# What is the least dense exoplanet?

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An exoplanet with density 0.31 grams per cubic centimeter has been found. Is this the least dense exoplanet we know of?

The article you link to refers to Borsato et al. 2019, which attempted to rectify discrepancies in the measured properties of planets in the Kepler-9 system between transit timing variation measurements and radial velocity measurements. They arrived at $$hosim0.31^{+0.05}_{-0.06} ext{ g cm}^{-3}$$ for Kepler-9c. However, Borsato et al.'s Figure 10 shows that there are other exoplanets in this mass regime with substantially lower densities, e.g. WASP-107b, which comes in at about $$hosim0.19 ext{ g cm}^{-3}$$:

Even WASP-107b, however, doesn't hold the record for the least dense exoplanet. The three planets in the Kepler-51 system, Kepler-51b, Kepler-51c, and Kepler-51d, may hold that record. Multiple groups (Masuda 2014, Roberts et al.) have found densities of around $$hosim0.03 ext{ - }0.06 ext{ g cm}^{-3}$$ for both planets.

Title Planet Star Notes
Most distant SWEEPS-04 / SWEEPS-11 SWEEPS J175902.67-291153.5 / SWEEPS J175853.92-291120.6 At 27,700 light years for both of these planets, these are the most distant extrasolar planets ever discovered, but certainly not the most distant overall.

An analysis of the lightcurve of the microlensing event PA-99-N2 suggests the presence of a planet orbiting a star in the Andromeda Galaxy.

In late January 2018, a team of scientists led by Xinyu Dai claimed to have discovered a collection of about 2,000 rogue planets in the quasar microlens RX J1131-1231, which is 3.8 billion light-years distant. The bodies range in mass from that of the Moon to several Jupiter masses.

## ESA’s exoplanet watcher Cheops reveals unique planetary system

ESA’s exoplanet mission Cheops has revealed a unique planetary system consisting of six exoplanets, five of which are locked in a rare rhythmic dance as they orbit their central star. The sizes and masses of the planets, however, don’t follow such an orderly pattern. This finding challenges current theories of planet formation.

The discovery of increasing numbers of planetary systems, none like our own Solar System, continues to improve our understanding of how planets form and evolve. A striking example is the planetary system called TOI-178, some 200 light-years away in the constellation of Sculptor.

Astronomers already expected this star to host two or more exoplanets after observing it with NASA’s Transiting Exoplanet Survey Satellite (TESS). New, highly precise observations with Cheops, ESA’s Characterising Exoplanet Satellite that was launched in 2019, now show that TOI-178 harbours at least six planets and that this foreign solar system has a very unique layout. The team, led by Adrien Leleu of University of Geneva and the University of Bern in Switzerland, published their results today in Astronomy & Astrophysics.

One of the special characteristics of the TOI-178 system that the scientists were able to uncover with Cheops is that the planets – except the one closest to the star – follow a rhythmic dance as they move in their orbits. This phenomenon is called orbital resonance, and it means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits.

A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede. For every orbit of Europa, Ganymede completes two orbits, and Io completes four (this is a 4:2:1 pattern).

In the TOI-178 system, the resonant motion is much more complex as it involves five planets, following a 18:9:6:4:3 pattern. While the second planet from the star (the first in the pattern) completes 18 orbits, the third planet from the star (second in the pattern) completes nine orbits, and so on.

Initially, the scientists only found four of the planets in resonance, but by following the pattern the scientists calculated that there must be another planet in the system (the fourth following the pattern, the fifth planet from the star).

“We predicted its trajectory very precisely by assuming that it was in resonance with the other planets,” Adrien explains. An additional observation with Cheops confirmed that the missing planet indeed existed in the predicted orbit.

After they had uncovered the rare orbital arrangements, the scientists were curious to see whether the planet densities (size and mass) also follow an orderly pattern. To investigate this, Adrien and his team combined data from Cheops with observations taken with ground-based telescopes at the European Southern Observatory’s (ESO) Paranal Observatory in Chile.

But while the planets in the TOI-178 system orbit their star in a very orderly manner, their densities do not follow any particular pattern. One of the exoplanets, a dense, terrestrial planet like Earth is right next to a similar-sized but very fluffy planet ­­– like a mini-Jupiter, and next to that is one very similar to Neptune.

“This is not what we expected, and is the first time that we observe such a setup in a planetary system,” says Adrien. “In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory.”

Catastrophic events such as giant impacts could normally explain large variations in planet densities, but the TOI-178 system would not be so neatly in harmony if that had been the case.

