Astronomy

Detecting habitable planets

Detecting habitable planets


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From my understanding we detect planets by measuring dips in light intensity from the star the habitable planet is orbiting when it passes by it. There are 2 things I don't understand about this method:

  1. Planets in a solar system tend to orbit their star in one disc like plane. Can we only detect planets if this plane is in-alignment with our sensors here on earth/ space. I assume if someone looked at our sun from the 'bottom' they would never see a planet cross it. If so this would exclude a big portion of the stars we are looking at?

  2. Our orbit is 1 year. I assume the orbits of habitable plants can vary and having a short/ close to 1 year orbit is not a criteria for habitability. Even so we have to measure 1 dip every year or 10. How can we tell that this a planet with a regular yearly orbit or just anything passing between the star and us. So maybe a better question to ask for number 2 is what can we find out from these dips?


  1. Yes and yes. Transit detection is only effective for that (small) fraction of planets that pass between the star and our line of sight. Most planets will go undetected by this method.

  2. Also correct. The detection of multiple transits is required to find planets. Even then the dips can be caused by other things (e.g. grazing eclipse binary star systems). This means to find Earthlike planets in Earthlike orbits would require years of observation. But habitable zones around less luminous stars are closer-in and have shorter orbital periods. It is these stars that planet-hunting missions like TESS are focused on.


It isn't the only way, Aaron. See the Wikipedia Methods of detecting exoplanets article. The first method listed is radial velocity. That's where we measure the Doppler shift of the star's "wobble".

The second method is transit photography. And as you say it has a drawback. In fact there's more than one:

"This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit (in small stars, the radius of the planet is also an important factor). About 10% of planets with small orbits have such an alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a Sun-sized star at 1 AU, the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot guarantee that any particular star is not a host to planets…

The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems… "


29 Habitable Alien Worlds Could Detect Life on Earth, Scientists Estimate

Astronomers have discovered thousands of planets outside of our solar system over the past few decades, revealing a multitude of captivating worlds within our galaxy. This golden age of exoplanet discovery raises an alluring question: If we can spot worlds orbiting stars from Earth, could speculative alien civilizations spot Earth from their own vantage points?

Most exoplanets are discovered as they pass in front of their host star from our perspective, causing a very slight dip in stellar brightness that can be detected by telescopes on Earth. This technique, known as the transit method, has enabled scientists to spot and even characterize basic details about these worlds, including whether they might be potentially habitable.

Now, a pair of exoplanet researchers have inverted this process by cataloging which star systems within roughly 300 light years of our solar system are in the perfect spot to witness Earth crossing in front of the Sun. As it turns out, a whopping 1,715 stars 𠇊re in the right position to have spotted life on a transiting Earth since early human civilization (about 5,000 years ago), with an additional 319 stars entering this special vantage point in the next 5,000 years,” according to a study published on Wednesday in Nature.

𠇎verything moves in the universe,” said Lisa Kaltenegger, an associate professor of astronomy and director of the Carl Sagan Institute at Cornell University who co-authored the new study, in a call. “The cosmos is dynamic—we are moving around the Sun, the Sun moves around the center of the galaxy—so this vantage point or this cosmic front seat, so to say, to see the Earth backlit or as a transiting planet, has to be at a point that is both gained and also lost.” 

“It’s a little bit like ships in the night passing each other, and some see each other and some don’t,” she added. 

To explore these relationships between exoplanets, Kaltenegger teamed up with Jaqueline Faherty, a senior scientist at the American Museum of Natural History, who is well-versed in utilizing the immense observations collected by the European Space Agency’s Gaia satellite. Gaia, launched in 2013, is currently building the most comprehensive map of space objects in the Milky Way. 

The sheer magnitude of Gaia’s catalogue, which tracks millions of stars and their movements, allowed the researchers to pinpoint systems in what they called the Earth transit zone (ETZ) over the past 5,000 years, along with those that will enter it during the next  5,000 years. The list of those systems is compiled at this link.

While past studies have identified star systems that can spot Earth transits in the present, Kaltenegger and Faherty are the first researchers to expand this temporal aperture out to a range of 10,000 years spanning the past and (fingers crossed) future of human civilization. The results revealed an abundance of systems that have occupied the right position to watch Earth transits for millennia at a time.

In other words, if there are any hypothetical aliens that live in the star systems identified in the study, they would have had ample opportunity to spot Earth in front of the Sun and perhaps even identify signs of life and intelligence on our world, such as by identifying radio signals. 

“What we showed in our paper is that most stars have this vantage point [to see Earth transits] for at least 1,000 years, and a lot of stars actually have it for more than 10,000 years,” Kaltenegger said. “We couldn&apost say anything more than that because our timeline is 10,000 years, but it was interesting that this vantage point holds for generations of astronomers, or generations of alien astronomers” that 𠇌ould develop technology to find us.”

The new study also spotlights subpopulations within that list that might be particularly interesting in the search for extraterrestrial intelligence (SETI). For instance, Kaltenegger and Faherty found that human-made radio waves have already reached 75 of the closest stars on the list, some of which host potentially habitable exoplanets. 

By combining these observations with a likely rate of rocky planets in the habitable zone of star systems, the team concluded that 𠇊n estimated 29 potentially habitable worlds that could have seen Earth transit and could also detect radio waves from our planet,” according to the study.

This subcategory is especially important for SETI because it hints at the star systems that might be good candidates for attempts at two-way interstellar communication, assuming they are inhabited. 

