST-1. The image does not show the planets' orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look. Image Credit: NASA/JPL-Caltech
› Full image and caption

The next step in exploring TRAPPIST-1 will be NASA's James Webb Space Telescope, which will be able to delve into the question of whether these planets have atmospheres and, if so, what those atmospheres are like. A recent study using NASA's Hubble Space Telescope found no detection of hydrogen-dominated atmospheres on planets TRAPPIST-1d, e and f -- another piece of evidence for rocky composition -- although the hydrogen-dominated atmosphere cannot be ruled out for g.

Illustrations of these worlds will change as ongoing scientificinvestigations home in on their properties.

"Our conceptions of what these planets look like today may change dramatically over time," said Robert Hurt, senior visualization scientist at the Spitzer Science Center. "As we learn more about these planets, the pictures we make will evolve in response to our improved understanding.


The seven rocky planets of TRAPPIST-1 seem to have very similar compositions

A new international study led by astrophysicist Eric Agol from the University of Washington has measured the densities of the seven planets of the exoplanetary system TRAPPIST-1 with extreme precision, the values obtained indicating very similar compositions for all the planets. This fact makes the system even more remarkable and helps to better understand the nature of these fascinating worlds. This study has just been published in the Planetary Science Journal.

The TRAPPIST-1 system is home to the largest number of planets similar in size to our Earth ever found outside our solar system. Discovered in 2016 by a research team led by Michaël Gillon, astrophysicist at the University of Liège, the system offers an insight into the immense variety of planetary systems that probably populate the Universe. Since their detection, scientists have studied these seven planets using multiple space (NASA's Kepler and Spitzer telescopes) and ground-based telescopes (TRAPPIST and SPECULOOS in particular). The Spitzer telescope alone, managed by NASA's Jet Propulsion Laboratory, provided more than 1,000 hours of targeted observations of the system before being decommissioned in January 2020.

Hours of observations that enabled to refine the information we have on the exoplanetary system. "Since we can't see the planets directly, we analyze in detail the variations of the apparent brightness of their star as they 'transit' it, i.e. as they passes in front of it," explains Michaël Gillon." Previous studies had already enabled astronomers to take precise measurements of the masses and diameters of the planets, which led to the determination that they were similar in size and mass to our Earth and that their compositions must have been essentially rocky. "Our new study has greatly improved the precision of the densities of the planets, the measurements obtained indicating very similar compositions for these seven worlds," says Elsa Ducrot, a doctoral student at ULiège. "This could mean that they contain roughly the same proportion of materials that make up most rocky planets, such as iron, oxygen, magnesium and silicon, which make up our planet. "After correcting for their different masses, the researchers were able to estimate that they all have a density of around 8% less than the Earth's, a fact that could have an impact on their compositions.

A different recipe

The authors of the study put forward three hypotheses to explain this difference in density with our planet. The first involves a composition similar to that of the Earth, but with a lower percentage of iron (about 21% compared to the 32% of the Earth). Since most of the iron in the Earth's composition is found in the Earth's core, this iron depletion of the TRAPPIST-1 planets could therefore indicate cores with lower relative masses. The second hypothesis implies oxygen-enriched compositions compared to that of our planet. By reacting with iron, oxygen would form iron oxide, better known as 'rust'. The surface of Mars gets its red colour from iron oxide, but like its three terrestrial sisters (Earth, Mercury, and Venus), it has a core of unoxidised iron. However, if the lower density of the TRAPPIST-1 planets was entirely due to oxidised iron, then the planets would be 'rusted to the heart' and may not have a real core, unlike the Earth. According to Eric Agol, an astrophysicist at the University of Washington and lead author of the new study, the answer could be a combination of both scenarios -- less iron in general and some oxidised iron.