“The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” explains co-author Yann Alibert from the University of Bern.

Revealing the complex architecture of the TOI-178 system, which challenges current theories of planet formation, was made possible thanks to almost 12 days of observations with Cheops (11 days of continuous observations, plus two shorter observations).

“Solving this exciting puzzle required quite some effort to plan, in particular to schedule the 11-day continuous observation needed in order to catch the signatures of the different planets,” says ESA Cheops project scientist Kate Isaak. “This study highlights very nicely the follow-up potential of Cheops – not only to better characterise known planets, but to hunt down and confirm new ones.”

Adrien and his team want to continue to use Cheops to study the TOI system in even more detail.

“We might find more planets that could be in the habitable zone – where liquid water might be present on the surface of a planet – which begins outside of the orbits of the planets that we discovered to date,” says Adrien. “We also want to find out what happened to the innermost planet that is not in resonance with the others. We suspect that it broke out of resonance due to tidal forces.”

Astronomers will use Cheops to observe hundreds of known exoplanets orbiting bright stars.

“Cheops will not only deepen our understanding of the formation of exoplanets, but also that of our own planet and the Solar System,” adds Kate.

‘Six transiting planets and a chain of Laplace resonances in TOI-178’ by A. Leleu et al. appears in Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202039767

Cheops is an ESA mission developed in partnership with Switzerland, with a dedicated consortium led by the University of Bern, and with important contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden and the UK.

ESA is the Cheops mission architect, responsible for procurement and testing of the satellite, the launch and early operations phase, and in-orbit commissioning, as well as the Guest Observers’ Programme through which scientists world-wide can apply to observe with Cheops. The consortium of 11 ESA Member States led by Switzerland provided essential elements of the mission. The prime contractor for the design and construction of the spacecraft is Airbus Defence and Space in Madrid, Spain.

The Cheops mission consortium runs the Mission Operations Centre located at INTA, in Torrejón de Ardoz near Madrid, Spain, and the Science Operations Centre, located at the University of Geneva, Switzerland.

## Cotton-candy planets

Calculating the density of a planet requires a return to high school physics. The density of an object is its mass divided by its volume the volume is determined by its radius. Hubble's precise measurements helped the researchers to better constrain the mass of the exoplanets. To find their radius &mdash and thus their volume &mdash scientists compare the size of the planet to its star. By revisiting what was known about the star, Roberts and her colleagues were able to determine a more precise radius.

Kepler-51b has a mass about twice that of Earth and a radius about seven times larger, and it orbits its star every 45 days. With its 130-day orbit, Kepler-51d is a bit larger, about 7.5 times as massive as Earth with a radius nearly ten times that of our planet. The third sibling, Kepler-51c, takes 85 days to travel around the star and has about four times the radius of Earth.

By combining the updated mass with the revised radius, the researchers could calculate that the densities of the planets ranged from 0.03 grams to 0.06 grams per cubic centimeter. That's a tenth as dense as Saturn, the solar system's least dense planet, and one that would float in water if you could find a bathtub big enough.

Figuring out what that would mean in real-world terms would take a bit more work, but Roberts was determined.

Her first thought was of marshmallows. She melted a batch in the microwave but found the white treats were still too dense.

"That was just a horrible mess," she told Space.com.

For her cotton-candy deductions, she headed to a grocery store and bought containers of cotton candy. The freshly spun material wasn't dense enough, but she hoped the prepackaged tubs might work. She measured the volume of the container and weighed the material to calculate its density, which was a close match for the super-puffs.

"I bought so many of these tubs, the [cashier] was like, you must be a cotton candy fan," Roberts said.

"I was like, it's for science."

## Astronomy: The Strange Case Of The Exoplanet “Cousins”

Nature versus nurture refers to a long-standing debate among scientists who are trying to find out if human behavior is determined by the environment or is merely the result of a person’s genes. Planets and people can have a lot in common, and the atmospheres of a duo of hot Jupiter exoplanets is a case in point.