Some of the most promising locations highlighted in the study include the Trappist-1 system, which contains seven Earth-sized planets. This system is close enough to Earth to have received human-made radio waves and will be in the right position to witness Earth transits in 1,642 years. Ross-128, a system located just 10 light years away, exited the ETZ about 900 years ago, while Teegarden’s Star, located 12 light years away, will enter this zone in 2050.

Kaltenegger and Faherty point out that there is a lively debate about whether humans should attempt to contact speculative alien civilizations on potentially habitable exoplanets, given that we know nothing about their technological capabilities, motives, or indeed, if they exist at all. 

However, the researchers note that this point may be moot to some extent, because biological activity on Earth has been visible for eons and technological activity has been evident for at least a century.

“There are so many exoplanets that could have found us already as an interesting life-bearing planet,” Kaltenegger said. “They wouldn&apost know𠅊nd we wouldn&apost know if we found oxygen and methane somewhere else—which stage that life is in. But the ones within 100 light years would know that there&aposs a technologically advanced civilization, if they could actually find radio waves.” 

Nobody knows if any extraterrestrial lifeforms exist within the ETZ, but the new research reveals that our biological and technological footprints are exposed to any prospective aliens that might have the wherewithal to search for them. Regardless of whether you find that to be a comfort or a concern, it’s good to know exactly where in space we should be looking to see signs of those looking back at us.

Given the long distances between star systems and the corresponding delays in potential communication between humans and intelligent aliens, it’s also useful to think about these interactions on intergenerational timescales.

“When you think of the evolution of our planet through time, how the Sun changes through time, and how we have a certain amount of time in the habitable zone, I think the whole universe becomes much more interesting because you can glimpse the past, present, and future of what&aposs happening around you even though we have this tiny, tiny life,” Kaltenegger said. 

For Kaltenegger, imagining how aliens might view Earth from afar is an extension of the legacy of Carl Sagan, who pioneered interdisciplinary research into these ideas at Cornell for decades. One of Sagan’s most famous reveries, entitled “Pale Blue Dot,” was inspired by an image of Earth captured in the outer solar system by the Voyager mission. Our planet from that distance looks like 𠇊 mote of dust suspended in a sunbeam,” he said. 

That description could also apply to observations of Earth transits from the star systems catalogued in the new study, adding a new scale to the classic image of our pale blue dot.

“I wish I could talk to Carl Sagan,” Kaltenegger said. “I wish I had met him when he was alive. I sit in his office, so sometimes when I look out of the window, I think: ‘this is what he would have seen. I wonder what he thought about when he was standing here.’”

“We stand on the shoulders of giants,” she concluded. 𠇊ll these people that came before us have put small puzzle pieces into place that let us understand what is around us.”

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Other Cool Science with ATLAS

Two decades ago, astronomers believed that most stars have planets but not one extrasolar planet had been discovered. Today's advanced telescopes have uncovered over 1,000 extrasolar planets and the number is growing rapidly. Two primary methods are used to identify extrasolar planets: the back-and-forth motion of the star due to the orbiting planet and the slight dimming of the star as a planet passes in front. These detection techniques are biased in favor of giant planets like Jupiter that exist close to the parent star. The techniques are not well suited to detecting Earth-sized planets in the habitable zone - the range of distances from the star where life as we know it might thrive.

Only one or two of the 1,000 known planets might be habitable. And we have no means of determining if they actually are habitable (do they have water in liquid form, for example?) or of determining if life exists. Without a new approach, two decades from now we will know much more about how planets form and how common they are but nothing about whether any extrasolar planet is actually habitable.

ATLAS has a special capability to leap-frog the existing barriers in determining whether a planet is habitable because it will look at 10,000 white dwarf stars every night. White dwarf stars are the collapsed remnants of stars that have burned up their supply of nuclear fuel our Sun will become a white dwarf in about 5 billion years. Although nuclear reactions in the center of the star have ceased, white dwarfs take up to 10 billion years to cool down. Prof. Eric Agol at the University of Washington has shown that the key ingredients to evolution of life are present in such systems (ApJ 713: 31, 2011) but it is unknown whether these white dwarf stars have planets and whether the planets could have liquid water necessary for life.

Detecting a habitable planet around a white dwarf is far easier than around a normal star because the collapsed star is nearly the same size as Earth. Here an extrasolar planet transit is a dramatic event - like a solar eclipse by the Moon. More important, the light that passes through the planet's atmosphere during eclipse can be a substantial fraction of all the light that we observe on Earth, even for partial eclipses (1% for a white dwarf versus 0.0001% for a normal star). In such an event, it's possible to detect strong molecular lines from oxygen and water during the brief eclipse using the world's largest telescopes or perhaps the future James Webb Space Telescope (JWST).

While astronomers think it is more plausible that a habitable planet would be found around normal, main-sequence stars, it is completely unknown whether white dwarf stars have planets. ATLAS is a way to find out.


The habitable zone near a white dwarf star is shown shaded in blue
as a function of the white dwarf's age and distance from the star.

Astronomers Discover New Method to Detect Potentially Habitable Planets

According to ScienceDaily, this new research was conducted by a team of scientists in the Netherlands, who used a Low-Frequency Array (LOFAR) radio telescope to study emissions from aurorae, caused by the interaction between a planet and its star's magnetic field, particularly for red dwarf stars.

Red dwarfs are known to have strong magnetic fields that might heat up and erode the atmosphere of a habitable planet if they become exposed to this type of activity, even though the stars themselves are much smaller and cooler than the Sun. However, the radio emissions associated with this process allows scientists to probe the planet-star interaction.