The third hypothesis put forward by the researchers is that the planets are enriched with water compared to the Earth. This hypothesis would agree with independent theoretical results indicating a formation of the TRAPPIST-1 planets further away from their star, in a cold, ice-rich environment, followed by internal migration. If this explanation is correct, then water could account for about 5% of the total mass of the four outer planets. In comparison, water accounts for less than one tenth of 1% of the total mass of the Earth. The three inner planets in TRAPPIST-1, located too close to their stars for water to remain liquid under most circumstances, would need hot, dense atmospheres like on Venus, where water could remain bound to the planet in the form of vapour. But according to Eric Agol, this explanation seems less likely because it would be a coincidence that all seven planets have just enough water present to have such similar densities.

"The night sky is full of planets, and it is only within the last 30 years that we have been able to begin to unravel their mysteries," rejoices Caroline Dorn, astrophysicist at the University of Zurich and co-author of the article. "The TRAPPIST-1 system is fascinating because around this unique star we can learn about the diversity of rocky planets within a single system. And we can also learn more about a planet by studying its neighbours, so this system is perfect for that.


New research shows that the TRAPPIST-1 planets are even more Earth-like than we thought

Researchers from Bern University have performed the most accurate calculation of the density of the seven planets around the star TRAPPIST-1, making them the best-studied planets outside our own solar system. They found that all the planets are rocky and contain 5% water — much more than what Earth has.

An artistic depiction of what the seven planets might look like, based on available information about their size, density, and distance to the star. Credits: NASA/JPL/Caltech.

TRAPPIST-1, also technically designated as 2MASS J23062928-0502285, is a seemingly inconspicuous ultra-cool red dwarf star. However, astronomers have taken a special interest in it, as it seemingly harbors seven Earth-like planets, several of which have the potential to host life. Now, a new study offers an unprecedented view into the physical and chemical characteristics of these planets.

In order to assess their density, astronomers took advantage of something called “transit timing variations,” a method which has also been used to detect planets. Essentially, when a planet passes in front of a star, it causes a dip in luminosity. By studying that dip, you can identify a planet and its size. But astronomers went even deeper.

The TRAPPIST planets are located very close to each other — if you had an observer on one of them, they’d have a pretty awesome view of at least a couple other planets. But this also means that they attract each other gravitationally. If a single planet were to rotate around the star, or if the planets were too far apart for gravity to make a significant difference, a planet would always cross in front of its host star at consistent intervals of time. But because they’re so closely packed, they change the timing of each other’s “years” ever so slightly. This allowed astronomers to deduce the mass of the planets with an uncertainty less than 10%, and with the size and the mass, they calculated the density.

“We now know more about TRAPPIST-1 than any other planetary system apart from our own,” said Sean Carey, manager of the Spitzer Science Center at Caltech/IPAC in Pasadena, California, and co-author of the new study. “The improved densities in our study dramatically refine our understanding of the nature of these mysterious worlds.”

But while the planets might be similar to Earth, TRAPPIST-1 is extremely small by stellar standards, being only 9 percent as massive as our Sun. In turn, this means that it’s much cooler than our star, and in order for the planets to have Earth-like temperatures, they’d first need to be much closer to the star. This has indeed been confirmed by observation, as several of the planets lie closer to TRAPPIST-1 than Mercury to the Sun. In fact, a couple of them are actually much more illuminated than the Earth, while another planet, Trappist-d, matches Earth near-perfectly in terms of illumination.

This graph presents known properties of the seven TRAPPIST-1 exoplanets (labeled b through h), showing how they compare to the inner rocky worlds in our own solar system. Credit: NASA/JPL-Caltech.

The fact that the planets have up to 5% water is also an intriguing sign, especially considering that on Earth, water accounts for only 0.02% of the planet’s mass.

However, just because these planets have the right density, structure, and distance from their star, doesn’t make them habitable. It makes them potentially habitable, which is a big distinction.

“Densities, while important clues to the planets’ compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life,” said Brice-Olivier Demory, co-author at the University of Bern.

This illustration shows the seven Earth-size planets of TRAPPIST-1. The image does not show the planets’ orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look. Credit: NASA/JPL-Caltech.