Nature Vs Nurture: The Strange Case Of The Exoplanet “Cousins”
By Judith E Braffman-Miller

Nature versus nurture refers to a long-standing debate among scientists who are trying to find out if human behavior is determined by the environment or is merely the result of a person’s genes. Planets and people can have a lot in common, and the atmospheres of a duo of hot Jupiter exoplanets is a case in point. These two worlds serve as examples of how nature versus nurture operates when it comes to these two “cousin” exoplanets. In a one-of-a-kind experiment, planet-hunting astronomers used NASA’s Hubble Space Telescope (HST) to observe the hot Jupiter “cousins”, and because these two distant, gaseous and broiling worlds are virtually identical in both size and temperature, circling their nearly identical parent-stars at the same distance, the astronomers thought that their atmospheres would also be alike. What they found surprised them–one of these kindred worlds is cloudier than the other, and the difference between these distant worlds is now a delightful mystery just waiting to be solved by curious planetary scientists who are trying to understand why this difference exists between two such closely related worlds.

Lead scientists Dr. Giovanni Bruno of the Space Telescope Science Institute (STSI) in Baltimore, Maryland, explained in a June 5, 2017 STSI Press Release that “What we’re seeing in looking at the two atmospheres is that they’re not the same. One planet–WASP-67b is cloudier than the other–HAT-P-38b. We don’t see what we’re expecting, and we need to understand why we find this difference.”

The planetary scientists used HST’s Wide Field Camera 3 to observe the two “cousin” exoplanets’ spectral fingerprints, which measure chemical composition. “The effect that clouds have on the spectral signature of water allows us to measure the amount of clouds in the atmosphere. More clouds mean that the water feature is reduced,” Dr. Bruno added.

“This tells us that there had to be something in their past that is changing the way these planets look,” he continued to explain.

From a historical perspective, the hunt for distant alien worlds, located within the families of stars beyond our own Sun, proved to be a difficult endeavor. The discovery of the first exoplanets a generation ago clearly represents one of humanity’s greatest accomplishments. Spotting a giant planet, such as our own Solar System’s banded behemoth, Jupiter, has been compared to observing light skipping off a gnat that is flying in front of the 1,000-watt light bulb of a street lamp–when the observer is 10 miles away.

The smaller the exoplanet, the harder it is to discover. For example, if an alien astronomer, belonging to a technologically advanced civilization, went on the hunt for other planets in remote regions of our Milky Way Galaxy, it would have a difficult time finding our small planet. This is because our Earth would appear as only a faint and insignificant speck in the vastness of space. Indeed, our planet is very well hidden from prying alien astronomers because the glare of our Star overwhelms it.

The first detection of an exoplanet occurred back in 1988. However, the first confirmed discovery came in 1992, with the detection of some bizarre and hostile planets circling a dense, city-sized stellar corpse called a pulsar. Pulsars are the lingering relics of massive stars that have perished in the terrible fury of a supernova explosion. This furious, fatal, final blaze of glory marks the violent and catastrophic end of the star-that-was.

Astronomers detected the first exoplanet in orbit around a still “living” star, like our own Sun, back in 1995. However, this historic discovery left confusion in its wake. The newly discovered alien world, dubbed 51 Pegasi b, was unlike anything planetary scientists thought could exist. 51 Peg b is a hot Jupiter–a giant gaseous world, like our Solar System’s Jupiter, that closely hugs its parent-star in a roasting orbit that is much closer to its stellar parent than Mercury’s orbit around our Sun. Before the discovery of 51 Peg b, most astronomers thought that giant gaseous planets could only exist much farther away from their stars–comparable to Jupiter’s distance from our Sun. Jupiter is located in the cold outer region of our Solar System.

The original technique used by astronomers back in 1995–the Doppler Shift method–favors the discovery of giant planets circling around their parent-stars in close, broiling orbits. The Doppler Shift method looks for a tiny wobble induced on a star by an orbiting planet–the larger the planet, the greater the wobble, and the easier it is for planet-hunting astronomers to spot.

As of June 1, 2017, 3,610 exoplanets, inhabiting 2,704 planetary systems, have been discovered–and 610 multiple planetary systems have also been verified. Since 2004, the European Southern Observatory’s (ESO’s) High Accuracy Radial velocity Planet Searcher (HARPS) 3.6 meter telescope, has detected approximately 100 exoplanets, and since 2009, NASA’s Kepler Space Telescope has discovered over two thousand. Kepler has also spotted a few thousand candidate planets, out of which only about 11% may prove to be false-positives. Planet-hunting astronomers estimate that about 1 in 5 stars similar to our Sun are orbited by an “Earth-sized” planet located in the habitable zone surrounding their star. The habitable zone of a star is that Goldilocks region where temperatures are not too hot, not too cold, but just right for water to exist in its life-sustaining liquid phase. Where liquid water exists, life can potentially evolve as well. If there are 200 billion stars inhabiting our Galaxy, it may be that there are 11 billion potentially habitable Earth-sized worlds in our Milky Way. This already huge number could rise even further if planets circling the numerous, long-lived red dwarf stars are included in the estimate. Red dwarf stars are the smallest, coolest, and most abundant true stars dwelling in our Galaxy. Red dwarfs are even smaller than our own small Sun, and they can potentially remain on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution for trillions of years. For this reason, it is generally thought that there are (as yet) no red dwarf relics inhabiting the Cosmos. This is because our Universe is a “mere” 13.8 billion years old, and no red dwarf has had enough time to die since the Big Bang.