"The motion of the planet through a red dwarf's strong magnetic field acts like an electric engine much in the same way a bicycle dynamo works," said Harish Vedantham, the lead author of the Nature Astronomy study. "This generates a huge current that powers aurorae and radio emission on the star."

Have you seen Rogue One: A Star Wars Story?

As this is the first time that astronomers have been able to detect and decipher these signals, solar-system studies are expected to expand into new territories, as scientists may now be able to use these novel techniques to potentially discover exoplanets in habitable zones by finding similar emission from other stars.

"We now know that nearly every red dwarf hosts terrestrial planets, so there must be other stars showing similar emission," said Joe Callingham, who was a co-author on the recent Nature Astronomy paper. "We want to know how this impacts our search for another Earth around another star."


New Model Could Help I.D. Potentially Habitable Alien Planets

A trio of super-Earths found in the habitable zone of the star Gliese 667C, two probably rocky planets in the Goldilocks zone around Kepler-62 and possible super-Earths orbiting Tau Ceti and HD 40307 at just the right distance for liquid water to exist on their surfaces, albeit under certain conditions.

These are all just from the past twelve months. Should those exoplanet hunters who are seeking out Earth 2, a planet where life as we know it could possibly exist, start to feel excited? [The Strangest Alien Planets (Photos)]

Not yet. Our knowledge of these planets is woefully incomplete. However, the times may be changing. While we cannot yet determine whether a planet is hospitable to life, David Kipping of the Harvard&ndashSmithsonian Center for Astrophysics has led a team of astronomers to develop a new theoretical model that can tell us with one swift glance whether a super-Earth &mdash a world with two to 10 times the mass of our planet and up to twice the diameter &mdash has an atmosphere that might not be suitable for life.

Consequently, we could rule such worlds out of our search for analogs to Earth. It’s all about whether a planet has an atmosphere and how that atmosphere is connected to the relationship between a planet’s mass and diameter.

The two main exoplanet detecting techniques are beautifully complementary. When a planet transits its star &mdash that is, passes in front of its star, blocking a fraction of the starlight &mdash we can determine the diameter of the planet from the size of the transit. Meanwhile, that orbiting planet also exerts a gravitational tug on its parent star. If we can detect that tug we can calculate the planet’s mass based on the extent by which the planet is pulling on the star.

The only problem is that not all planets orbit their star at an appropriate angle for us to see a transit, while some exoplanets and their stars are too distant and faint for us to accurately measure their "radial velocity" tug (many of the Kepler spacecraft’s candidate planets fall into this category).

However, for those worlds where we are fortunate to know both properties, we can work out a planet’s volume and then divide the mass by the calculated volume to determine the planet’s density, which tells us whether it is likely rocky, gaseous or icy.

The computer model that Kipping has developed, along with Harvard’s Dimitar Sasselov and Princeton’s David Spiegel, allows an astronomer to plug in these numbers for mass and radius and, with the knowledge of the density, figure out if a planet &mdash in particular a super-Earth &mdash has a light but extended atmosphere or a relatively thin, heavy atmosphere.

That’s important because Earth’s atmosphere is the latter kind &mdash a 100 kilometer (62 mile) layer filled with the likes of nitrogen, oxygen, carbon dioxide, argon, water vapor and neon that contributes just 1.5 percent of Earth’s radius. We don’t know if an extended atmosphere of mostly hydrogen and helium &mdash similar to Uranus’ or Neptune’s atmospheres but warmer &mdash could support life, and so searches for Earth’s twin may want to avoid such worlds.

Solid, liquid or gas

The way Kipping, Sasselov and Spiegel’s model makes use of a graph that plots a planet’s mass against its radius, and where a world falls on that graph, tells us whether it is solid rock, partly watery or has a significant fraction of gas.

"There’s a full range of models that we think a super-Earth can be built out of," Kipping said. "You can make them out of iron, or out of silicate, or out of water, or some mixture of those things."

However, when a planet transits a star, not only does the solid body of the planet block some of the starlight, but so too does its atmosphere. By simply detecting the planet’s silhouette we cannot automatically figure out which part is solid and which part is gaseous atmosphere. The mass-radius diagram, however, offers a way around this problem. [9 Exoplanets That Could Host Alien Life (Countdown)]

Kipping and his cohorts have calculated theoretical limits &mdash boundary conditions &mdash for each type of planet. The lower boundary condition denotes a super-Earth made of solid rock with an iron core and lacking an atmosphere. The top boundary signifies a planet made entirely of water that, Kipping said, is probably impossible &mdash there needs to be a solid core in there somewhere &mdash and thus you cannot get a super-Earth less dense than a water-world (purely gaseous planets, it is thought, cannot exist as small as super-Earths and even Neptune-type worlds have a large rocky core lurking inside them).

Therefore, if you discover a planet and plot its mass against its radius only to discover that it resides on the graph above the impossible pure-water line, then the only way to explain its apparent density given its radius is that it must have a large atmosphere.

Such mass-radius models have been around for a while, but what makes Kipping’s different is that they are based on a new understanding of the physics of materials placed under the enormous amounts of pressure that the interior of a super-Earth would impose on them. Dimitar Sasselov, along with his student Li Zeng, was able to create superior models of the interior of super-Earths using new laboratory technology that is able to simulate those pressures.

They published their work in the March 2013 issue of the Publications of the Astronomical Society of the Pacific and Kipping’s mass-radius diagram, itself to be published in the Monthly Notices of the Royal Astronomical Society, is modeled around those interior structures derived by Sasselov and Zeng.