For instance, we have no idea how their surfaces might look like. Mars and the Moon have quite similar densities, but they look completely different. It’s tantalizing to think what the surface of these planets might be like, but for now, we just don’t have enough information to make any assumption. The next step will be using NASA’s James Webb Space Telescope to figure out whether these planets have atmospheres and if yes, what these atmospheres are like.

“Our conceptions of what these planets look like today may change dramatically over time,” said Robert Hurt, senior visualization scientist at the Spitzer Science Center. “As we learn more about these planets, the pictures we make will evolve in response to our improved understanding.

Journal Reference: S. Grimm et al.: The nature of the TRAPPIST-1 exoplanets, Astronomy and Astrophysics, 05.02.2018, in press.


Stellar characteristics [ edit | edit source ]

TRAPPIST-1 is an ultra-cool dwarf star of spectral class M8.0±0.5 that is approximately 8% the mass of and 11% the radius of the Sun. Although it is slightly larger than Jupiter, it is about 84 times more massive. High-resolution optical spectroscopy failed to reveal the presence of lithium, suggesting it is a very low-mass main-sequence star, which is fusing hydrogen and has depleted its lithium, i.e., a red dwarf rather than a very young brown dwarf. It has a temperature of 2,516 K (2,243 °C 4,069 °F), and its age has been estimated to be in the range of 3 to 8 Gyr. In comparison, the Sun has a temperature of 5,778 K (5,505 °C 9,941 °F) and an age of about 4.6 Gyr. Observations with the Kepler K2 extension for a total of 79 days revealed starspots and infrequent weak optical flares at a rate of 0.38 per day (30-fold less frequent than for active M6-M9 dwarfs) a single strong flare appeared near the end of the observation period. The observed flaring activity possibly changes the atmospheres of the orbiting planets on a regular basis, making them less suitable for life. The star has a rotational period of 3.3 days.

High-resolution speckle images of TRAPPIST-1 were obtained and revealed that the M8 star has no companions with a luminosity equal to or brighter than a brown dwarf. This determination that the host star is single confirms that the measured transit depths for the orbiting planets provide a true value for their radii, thus proving that the planets are indeed Earth-sized.

Owing to its low luminosity, the star has the ability to live for up to 12 trillion years. It is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. Its luminosity is 0.05% of that of the Sun (L☉), most of which is emitted in the infrared spectrum, and with an apparent magnitude of 18.80 it is not visible to the naked eye from the Earth.


An Update on the Potential Habitability of TRAPPIST-1. No Aliens yet, but We’ve Learned a lot.

One year ago, I wrote an article about the remarkable discovery of the TRAPPIST-1 planetary system, a system of seven temperate terrestrial planets orbiting an ultra-cool red dwarf star. This was an enormous astronomical discovery because these low-mass stars are the most numerous ones in our galaxy, and the discovery of potentially habitable planets around one of them led many people to speculate about the existence of life there and elsewhere in our galaxy around similar stars.

This announcement also inspired a lot of additional studies by astronomers worldwide, who have used additional instruments and run complex models to better understand this planetary system and its potential for hosting life.
One year later, it seems to me that the time is right to give you an update on what we’ve learned about this planetary system, which is located only 41 light-years from Earth.

Better Understanding of the Planetary System
Between December 2016 and March 2017, additional data on TRAPPIST-1 were collected using the Kepler spacecraft in the K2 program. Kepler was designed to measure transits of exoplanets, but observations of TRAPPIST-1 were a huge challenge even for this remarkable planet-hunting spacecraft because TRAPPIST-1 is very faint in visible light. During its lifetime, astronomers have learned a lot about Kepler’s many capabilities, including better ways to reach the sensitivity necessary to detect the signatures of TRAPPIST-1-type transits (typically 0.1% the flux of the star). The authors of an article published in May 2017 in Nature were able to constrain the orbital period of the outermost planet, TRAPPIST-1h (P=18.766 days). Their work shows that the seven planets are, as suspected, in three-body resonances in a complex chain that suggests good stability over a very long period of time.