The least massive exoplanet discovered so far is Draugr (PSR B1257+12A or PSR B1257+12B), which weighs-in at only twice the mass of our planet’s Moon. In contrast, the most massive known exoplanet is DENIS-P J082303.11-491201 b, and it is about 29 times more massive than Jupiter. However, according to some definitions of a planet, this extremely large world is too massive to be a planet, and may be a type of failed star called a brown dwarf. Brown dwarfs are relatively small distant worlds that probably form the same way as their true stellar kin, but never manage to attain the mass necessary to light their nuclear-fusing fires. These stellar failures are really a pretty purple-pink color called magenta, and they are born as a result of the collapse of a dense pocket embedded within the swirling, undulating folds of a cold, giant molecular cloud–just like their more successful stellar relatives.

Some exoplanets cling closely to their parent-star in such tight, broiling orbits that they require only a few hours to complete a single orbit. However, there are other alien planets that take thousands of years to circle their star. Indeed, some exoplanets are so far from their parent-star that it is sometimes very hard for astronomers to determine whether they are really bound to it gravitationally. Almost all of the exoplanets discovered so far are inhabitants of our own Milky Way Galaxy, but there have also been detections of a handful of intriguing, but still unconfirmed, extragalactic exoplanets. The nearest exoplanet to Earth is dubbed Proxima Centauri b, which circles Proxima Centauri, the closest star to our own Sun. Proxima Centauri b is “only” 4.2 light-years from Earth.

There is also a heavy population of so-called rogue planets, which do not belong to the family of any star at all, but wander through the wilderness of interstellar space without a parent-star to call their own. Alas, these lonely, solitary alien worlds were probably once members of a planetary system, but were rudely evicted by the gravitational nudges of sibling worlds, or by the gravitational disruption caused when a traveling star passed too close to their own stellar parent. Astronomers tend to consider these lonely worlds separately, especially if they are gas-giant planets. If this is the case, these rogue planets are frequently classified as sub-brown dwarfs. The rogue planets that roam our Milky Way may number in the billions.

Nature Vs. Nurture: The Strange Case Of The Exoplanet “Cousins”

The two mismatched “cousin” exoplanets–one cloudy and one clear–circle around their yellow dwarf stars once every 4.5 Earth days. Both exoplanets hug their parent-star tightly, much more closely than Mercury hugs our Sun. However, long ago, the planets likely migrated inward toward the glaring fires and searing-heat of their stellar parent from the more distant regions where they were born.

It is possible that one planet formed differently from the other as the result of a different set of circumstances. “You can say it’s nature versus nurture. Right now, they appear to have the same physical properties. So, if their measured composition is defined by their current state, then it should be the same for both planets. But that’s not the case. Instead, it looks like their formation histories could be playing an important role,” explained study co-investigator Dr. Kevin Stevenson in the June 5, 2017 the STSI Press Release.

The clouds of this distant duo of searing-hot Jupiter-like gas-giants are not like the clouds we see on Earth. Instead, these very alien clouds are likely alkali clouds. This means that they are probably made up of molecules such as sodium sulfide and potassium chloride. The average temperature on each of these broiling planets is over 1,300 degrees Fahrenheit.

The two exoplanets are also tidally locked. This means that they always show the same side facing their stellar parent. The two worlds have an extremely hot day-side and a cooler night-side.

The team of astronomers have only just begun to learn what factors are important in causing some exoplanets to be cloudy, in contrast to others that are clear. In order to gain a better understanding of what the planets’ mysterious pasts may have been like, scientists will need future observations with the HST and the soon-to-be-launched James Webb Space Telescope.

The team’s results were presented on June 5, 2017 at the 230th meeting of the American Astronomical Society in Austin, Texas.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.

## Because exoplanets don’t care what humans think is impossible

Super-puff planets are the least dense of all exoplanets. Even these, however, were thought to have a lower limit to their densities. This new study challenges these old assumptions.