What does the model tell us about super-Earths we have already discovered? Kipping, Spiegel and Sasselov concentrated on GJ 1214b, a world with six and a half times the mass and two and a half times the diameter of our planet that is orbiting a red dwarf star 47 light years away.

Prior to now the planet had been a puzzle &mdash no matter what wavelength it was observed in, the size of the planet was always the same, which shouldn’t happen because an atmosphere should be more opaque to some wavelengths than others. Was its atmosphere extended and topped with thick, opaque clouds, or was its atmosphere thin enough not to be noticed? Employing the mass&ndashradius diagram settles the matter.

"Our method says that 20 percent of this planet’s radius is pure atmosphere, which strongly favors the idea of a very light, extended hydrogen-helium atmosphere with clouds on top," Kipping said. "So we are able to come into this discussion with these two possibilities and say which is more likely, just based on the simple measurement of the mass and the radius of the planet."

Uncertain data

Another intriguing world is Kepler-22b, which was the first habitable zone planet to be discovered by NASA’s Kepler spacecraft. Around 620 light-years from Earth, it orbits a sun-like star at a distance of 0.85 astronomical units (one astronomical unit is the average distance between Earth and the sun, 149.6 million km) and has a diameter two and a half times that of our planet. [Gallery: A World of Kepler Planets]

"We tried to apply our technique to this planet but unfortunately the mass measurement is very poor because it is a very distant star," Kipping said. "What we found was that the data was unable to say one way or another what kind of planet it is it sits right on the blue [water-world] line, so we can’t tell whether it is a rocky planet with an extended atmosphere or a water-world with very little atmosphere."

Unfortunately that’s also the story for the rest of the potentially habitable planets discovered so far, a list of which is maintained by Professor Abel Mendez of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, in the form of the Habitable Exoplanets Catalog.

One dozen planets currently reside on the list, meeting the criteria of being (probably) rocky and existing within their star’s habitable zone. However, as we found with Kepler-22b, in most cases either the mass or the radius is little more than an estimate, and as such the majority tend to sit on that boundary condition.

"Astronomers estimate mass or radius from the assumption that smaller planets are more rocky in composition and those larger planets close to two Earth radii are water-worlds," Mendez said. "This seems to be a good estimate for most cases but there is a lot of uncertainty for example, Kepler-11f has just over two Earth masses but it is a gas planet, while Kepler-20b with about nine Earth masses is rocky."

Kipping’s mass&ndashradius diagram is only half the job. Without good data the new mass-radius relationship is limited in what it can tell us. For Kepler planets, better mass measurements from radial velocities are required, but this is tricky given that most of the stars around which Kepler discovers planets are faint and distant.

For those worlds discovered by radial velocity, we need more luck in observing transits to give us their diameter. The approval of the Transiting Exoplanet Survey Satellite (TESS), which is scheduled to launch in 2017 and will systematically survey all the brightest stars in the sky for transiting planets, will be a massive boon to the field.

"The TESS mission promises to dramatically change this picture," said Heather Knutson, a planetary astronomer at the California Institute of Technology whose research is focused in the area of exoplanet atmospheres. "At the moment there are currently only three transiting super-Earths that are suitable for detailed characterization and all three have been observed with either the Spitzer or Hubble space telescopes, or both. In the era of TESS we will have far more super-Earths than we can reasonably study and Kipping’s criterion will provide a useful means to select targets that are likely to have detectable atmospheric signatures."

The launch of the James Webb Space Telescope (JWST) a year after TESS will also dramatically boost the nascent science of exoplanetary atmospheric investigations. JWST, with its 6.5-meter mirror, will extend its observations well into the near-infrared, perfect for picking up the tenuous signatures of water, methane, oxygen, carbon monoxide and carbon dioxide in atmospheres, which could be interpreted as biosignatures depending upon their concentrations. TESS will identify the planets, the mass-radius model will decide which ones we want to observe, and JWST will tell us about them. It is going to be an exciting time and the wait will be excruciating for scientists. [See a video about the JWST]

"At this point almost anything is possible!" Knutson said.

Finding a potentially habitable planet

There are many factors that go into making a planet habitable, from the presence of a magnetic field to protect its atmosphere to the question of whether it has plate tectonics to recycle carbon. A stable rotational axis, a moderate impact rate and sufficient gravity are also plausible necessities.

Yet, the possession of an atmosphere, especially one that contains some form of greenhouse gas, is one of the most crucial factors, essential for maintaining cozily warm temperatures that permit all-important liquid water to exist on its surface. That said, the range of suitable atmospheres may not be as narrow as we may think.

"I don’t think that thick hydrogen&ndashhelium atmospheres will rule out the potential for life on these planets as long as the pressure at the surface/water transition allows for liquid water," Mendez said.

So a super-Earth, with a thick envelope of hydrogen swathing a rocky core deep down could still have watery conditions at depths where the pressure, according to Mendez, drops below 10,000 atmospheres, although of course temperature will also have a say where and if this transition point occurs.

There is one more intriguing possibility. On Earth, convection currents and air flows are strongly influenced by what is on the surface, be it oceans, continents or mountains. Could a careful study of the atmosphere of a super-Earth tell us things about the terrain below that are otherwise beyond the capabilities of our telescopes?

"Yes, potentially, but the atmosphere would need to be thin enough for our observations to detect the atmosphere flows from the region close to the surface," said Knutson, who also points out that a thin atmosphere will be transparent enough for us to spectroscopically measure the surface of the planet and determine whether there are oceans, desert or even plant life.