Keep in mind that we do not see the planets but detect only their shadow using the transit technique that gives us a good estimate of a planet’s size and its orbit. However, to truly understand the nature of a planet, we also need to determine its density, and hence its mass. In an effort to estimate mass in multiple systems, astronomers have used a technique called transit-timing variations (or TTV). This technique consists of measuring a small shift in the timing of a transit caused by gravitational interaction with the other planets in the system. Using a new algorithm and a complete set of data, including data from both TRAPPIST and K2, a team of scientists has significantly improved the density measurements of the TRAPPIST-1 planets, which range from 0.6 to 1.0 times the density of Earth, or a density measurement similar to what we see in the terrestrial planets in our solar system. If we also consider the amount of light we receive from these planets, TRAPPIST-1 e is probably the most Earth-like one in the system. A paper published in February 2018 also included a discussion of the interior of these planets and suggested that TRAPPIST-1 c and e have large rocky interiors and -b, -d, -f, -g should have thick atmospheres, oceans, or icy crusts.

Figure 2: Revised density and incident flux received by the TRAPPIST-1 planets (in red) compared to our solar system’s terrestrial planets (from Grimms et al. 2018)

To understand a planetary system, we need accurate information about its most massive object, its star. Stellar astronomers have improved their knowledge of TRAPPIST-1’s star and now estimate its age to be between 5 and 10 billion years, which makes it older than our sun. This estimate is based on various methods, including the study of its activity, its rotation rate, and its location in the Milky Way. Its mass has also been revised to 9% the mass of our sun, which slightly affects the distance of the planet from the host star.

While observing the TRAPPIST system, astronomers have also detected strong star- like flares (seen, for instance, toward the end of the K2 observations). UV monitoring by the Hubble Space Telescope and by XMM/Newton combined with modeling revealed that the inner planets may have lost a large amount of water, but the outermost ones probably retain most of theirs. The complexity of these outgassing models and interactions with the stellar wind, when combined with planetary masses, are key to understand the nature of TRAPPIST-1’s planets and their potential habitability.

Dynamicists, who represent another important astronomical subdiscipline, have also taken an interest in this complex system. With seven planets surrounding a low-mass star, one can legitimately wonder about system stability. Their models show us that the system can be stable over billions of years, which is outstanding news if you want life to flourish there.

New Experiments and Innovative Ideas
We now have unambiguous proof of the existence of the TRAPPIST-1 planets, and we know about their orbits, their size, and their mass, but a lot still remains to be learned before we can claim that they have liquid water on their surface, and we need to know far more than that before we can conclude that these planets might be habitable, or inhabited.

One of the key challenges to computing the surface temperature of a planet is the existence and composition of its atmosphere. The atmosphere can act like a blanket, warming up the planetary surface. Using the Hubble Space Telescope, astronomers have attempted to detect the presence of rich hydrogen-dominated atmospheres around TRAPPIST-1 planets d, e, f, and g. Multi-color transit events taken in the near-infrared have ruled out such an atmosphere for planets d, e, and f. A H2-dominated atmosphere would lead to high surface temperatures and pressures, which are incompatible with the presence of liquid water. This negative detection suggests that these planets could have an Earth-like atmosphere with a temperate surface climate, which is more good news if, like me, you’re interested in habitability.

Figure 3: The Hubble observations revealed that the planets do not have hydrogen-dominated atmospheres. The flatter spectrum shown in the lower illustration indicates that Hubble did not spot any traces of water or methane, which are abundant in hydrogen-rich atmosphere (Credit: NASA, ESA and Z. Levy (STScI)

If life appeared on one TRAPPIST-1 planet at a time when it was hospitable, what are the chances that it spread throughout the entire system? Two astronomers discussed this hypothesis in a short article published in June 2017 and used a simple model for lithopanspermia (the transfer of organisms in rocks from one planet to another) to discover that the likelihood of that happening is orders of magnitude higher than for the Earth-to-Mars system. In compact TRAPPIST-1, the probability of impact is higher and the transit time between planets is shorter, which makes contamination among planets more likely. They concluded that the probably of abiogenesis (the appearance of life) is enhanced for TRAPPIST-1. Of course, this is pure speculation based on physical considerations that need to be backed up by observations, but it reinforced the importance of finding such compact mini-planetary systems elsewhere the galaxy.