Piaulet’s team analyzed data recorded at the Keck Observatory in Hawaii, in order to better understand the mass and composition of this exoplanet, discovered in 2017.

They utilized the radial velocity method, examining the wobble created by a star as a planet orbits around it. The team determined that WASP-107b is centered on a solid core just four times more massive than Earth.

This would suggest that over 85% of the mass of the world is contained in its atmosphere. Neptune — the most similar planet in our solar system — holds just 10 to 15% of its mass in its atmosphere.

“We had a lot of questions about WASP-107b. How could a planet of such low-density form? And how did it keep its huge layer of gas from escaping, especially given the planet’s close proximity to its star?” Piaulet descibes.

Examination of the atmosphere of WASP-107b, using the Hubble Space Telescope, showed little methane in the air surrounding this unusual exoplanet, where researchers expected to find higher concentrations.

Revolving around the sun, in an orbit similar to our own, is NASA’s Kepler Spacecraft. Launched in 2009 and named after the Renaissance computer science homework help astronomer Johannes Kepler, Kepler’s mission, like that of many ground-based telescopes, is to survey a portion of the Milky Way and discover exoplanets – planets outside of our solar system. Ultimately we hope to find a habitable world. The impetus to find such a world echoes a familiar predicament that recently came to light in the Pixar film Wall-E. What if Earth one day becomes uninhabitable and humans must move to a new home? Ideally, we would rather find a planet like our own than spend our days in a free-floating spaceship. Colonization aside, simply knowing if life exists elsewhere would answer one of humanity’s most fascinating questions: what else is out there?

As of July 13 2013, nearly 1000 exoplanets have been identified, a quarter of which may be habitable [1,2,4]. The number of planet candidates, or planets that have been detected by one method but not yet been confirmed by another, rank in the thousands, and each month, the Kepler telescope alone spots hundreds more. Scientists estimate that each star in our galaxy has at least one orbiting planet. This means that our galaxy is likely home to at least 100 billion exoplanets and potentially, 17 billion earth-sized planets, sometimes referred to as twin Earth or Earth 2.0 [1,2,3].

### How do we find exoplanets?

If you peer through a telescope onto a clear night sky, you may be able to spot the rings of Saturn. In terms of distance, Saturn is 1.5 light hours away, meaning light from Saturn’s rings takes 1.5 hours to reach us. The nearest exoplanet candidate, Alpha Centauri Bb, is 4.4 light years away [], meaning light from that planet takes 4.4 years to reach us. As result, less than 5% of exoplanets can be seen with telescopes.

How, then, have we been able to find the rest of the exoplanets? While not used by Kepler, most exoplanets have been found using a technique called radial velocity, or Doppler spectroscopy. The principle behind this technique is that we can detect tiny changes in a star’s radial velocity caused by an orbiting planet. A star’s radial velocity is the speed it moves towards or away from an observer. The gravitational pull of an orbiting planet tugs on the parent star and causes the star to ‘wobble’ or move around their common center of gravity. As the planet moves around the star, the observer sees a shift in the star’s radial velocity. This shift in radial velocity is dependent on the size of the planet and its distance from the star. A larger planet that is closer to its parent star will cause the radial velocity to shift more. Jupiter, for instance, shifts our sun by 12 m/s while the earth shifts it by less than 0.1m/s because Earth is much smaller than Jupiter [3,5]. Below, you can see how a planet’s movement around a star causes it to appear to be moving toward or away from an observer.

Image by Reyk via Wikimedia Commons.

Current Doppler spectroscopy instruments are only precise to

1m/s. This means scientists cannot use this method to discover smaller planets that shift their parent star’s radial velocity by less than 1 m/s, and this method biases scientists to finding larger planets. The up-side of Doppler spectroscopy is that it does not require more extensive equipment than a well-tuned spectrometer, an instrument that detects light emissions (hence no need for huge telescopes or even space telescopes). The reason for this simplicity is we are not actually directly observing the star but looking for shifts in the star’s light emission that could suggest a wobble. The down-side of Doppler spectroscopy is that it requires time, approximately 500 to 1000 observations per wobbly star [] and can only be used with stars whose wobble causes them to move closer and further away from us as opposed to side to side shown here.

Image by Reyk via Wikimedia Commons.