"When we get these new super telescopes in the future [such as the Thirty Meter Telescope, the Giant Magellan Telescope and the European Extremely Large Telescope] we’ll be able to go down to sort of Earth-like atmospheres," Kipping said. "In special cases we could probably go down to these very small atmospheres that are potentially life-harboring."

But we’re getting ahead of ourselves the new mass&ndashradius model only provides us with a way of saying which planets don’t have a thin atmosphere. If we come to the conclusion that a super-Earth does not have an extended atmosphere, then it might be worth pointing JWST at it to measure the spectrum of any atmosphere present and see whether it is analogous to Earth’s atmosphere. [7 Ways to Discover Alien Planets (Countdown)]

"If you are really hunting Earth-like planets and our method tells you it has a big extended atmosphere, then you are probably wasting your time," Kipping said. "So it’s a way of making our searches for Earth-analogues more efficient."

With TESS and JWST and the next generation of extremely large telescopes on the horizon, Kipping’s new model is timely indeed. The way things are going, the next decade might be the decade of the super-Earth. All the hints are it is going to be an exciting time.


Astronomers define the 'really habitable zone': Planets capable of producing gin and tonic

Gin and tonic. Credit: By NotFromUtrecht – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8529628

A hospitable star that doesn't kill you with deadly flares. A rocky planet with liquid water and an agreeable climate. Absence of apocalyptic asteroid storms. No pantheon of angry, vengeful and capricious gods. These are the things that define a habitable planet.

Now, some scientists are adding one more criterion to the list: gin and tonic.

Exoplanets are a hot topic in space science right now. We know of about 4,000 confirmed exoplanets, with many more on the way. We've come a long way from a few decades ago, when as far as we knew, our solar system was the only one with a habitable world. What else were we supposed to think?

The Kepler mission changed all that. Our knowledge of exoplanets grew in leaps and bounds, and along with the discovery of all those distant planets, we began to refine our criteria for what a habitable world might look like.

Water, safety from stellar radiation, and an agreeable climate were just the beginning. It's time to refine our understanding of habitable, and to start to add some other essentials with the list. According to one team of researchers, it's time to introduce the concept of the "really habitable zone" (RHZ).

For these authors, an exoplanet is only in the RHZ if it can provide gin and tonic. Once a planet can provide that, it moves from habitable to really habitable. Maybe a planet without gin and tonic isn't really habitable at all maybe it's more of place where we'd like to send people we don't like.

The new paper is titled "Defining the Really Habitable Zone." The lead author of the paper is Marven F Pedbost.

So far, the science of the RHZ is unproven. But that's not deterring these intrepid researchers. As they say in the introduction: "In common with much of the work in the exoplanet field, we rely throughout on assumptions which are difficult if not impossible to test and present some plots which astronomers can use in their own talks, stripped of all caveats."

There's some background to the idea of the RHZ. "The inquiry into the existence of life, however, is an extremely complex topic involving numerous convoluted considerations, making it an ideal theme for telescope and grant funding applications, but a less practical question to answer. Instead, the community has formed a handshake agreement to instead investigate the more loosely defined question of habitable zones."

Universe Today readers are familiar with the idea of a habitable zone. It basically means liquid water. All other considerations aside, we know that all life on Earth needs liquid water, so we search for other worlds that have it. If a planet, or a moon, has liquid water, we say it's in the habitable zone, or more cautiously, the potentially habitable zone.

A whole bunch of other conditions have to be met before life can exist. But like the authors say, it's very convoluted. So why not just ignore it and jump ahead to the really habitable zone, where abundant gin and tonics are waiting for us to arrive and drink them?

From the paper. The BHZ is the Boring Habitable Zone, where there’s likely water but no gin and tonics. The Blue region is the Really Habitable Zone, where exo-gin, exo-citrus, and exo-juniper are likely abundant. Image Credit: Pedbost et al, 2020

What do you need to make a gin and tonic? According to the authors of the paper, we need several things.

"To proceed, we define the Minimum Acceptable Gin and tonIC, or MAGIC (Cook 2019) 3. A MAGIC must contain: gin, tonic, ice and some sort of citrus."

Gin aficionados know that it's flavoured with 'botanicals," which is nowhere defined clearly.

"Gin, in essence, is alcohol which has been flavoured with a wide variety of 'botanical' species," write the authors. "A precise definition of 'botanical' is lacking, so we assume it is the equivalent of a astronomer's use of 'metal' – including almost everything in the universe apart from a few common ingredients. Everything is a metal, apart from hydrogen and helium, and everything is a botanical apart from water and alcohol."

Now we're getting somewhere.

Spectroscopic analysis shows that gin contains juniper as a primary botanical. Juniper grows in a variety of conditions on Earth. But how hardy and widespread is exo-juniper? According to the authors, "… we should expect exo-juniper to exist on a wide range of planets." Sounds good!

Citrus isn't quite as hardy as juniper, so exo-citrus worlds may be rarer than exo-juniper worlds. From the paper: "In contrast to juniper-related considerations, the region around a star where the conditions are adequate for the growing of lemons or limes, fundamental ingredients required for the gin and tonic drink, is sensitive to a number of factors. These necessary citrus fruits thrive in temperatures ranging from 21 to 38? C (botanist, priv. comm.) and require a steady supply of H2O, hereafter water."

The paper contains much more detail of course, so we encourage interested readers to read it closely. We also encourage readers to read the team's other important paper, "Galaxy Zoo: an unusual new class of galaxy cluster." That paper contains the same level of scientific rigor and ground-breaking analysis.

This is just the beginning of the scientific reckoning with the RHZ. Other papers are bound to follow.