Life can exist on moons as well as planets, and a moon can be a significant contributor to the presence of life because its sheer presence can stabilize the planet’s axis of rotation and create tidal pools that may be necessary for complex molecules to form and interact. No moons have been detected around the TRAPPIST-1 planets, even though the Spitzer observations were able to detect a moon as large as Earth’s. Theoretical study shows that the inner planets (-b to -e) are unlikely to have small moons because of the proximity of their star and other planets. We are not yet able to detect the presence of a small moon circling one of the outermost planets, and will not be able to detect one without using bigger telescopes in space and on the ground.

Induction heating is a process used on Earth to melt metal. It occurs when we change the magnetic field in a conducting medium, which then dissipates the energy through heat. Astronomers have known for a few years that M-type stars like TRAPPIST-1 have a strong magnetic field. A group of astronomers studied the effect of such a strong magnetic field on the interior of planets in a system tilted with respect to the magnetic field of their star. Assuming a planetary interior and composition similar to Earth, they determined that the three innermost planets (-b, -c, -d) should experience enhanced volcanic activity and outgassing, and in some extreme cases have developed a magma ocean with plate tectonics and large-scale earthquakes, comparable to Io, a satellite of Jupiter. Again, this result is extremely model-dependent since we don’t yet have a clear idea of the internal composition of those planets, which will directly affect the strength of the induction heating. However, if they are truly Earth-like in composition, they could be a hellish version of our own planet.

Other scientists have also discussed the existence of significant plate tectonics and intense earthquakes in this system due to tidal stress introduced by planet-to-star and planet-to-planet interactions. If the activity is right, some of the TRAPPIST-1 planets could indeed be similar to Earth with the equivalent of continental plates, ocean floors, and active volcanoes, but one day we will need to take a picture to confirm this.

What’s next?
I have summarized some of the latest articles published over the past two years about the wonderful TRAPPIST-1 system. This list is not exhaustive and I probably missed some interesting ideas and new hypotheses about this complex system.
But one thing is crystal-clear: My readings have left me (and a lot of other people) stoked about what we might find from additional observations with large ground-based telescopes, including an Extremely Large Telescope (like the TMT, ELT, or GMT), or the James Webb Space Telescope (JWST). Each of these facilities is needed to constrain our models and refine our understanding of this system. For instance, long-term monitoring of the system with these facilities will place further constraints on the presence of moons in the system. Using the accurate photometry made possible by JWST, astronomers hope to constrain planetary masses and orbits to a great accuracy, derive the composition of their atmospheres, construct crude temperature maps of all of the planets in the TRAPPIST-1 system.
After 2020, if everything goes well with JWST and if the space telescope provides the superb data that we expect, we might have a crude map of the TRAPPIST-1 planets, similar to the rough image of Pluto made with Hubble Space Telescope and later validated by the New Horizons Spacecraft.

Figure 4: A comparison between images of Pluto obtained by New Horizons by direct imaging and the Hubble Space Telescope by lightcurve reconstruction. Credit: NASA (Picture combined and labeled by S. Hariri)

In less than two decades, nearby planetary systems like TRAPPIST-1 will become our cosmic backyard, and if everything goes as planned with missions like TESS, PLATO, ARIEL, and JWST as well as the ELTs, we will soon learn the secrets of those exotic worlds which, I am convinced, will surprise us by their diversity, just as our own solar system has surprised us over the past two decades, surprises us today, and will surely continue to surprise us in the future.
Clear skies,

If you want to learn more about the TRAPPIST-1 system, check out some of those articles (all available for free on ArXiV).