The other dominant exoplanet detection method is the transit method. As a planet passes between its parent star and an observer, the star’s observed brightness dims. This dip in stellar brightness is what space telescopes like Kepler pick up. Jupiter-sized planets can block up to 1% of the star’s light whereas Earth-sized planets may only block 0.01%. Detection of smaller planets, like Earth, is heavily dependent upon precision and timing. The method as a whole has been likened to spotting a seagull fly across a lighthouse beam []. Below is a figure adapted from Science magazine that shows a planet moving across a star and the resulting variations in stellar brightness and radial velocity (Stellar RV) []. In panel B, note the dip in stellar brightness and the ‘wobble’ on the radial velocity curve as the planet passes across.

Figure 1. Illustration of a planetary orbit and its effect on stellar brightness and RV. (A) The planet orbits the parent star counterclockwise and eclipses the star. (B) The observer detects the planet by either the dip in the stellar brightness (transit method) or the wobble in the stellar RV (Doppler spectroscopy). Both methods are necessary to confirm a planet’s existence. From A. W. Howard, “Observed Properties of Extrasolar Planets”, Science 340, 572 (2013). Reprinted with permission from AAAS.

Doppler spectroscopy and the transit method are complementary. The transit method frequently produces false positives (‘planet’ signals that are due to something else) and relies on ground-based methods like Doppler spectroscopy to confirm a planet’s existence. Moreover, while planet size and orbit can be inferred from transit data, mass, most often, cannot. Mass, however, can be measured from follow up spectroscopy observations []. Together, the two methods provide the bulk of a planet’s physical attributes—mass and radius. Density can then be calculated by dividing the planet’s mass by its volume.

Once scientists know mass, radius, and density, they can make educated guesses about an exoplanet’s atmosphere and core composition. For example, an exoplanet with a low density (

0.5 g/cm 3 compared to Earth’s 5.5 g/cm 3 density) but a relatively large mass (8 times as heavy as Earth) may suggest a substantial amount of lighter gasses (like hydrogen and helium) in the atmosphere. An exoplanet with a high density (

9 g/cm 3 ) and a relatively small mass (4 times as heavy as Earth) is more likely to have a rock/iron core with little to no atmosphere []. Exoplanets with densities close to that of water may be ‘water worlds’ covered in oceans or sheets of ice [].

Direct imaging is needed to truly determine the makeup of a planet. This approach gathers light from the planet itself and disperses it into a spectrum []. Coronagraphs (telescopic attachments designed to block light) are used to block out light from the parent star. The planet’s emissions of infrared light are then measured, as planets are usually brighter in infrared than they are in visible light. Nevertheless, gathering planet light is not always easy. The parent star is millions to billions of times brighter than the planet and plucking a planet out from its star’s glare is sometimes impossible. This is much easier if it is a large planet in a wide orbit around a dim star. Exoplanets like this are usually large and gas-filled, and are what we find most often.

### Other Worlds

While astronomers watch intently for exoplanets showing up as small blips in stellar radial velocity or stellar brightness, the general public envisions exoplanets as the artist renders them, orbs aglow in the finest stardust. But what are some of these worlds really like? In our search for the habitable, we have come across worlds that are truly alien. There are pulsar planets that orbit fast spinning pulsar stars left behind after a supernova. These stars are very dense and emit pulses of electromagnetic radiation. Planets orbiting pulsar stars are trapped by the star’s strong gravitational field and become very dense themselves one such planet is said to be made of pure diamond. There are rogue planets that orbit no star and float aimlessly through space, homeless and exceptionally cold. There are also circumbinary planets that orbit binary stars or two star systems and thus see two suns. And, in stark contrast to the water worlds mentioned above, there are also volcanic worlds, planets so tethered to their parent stars that they see nothing but 4000°F heat and rain of molten rock [].

Exoplanets also come in an assortment of sizes, from the very large Jupiter-size planets, to the modest, Earth-size ones. Jupiter-size planets make up the majority of confirmed exoplanets. Puffy planets are hot, Jupiter-size planets that are so light they could float on water if given a large enough tub. One size down from the Jupiters are the Neptunes and the mini-Neptunes. One size down from the Neptunes are the Super-Earths and the Earth-size planets []. Below is a table that illustrates the current common mass classification of exoplanets in terms of size with respect to Earth. An exoplanet with enough pull, usually a Jupiter sized one, may even have an exomoon spinning faithfully around it.