For now, the last word belongs to the authors: "We suggest that efforts should be directed in the near future towards investigating only those planets whose orbits lie within the RHZ, and made unverified claims about the possibility of detecting relevant features. We're off for a drink."


Kepler Discovers its Smallest Habitable Zone Planets

The Kepler-62 system has five planets: 62b, 62c, 62d, 62e and 62f. The Kepler-69 system has two planets: 69b and 69c. Kepler-62e, 62f and 69c are the super-Earth-sized planets.

Two of the newly discovered planets orbit a star smaller and cooler than the sun. Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62f is likely to have a rocky composition. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth.

The third planet, Kepler-69c, is 70 percent larger than the size of Earth, and orbits in the habitable zone of a star similar to our sun. Astronomers are uncertain about the composition of Kepler-69c, but its orbit of 242 days around a sun-like star resembles that of our neighboring planet Venus.

Scientists do not know whether life could exist on the newfound planets, but their discovery signals we are another step closer to finding a world similar to Earth around a star like our sun.

"The Kepler spacecraft has certainly turned out to be a rock star of science," said John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. "The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity."

The Kepler space telescope, which simultaneously and continuously measures the brightness of more than 150,000 stars, is NASA's first mission capable of detecting Earth-size planets around stars like our sun.

Orbiting its star every 122 days, Kepler-62e was the first of these habitable zone planets identified. Kepler-62f, with an orbital period of 267 days, was later found by Eric Agol, associate professor of astronomy at the University of Washington and co-author of a paper on the discoveries published in the journal Science.

The size of Kepler-62f is now measured, but its mass and composition are not. However, based on previous studies of rocky exoplanets similar in size, scientists are able to estimate its mass by association.

"The detection and confirmation of planets is an enormously collaborative effort of talent and resources, and requires expertise from across the scientific community to produce these tremendous results," said William Borucki, Kepler science principal investigator at NASA's Ames Research Center at Moffett Field, Calif., and lead author of the Kepler-62 system paper in Science. "Kepler has brought a resurgence of astronomical discoveries and we are making excellent progress toward determining if planets like ours are the exception or the rule."

The two habitable zone worlds orbiting Kepler-62 have three companions in orbits closer to their star, two larger than the size of Earth and one about the size of Mars. Kepler-62b, Kepler-62c and Kepler-62d orbit every five, 12 and 18 days, respectively, making them very hot and inhospitable for life as we know it.

The five planets of the Kepler-62 system orbit a star classified as a K2 dwarf, measuring just two-thirds the size of the sun and only one-fifth as bright. At seven billion years old, the star is somewhat older than the sun. It is about 1,200 light-years from Earth in the constellation Lyra.

A companion to Kepler-69c, known as Kepler-69b, is more than twice the size of Earth and whizzes around its star every 13 days. The Kepler-69 planets' host star belongs to the same class as our sun, called G-type. It is 93 percent the size of the sun and 80 percent as luminous and is located approximately 2,700 light-years from Earth in the constellation Cygnus.

"We only know of one star that hosts a planet with life, the sun. Finding a planet in the habitable zone around a star like our sun is a significant milestone toward finding truly Earth-like planets," said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, Calif., and lead author of the Kepler-69 system discovery published in the Astrophysical Journal.

When a planet candidate transits, or passes in front of the star from the spacecraft's vantage point, a percentage of light from the star is blocked. The resulting dip in the brightness of the starlight reveals the transiting planet's size relative to its star. Using the transit method, Kepler has detected 2,740 candidates. Using various analysis techniques, ground telescopes and other space assets, 122 planets have been confirmed.

Early in the mission, the Kepler telescope primarily found large, gaseous giants in very close orbits of their stars. Known as "hot Jupiters," these are easier to detect due to their size and very short orbital periods. Earth would take three years to accomplish the three transits required to be accepted as a planet candidate. As Kepler continues to observe, transit signals of habitable zone planets the size of Earth that are orbiting stars like the sun will begin to emerge.

Ames is responsible for Kepler's ground system development, mission operations and science data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif., managed Kepler mission development.

Ball Aerospace & Technologies Corp. in Boulder, Colo., developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's 10th Discovery Mission and was funded by the agency's Science Mission Directorate.


A new method to search for potentially habitable planets

Imaging planets orbiting around nearby stars, which could potentially harbour life, has become a possibility thanks to the progress made in observational methods by an international team of astronomers. First candidate: Alpha Centauri, a system similar to ours, "only" 4.3 light years away. This study is the subject of a publication in the journal Nature Communications.

Efforts to obtain direct images of exoplanets - planets outside our solar system - have so far been hampered by technological limitations, which have led to a bias towards detecting planets much larger than Jupiter, around very young stars and far from the habitable zone, the area in which a planet may have liquid water on its surface, and thus potentially life. "The Earth itself illuminates us at the wavelengths used for detection, and the infrared emissions from the sky, the camera and the telescope itself tend to drown out the signals we want to detect," says Kevin Wagner, NASA Hubble/Sagan post-doctoral fellow at the University of Arizona's Steward Observatory and first author of the paper. But the good reason to focus on these wavelengths is that this is where an Earth-like planet, in the habitable zone around a sun-like star, will shine the brightest. "

In other words, if astronomers want to find planets whose conditions are suitable for life as we know it, they must look for rocky planets the size of the Earth, within the habitable zones around older stars, similar to our Sun. And to do this, they have developed a new system for imaging exoplanets in the mid-infrared in combination with a very long observation time. This system, which was able to achieve unprecedented sensitivity by using a deformable secondary mirror to correct for the distortion of light induced by the Earth's atmosphere, used a coronagraph developed - thanks to an ERC grant - by researchers at ULiège. A starlight-blocking device which they optimised for the mid-infrared spectrum of light in order to block the light from one star at a time.