Boss, Alan P., Alycia J. Weinberger, Sandra A. Keiser, Tri L. Astraatmadja, Guillem Anglada-Escude, and Ian B. Thompson. 2017. Astrometric Constraints on the Masses of Long-Period Gas Giant Planets in the TRAPPIST-1 Planetary System. The Astronomical Journal, Volume 154, Issue 3, article id. 103, 6 pp. (2017). 154. doi:10.3847/1538-3881/aa84b5.

Bourrier, V., J. de Wit, E. Bolmont, V. Stamenkovic, P. J. Wheatley, A. J. Burgasser, L. Delrez, et al. 2017. Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets. The Astronomical Journal, Volume 154, Issue 3, article id. 121, 17 pp. (2017). 154. doi:10.3847/1538-3881/aa859c.

Burgasser, Adam J., and Eric E. Mamajek. 2017. On the Age of the TRAPPIST-1 System. The Astrophysical Journal, Volume 845, Issue 2, article id. 110, 10 pp. (2017). 845. doi:10.3847/1538-4357/aa7fea. de Wit, J., H. R. Wakeford, N. Lewis, L. Delrez, M. Gillon, F. Selsis, J. Leconte, et al. 2018. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nature Astronomy, Volume 2, p. 214-219 2: 214–219. doi:10.1038/s41550-017-0374-z.

Grimm, S, B-O Demory, M Gillon, C Dorn, E Agol, A Burdanov, L Delrez, et al. 2018. The nature of the TRAPPIST-1 exoplanets. Astronomy & Astrophysics. doi:10.1051/0004-6361/201732233.

Kane, Stephen R., and Stephen R. 2017. Worlds Without Moons: Exomoon Constraints for Compact Planetary Systems. The Astrophysical Journal Letters, Volume 839, Issue 2, article id. L19, 4 pp. (2017). 839. doi:10.3847/2041-8213/aa6bf2.
Kislyakova, K. G., L. Noack, C. P. Johnstone, V. V. Zaitsev, L. Fossati, H. Lammer, M. L. Khodachenko, P. Odert, and M. Guedel. 2017. Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating. Nature Astronomy, Vol. 1, p. 878-885 (2017) 1: 878–885. doi:10.1038/s41550-017-0284-0.

Lingam, Manasvi, and Abraham Loeb. 2017. Enhanced interplanetary panspermia in the TRAPPIST-1 system. Proceedings of the National Academy of Sciences, vol. 114, issue 26, pp.6689-6693 114: 6689–6693. doi:10.1073/pnas.1703517114.

Luger, Rodrigo, Marko Sestovic, Ethan Kruse, Simon L. Grimm, Brice-Olivier Demory, Eric Agol, Emeline Bolmont, et al. 2017. A seven-planet resonant chain in TRAPPIST-1. Nature Astronomy, Volume 1, id. 0129 (2017). 1. doi:10.1038/s41550-017-0129.

Tamayo, Daniel, Hanno Rein, Cristobal Petrovich, and Norman Murray. 2017. Convergent Migration Renders TRAPPIST-1 Long-lived. The Astrophysical Journal Letters, Volume 840, Issue 2, article id. L19, 6 pp. (2017). 840. doi:10.3847/2041-8213/aa70ea.

Van Grootel, Valerie, Catarina S. Fernandes, Michaël Gillon, Emmanuel Jehin, Jean Manfroid, Richard Scuflaire, Adam J. Burgasser, et al. 2017. Stellar parameters for TRAPPIST-1. The Astrophysical Journal, Volume 853, Issue 1, article id. 30, 7 pp. (2018). 853. doi:10.3847/1538-4357/aaa023.

Zanazzi, J. J., and Amaury Triaud. 2017. Initiation of Plate Tectonics on Exoplanets with Significant Tidal Stress. eprint arXiv:1711.09898.


Watch the video: Exploring the TRAPPIST-1 System (May 2022).