### How close are we to ET?

Although we have discovered many different types of planets, we have not yet found extraterrestrial life. Nonetheless, current instruments are probing for the likes of ET in the habitable zones of planet-sustaining stars. The “habitable zone” is the region around a star where a planet can have the right temperature to maintain liquid water [7,8]. The zone changes with the type of planet and type of star a larger, brighter star, for instance, has a wider habitable zone and a drier, rockier planet may only be habitable at the inner edge of the zone. It is important to note that just because a planet is in the habitable zone does not mean it is habitable [8,10]. Venus, Mars, and Earth all fall within our sun’s habitable zone however, Venus is too hot and Mars is too cold but Earth, as Goldilocks might say, is just right. To search for life, scientists look for exoplanet emissions of biosignature gases that can be detected by spectrometers []. Biosignature gases are produced by life on the planet (microbes, plants, people, etc) and gather in the atmosphere at high levels. Today’s telescopes do not have the capacity to detect these gases but future telescopes may. Hence the race to find life is also a race to advance science.

A conventional habitable planet is covered in liquid water. Water is the crux of life as we know it but perhaps this is not the case for every planet, perhaps somewhere in the ether is a gas planet teeming with new life. Given the number of exoplanets in our galaxy, other possibilities may very well exist. As stated in a recent Science article, ‘planet habitability is planet specific’ [] and no universal rule can apply.

Weike Wang is a graduate student at the Harvard School of Public Health

#### References:

[] J. Schneider The Extrasolar Planets Encyclopedia http://exoplanet.eu/catalog.php.

[] Homepage for Kepler Telescope http://kepler.nasa.gov/

[] Exoplanets, Worlds Beyond our Solar System (space.com) http://www.space.com/17738-exoplanets.html

[] M. Cruz, R. Coontz, “Alien Worlds Galore”, Science 340, 565 (2013).

[] Y. Bhattacharjee, D. Clery “A Gallery of Planet Hunters”, Science 340, 566 (2013).

[] L. Wade, “A Glossary of Their Quarry”, Science 340, 570 (2013).

[] A. W. Howard, “Observed Properties of Extrasolar Planets”, Science 340, 572 (2013).

[] S. Seager, “Exoplanet Habitability” Science 340, 577 (2013).

## What is the Least Dense Exoplanet?

In order to calculate a crude, average density we need a mass and radius estimate or measurement for an exoplanet (and a spherical geometry is usually assumed). However, the majority of exoplanets do not have radius (or any kind of size) measurement. For example, on 3 August, 2012, out of 777 confirmed exoplanets, only 252 (just over 32%) had a radius measurements, and only 241 had both mass and radius measurements. Therefore, if we ask &ldquoWhat is the least dense exoplanet?&rdquo we must remember that we can only answer this question for a fraction of the confirmed exoplanets. Using the same example, on that date, the least dense exoplanet in the Extrasolar Planets Encylopedia catalog was WASP-17 b. This planet has about half of the mass of Jupiter contained within a size that is about twice that of Jupiter, so we could expect the density to be about a sixteenth of Jupiter. Using the exact numbers give a density of .0616$times that of Jupiter, or about .0148$ times that of Earth. This translates into about .082 < m g cm>^<-3>$, compared to Earth's$5.513 < m g cm>^<-3>\$. You will have to consult the original data in the table to figure out a reasonable estimate for the margin of error on the density, given the uncertainties on the mass and radius estimates (also see the important caveat at the end of this article).

You can get instant access to the book Exoplanets and Alien Solar Systems:

For a sphere, density is proportional to mass times radius cubed. If you would like to be able to find out yourself what is the least dense exoplanet found so far, here is how you do it. Go to the Extrasolar Planets Encylopedia catalog page. You will then have to download the data table using one of the options given just above the table header. Read the columns of data into an application or program that you are familiar with (for example, EXCEL). Then follow the instructions on the exoplanets density distribution page of this website.

## Because exoplanets don’t care what humans think is impossible

An artistic rendition of the exoplanet WASP-107b and its star, WASP-107. Some of the star’s light streams through the exoplanet’s extended gas layer. Image credit: ESA/Hubble, NASA, M. Kornmesser

Super-puff planets are the least dense of all exoplanets. Even these, however, were thought to have a lower limit to their densities. This new study challenges these old assumptions.

Piaulet’s team analyzed data recorded at the Keck Observatory in Hawaii, in order to better understand the mass and composition of this exoplanet, discovered in 2017.

They utilized the radial velocity method, examining the wobble created by a star as a planet orbits around it. The team determined that WASP-107b is centered on a solid core just four times more massive than Earth.

This would suggest that over 85% of the mass of the world is contained in its atmosphere. Neptune — the most similar planet in our solar system — holds just 10 to 15% of its mass in its atmosphere.