"We achieved the capability to directly image planets about three times the size of the Earth in the habitable zone of alpha Centauri," explains Olivier Absil, FNRS Research Associate and director of the PSILab (STAR Research Institute/Faculty of Sciences) at ULiège. Combined with efficient subtraction of thermal background noise, this method represents an improvement by a factor of 10 compared to existing capabilities for direct observation of exoplanets". Similar in effect to noise-canceling headphones, which allow soft music to be heard over a steady stream of unwanted jet engine noise, the technique allowed the team to remove as much of the unwanted noise as possible and detect the much fainter signals created by potential planet candidates inside the habitable zone.

Alpha Centauri, first candidate

Located only 4.3 light years away from our solar system, Alpha Centauri is a triple star system. It consists of two stars, Alpha Centauri A and B - which are similar in size and age to our Sun and orbit each other as a binary system - and Alpha Centauri C, better known as Proxima Centauri, a much smaller red dwarf that orbits its two sisters at a larger distance. "This system is the closest to ours," says Anne-Lise Maire, an astrophysicist at PSILab who also took part in the study. It proved to be an ideal candidate for testing our method, because Alpha Centauri A and B are similar to our Sun, but we don't know yet if there are planets orbiting either star. »

By moving one star on the coronagraph and one star off the coronagraph every tenth of a second, this technique has allowed researchers to observe each star for half the time, and more importantly it has allowed them to subtract one image from the next, which removes all but the noise of the camera and telescope. After removing the known artefacts created by the instrumentation and the residual light from the coronagraph, the final image revealed a light source designated as "C1", a plausible detection, which could be a planet the size of Neptune to Saturn, located at a distance from Alpha Centauri A similar to that between the Earth and the Sun, i.e. within the habitable zone. "At this stage however, without verification via a second observing campaign, we cannot exclude an instrumental artefact of unknown origin, or even the signature of an asymmetrical dust cloud," says Olivier Absil.

Another direct imaging campaign will be attempted in the coming years, and other methods (e.g. radial velocity measurements) could also provide an answer. In any case, these results demonstrate the potential of high-contrast imaging in the mid-infrared to enable the observation of Earth-like planets around nearby stars. "By 2028, the same type of coronagraph will equip the METIS instrument installed on the future ELT (Extremely Large Telescope), which will make it possible to image planets as small as the Earth in the habitable zone of alpha Centauri, and of a handful of other nearby stars," concludes Olivier Absil. The coronographs developed at the University of Liège are the result of more than 10 years of technological development, in partnership with the University of Uppsala, with funding from the European Research Council .

Finding a potentially habitable planet within Apha Centauri has been the goal of the initiative Breakthrough Watch/NEAR, which stands for New Earths in the Alpha Centauri Region, and involves researchers from the University of Liège. Breakthrough Watch is a global astronomical program looking for Earth-like planets around nearby stars. During the observation campaign, about 7 terabytes of data were collected. Data that the researchers made available to the public

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.


Detecting Life's Influence on Planetary Atmospheres

Biosignatures that vary in time and atmospheric gases that shouldn’t exist without life to replenish them could be two possible ways to detect life on exoplanets.

Finding any life that might exist on other planets is extremely challenging. Even in our own Solar System, where we can send probes and orbiters to worlds of interest such as Mars, it is difficult to assess if any microbial life is, or was ever, present. When studying exoplanets, we can only look at the starlight shining through a planet’s atmosphere in the hope that it will reveal absorption or emission lines that indicate gases produced by life. Detailed analyses of the atmospheres of exoplanets is still mostly in the realm of future telescopes, such as the James Webb Space Telescope ( JWST ), but understanding what to look for is an important step in the hunt for life on other planets.

Oxygen is produced by photosynthesis and is commonly thought to be a potential biosignature on other worlds, although it is also possible for oxygen to be produced from abiotic sources. Similarly, methane is produced by life and is a potential biomarker, but can also be produced by other means. Now, two recent papers discuss new ways of looking for biosignatures by studying how life can influence a planet’s atmosphere.

A paper by Stephanie Olson at the University of California, Riverside, and colleagues, discusses how seasonal changes in the atmosphere caused by life could be used as a biosignature. The second paper is by Joshua Krissansen-Tottonat the University of Washington, along with Olson and David Catling, and looked at potential biosignatures produced by atmospheric gases that can only co-exist in the presence of life.

The changing seasons

Signals from an exoplanet that vary over time, such as with the seasons, could help to rule out false positives or negatives that occur in single snapshot observations. By understanding how atmospheric gases vary over the course of a year on Earth, it will help inform scientists about what signals to look for on other planets.

“Rather that simply recognizing that a planet hosts life, we may be able to say something about how the activities of its biosphere vary in space and time,” says Olson.

Seasonality in the Earth’s atmosphere arises because of the interactions between the biosphere and the varying solar radiation reaching Earth at different points in its orbit. Seasonal variations shift the balance between two different reactions: photosynthesis and aerobic respiration. Photosynthesis occurs as carbon dioxide and water react to become organic matter and oxygen, and aerobic respiration causes the reverse reaction, producing carbon dioxide and water. The maximum production of oxygen occurs during the summer months when temperatures are warm.

The researchers examined the seasonal variations in carbon dioxide on Earth, a signal that could be detectable on other planets assuming that life elsewhere is also carbon-based. Carbon dioxide is an important atmospheric component on habitable worlds due to the role it plays in climate regulation via weathering.

They found that the seasonal carbon dioxide (CO2) signal would be dominated by land-based ecosystems, which are in direct contact with the atmosphere, indicating that CO2variability might not be detectable on ocean worlds. This is seen on Earth, where the ocean-dominated Southern Hemisphere has a weaker CO2 variability signal than the Northern Hemisphere. Carbon dioxide seasonality would be difficult to detect on other planets, but it is a powerful indicator of the presence of life since it is unlikely to occur on planets with an ocean unless life is present.

They also looked at the scenario of an exoplanet that is an analog of the early-Earth, where life existed but where there was still very little oxygen in the atmosphere. Weak oxygen signals are difficult to detect, but a varying ozone signature (ozone is a molecule built from three oxygen atoms) might be more visible in the spectrum of an exoplanet. Such a signal is more likely to be detected for a planet with less oxygen than the present day Earth because ozone can create a stronger signal than oxygen.

“Seasonality would be difficult to detect for a planet resembling the present-day Earth, at least in the case of oxygen,” explains Olson. “The reason is that baseline levels of oxygen are really high today, and so small seasonal fluctuations are very challenging to measure at our planets surface, and would be even more so on a distant planet.”

Atmospheres in disequilibrium

Krissansen-Totton, Olson and Catling also simulated early-Earth atmospheres, but this time looking for signatures of disequilibrium, meaning the presence of gases that would not ordinarily exist in an atmosphere without some active process, such as life, producing them. Earth has a large atmospheric disequilibrium today, but they calculated that a disequilibrium has existed since life formed on Earth and that the evolution of disequilibrium follows the rise in biogenic atmospheric oxygen.

In the Archean eon (4 to 2.5 billion years ago), a disequilibrium existed via the coexistence of carbon dioxide, nitrogen, methane, and liquid water, which ordinarily would react to create ammonium and bicarbonate, quickly removing the methane from the atmosphere without the presence of life to replenish it. Carbon dioxide and methane should be detectable in exoplanet spectra by JWST , particularly on planets orbiting red dwarfs. If these are detected, but no carbon monoxide is found, it could be a strong biosignature. This is because many of the non-biological scenarios that replenish methane would also be expected to produce carbon monoxide (CO), and because surface life consumes CO.

“This is a very easy metabolism to do if there’s CO and water around, then microbes can make a living by combining these species to make CO2and molecular hydrogen (H2),” says Krissansen-Totton.

The largest source of disequilibrium in the Proterozoic eon (2.5 to 0.54 billion years ago) was the coexistence of nitrogen, water and oxygen. Both oxygen and nitrogen are produced by life, and without life to replenish the oxygen, it would be converted to nitric acid in the ocean.

Recognizing signs of life that use different metabolic pathways might also be possible if the atmospheric gases are in an unusual disequilibrium, but it would be challenging to detect.

“Detecting microbes that oxidize iron in the ocean might be challenging since this particular metabolism does not generate any gaseous waste products,” says Krissansen-Totton. “Among the possible metabolisms that do produce waste gases are some promising possibilities. For example, laughing gas (N2O) is a biogenic gas that we would not expect to see in equilibrium in the atmospheres of lifeless planets. Similarly, various sulfur metabolisms might be detectable since they modify the abundances of organic molecules in a planet’s atmosphere to be out of equilibrium.”

Finding early-Earth analogues with signs of seasonality or disequilibrium might indicate that life is not only present, but has evolved in a similar manner to life on our own planet.

The Olson paper was supported by the NASA Astrobiology Institute, while the Krissansen-Totton research was also supported by NASA Astrobiology through the Exobiology & Evolutionary Biology Program and the Virtual Planetary Laboratory.

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Looking at Earth like an alien planet

Twenty-nine potentially habitable planets orbiting relatively nearby stars were in in a position to spot Earth in the past 5,000 years and possibly detect radio waves from our planet, according to a new study.

Why it matters: If intelligent life is out there, chances are it's searching for us too and any theoretical astronomers on these worlds would have been in a position to observe our planet in much the same way as Earthlings study distant stars and planets today.

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What they found: The new study, in the journal Nature, used a database of 331,312 stars within 300 light-years to show that 1,715 stars have been in a position to see Earth in the last 5,000 years, with 319 other stars expected to be able to see our world in the next 5,000 years.

If there are any alien astronomers out there on these potentially habitable worlds, they — in theory — could have seen the small dips in the Sun's light created when the Earth passes between the distant planet and our star a method for finding exoplanets used here on Earth.

The researchers also found about 75 stars are close enough to Earth that any radio waves sent out from our world could have reached them and possibly been detected, the same method used by SETI researchers to search for signs of intelligent life.

"We can't search everywhere, and so this is the best input target list now for anyone interested in potentially habitable worlds that can see us as a transiting planet," Cornell University astronomer Lisa Kaltenegger, an author of the new study, told Axios via email. "If someone had found us already, I wonder what they would think about us?"

Yes, but: Just because these theoretical alien astronomers might have Earth in a database of potentially habitable planets doesn't mean they would know for sure that we're here or that they could reach us.

Astronomers don't currently have the technology to confirm a truly Earth-like world somewhere out there in the universe, but future space telescopes being proposed now could allow researchers to detect habitable exoplanets in the future.

Some star systems with known potentially habitable worlds aren't yet able to see our planet, or our solar system has already moved out of view. Trappist-1, for example — which plays host to multiple potentially habitable planets — won't be able to see our Earth transit the Sun for another 1,642 years.

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