“We had a lot of questions about WASP-107b. How could a planet of such low-density form? And how did it keep its huge layer of gas from escaping, especially given the planet’s close proximity to its star?” Piaulet descibes.

Examination of the atmosphere of WASP-107b, using the Hubble Space Telescope, showed little methane in the air surrounding this unusual exoplanet, where researchers expected to find higher concentrations.

## Astronomers find an atmosphere around a nearby Earth-sized exoplanet! But what's it made of?

Gliese 1132 is a red dwarf, a small, cool, low-mass star. It’s so faint a star that even though it’s relatively close by at 39 light-years from Earth, you need a decent telescope to see it at all.

In 2005, though, when astronomers did point their telescopes at it, they discovered a planet orbiting it! Called GJ 1132b*, the exoplanet orbits the star nearly edge-on as seen from Earth, so once every orbit it passes directly in front of its star, what we call a transit. It blocks a small fraction of the starlight, revealing its presence as a slight dimming in the star. Knowing how much light was blocked and the size of the star, the size of the exoplanet can be determined. For GJ 1132b, it’s a smidge over 1.4 times the size of the Earth: a super-Earth.

That’s the size of the solid part, at least. New observations have done something astonishing: They’ve revealed the planet has an atmosphere, potentially a thick one composed of water, hydrogen and/or methane!

If the planet orbit is face-on to us, we see no transit. If it's edge-on, we do. Credit: Greg Loughlin

Still, this is pretty exciting: It’s only the second near-Earth-sized world found to have an atmosphere detected, and smaller and closer than the other (55 Cancri e is twice the diameter of Earth and 40 light years away). This makes it a good testbed for our observational capabilities, and for modeling the physical characteristics of the planet.

The atmosphere was detected in a clever way. Using the 2.2-meter MPG telescope in Chile, astronomers observed the star GJ 1132 using multiple filters simultaneously that only allowed specific colors of starlight to be seen. These covered the wavelengths of light we can see with our eyes, as well as infrared light. The exoplanet takes 1.6 Earth days to orbit, and they watched it pass a total of nine times in front of its star. This allowed them to get good data and see what’s what.

They found that in seven of the filters, the planet’s size was consistent. But in two filters (a near-infrared one called the z filter and another in the infrared called K) the star dimmed more than expected, meaning the planet was larger than expected. In the z filter the effect was particularly strong, and it just so happens that this color of light is preferentially absorbed by water vapor and methane! The simplest explanation is that in seven of the filters they were seeing the solid body of the planet dimming the starlight, but in those two remaining filters they’re seeing the atmosphere as well.

It’s hard to know exactly what the atmosphere is made of or how thick it is from these observations alone, but they are a big milestone along the way to that knowledge. And they are enough to give us some ideas.

For example, an Earth-like mix of materials in the solid planet itself (33% iron, 67% rocky silicates) is inconsistent with what they saw the planet must be less dense than Earth. The models suggest a wide range of potential composition it could be entirely made of rock (and would probably have a molten surface), or the surface could be rich in water. At that temperature the water would create a very thick atmosphere of water vapor and molecular hydrogen (H2). Interestingly, the GJ 1132b is close enough to its star to be tidally locked, spinning once for every time it goes around the star. Its year and day are the same length, so it shows one forever sunlit side to the star, with the other half facing away in eternal night. This would create some interesting weather on the planet!

But that’s a bit speculative. The beauty of this is that the star is bright enough, and the planet big enough, that more observations with different telescopes should be attainable to narrow down the range of potential conditions. Right now we don’t know a huge amount about it, but we may know a lot more soon.

And don’t let what we don’t know obscure what we do. The first exoplanet was discovered in 1992. Just 25 years ago! Since that time, we’ve found thousands more, from dinky rocks the size of Mercury to behemoths many times the mass of Jupiter. We’ve seen them around dim cool stars and big hot ones, orbiting them close in and far away. We’ve also seen multiple planets in systems, and several stars that host planets that are similar in size to Earth. And perhaps most excitingly, we’ve detected quite a few exoplanets orbiting at the right distance from their star to have liquid water on their surface.

We still haven’t found that first golden planet, one that is Earth-like. But we’re getting closer to being able to do so, and if it’s out there —and we think there are billions of them out there— we’ll find it.

* The star is the 1132 nd entry in the Gliese catalog of nearby stars. Wilhelm Gliese collaborated with Hartmut Jahreiss in 1979 to extend the catalog to more stars, so stars are given the abbreviation GJ.