Posted on by Matt Williams

Looking for Signs of Life on Distant Planets Just Got Easier

This illustration shows a star's light illuminating the atmosphere of a planet. Credits: NASA Goddard Space Flight Center

When it comes to searching for worlds that could support extra-terrestrial life, scientists currently rely on the “low-hanging fruit” approach. Since we only know of one set of conditions under which life can thrive – i.e. what we have here on Earth – it makes sense to look for worlds that have these same conditions. These include being located within a star’s habitable zone, having a stable atmosphere, and being able to maintain liquid water on the surface.

Until now, scientists have relied on methods that make it very difficult to detect water vapor in the atmosphere’s of terrestrial planets. But thanks to a new study led by Yuka Fujii of NASA’s Goddard Institute for Space Studies (GISS), that may be about to change. Using a new three-dimensional model that takes into account global circulation patterns, this study also indicates that habitable exoplanets may be more common than we thought.

The study, titled “NIR-driven Moist Upper Atmospheres of Synchronously Rotating Temperate Terrestrial Exoplanets“, recently appeared in The Astrophysical Journal. In addition to Dr. Fujii, who is also a member of the Earth-Life Science Institute at the Tokyo Institute of Technology, the research team included Anthony D. Del Genio (GISS) and David S. Amundsen (GISS and Columbia University).

Artist’s concept of the hot Jupiter WASP-121b, which presents the best evidence yet of a stratosphere on an exoplanet – generated using Engine House VFX. Credit: Bristol Science Centre/University of Exeter

To put it simply, liquid water is essential to life as we know it. If a planet does not have a warm enough atmosphere to maintain liquid water on its surface for a sufficient amount of time (on the order of billions of years), then it is unlikely that life will be able to emerge and evolve. If a planet is too distant from its star, its surface water will freeze; if it is too close, its surface water will evaporate and be lost to space.

While water has been detected in the atmospheres of exoplanets before, in all cases, the planets were massive gas giants that orbited very closely to their stars. (aka. “Hot Jupiters”). As Fujii and her colleagues state in their study:

“Although H2O signatures have been detected in the atmospheres of hot Jupiters, detecting molecular signatures, including H2O, on temperate terrestrial planets is exceedingly challenging, because of the small planetary radius and the small scale height (due to the lower temperature and presumably larger mean molecular weight).”

When it comes to terrestrial (i.e. rocky) exoplanets, previous studies were forced to rely on one-dimensional models to calculate the presence of water. This consisted of measuring hydrogen loss, where water vapor in the stratosphere is broken down into hydrogen and oxygen from exposure to ultraviolet radiation. By measuring the rate at which hydrogen is lost to space, scientists would estimate the amount of liquid water still present on the surface.

Artist’s impression of the “Venus-like” exoplanet GJ 1132b. Credit: cfa.harvard.edu

However, as Dr. Fujii and her colleagues explain, such models rely on several assumptions that cannot be addressed, which include the global transport of heat and water vapor vapor, as well as the effects of clouds. Basically, previous models predicted that for water vapor to reach the stratosphere, long-term surface temperatures on these exoplanets would have to be more than 66 °C (150 °F) higher than what we experience here on Earth.

These temperatures could create powerful convective storms on the surface. However, these storms could not be the reason water reaches the stratosphere when it comes to slowly rotating planets entering a moist greenhouse state – where water vapor intensifies heat. Planets that orbit closely to their parent stars are known to either have a slow rotation or to be tidally-locked with their planets, thus making convective storms unlikely.

This occurs quite often for terrestrial planets that are located around low-mass, ultra cool, M-type (red dwarf) stars. For these planets, their proximity to their host star means that it’s gravitational influence will be strong enough to slow down or completely arrest their rotation. When this occurs, thick clouds form on the dayside of the planet, protecting it from much of the star’s light.

The team found that, while this could keep the dayside cool and prevent water vapor from rising, the amount of near-Infrared radiation (NIR) could provide enough heat to cause a planet to enter a moist greenhouse state. This is especially true of M-type and other cool dwarf stars, which are known to produce more in the way of NIR. As this radiation warms the clouds, water vapor will rise into the stratosphere.

Artist’s impression of Proxima b, the closest exoplanet to the Solar System. In the background, the binary system of Alpha Centauri can be seen. Credit: ESO/M. Kornmesser

To address this, Fujii and her team relied on three-dimensional general circulation models (GCMs) which incorporate atmospheric circulation and climate heterogeneity. For the sake of their model, the team started with a planet that had an Earth-like atmosphere and was entirely covered by oceans. This allowed the team to clearly see how variations in distance from different types of stars would effect conditions on the planets surfaces.

These assumptions allowed the team to clearly see how changing the orbital distance and type of stellar radiation affected the amount of water vapor in the stratosphere. As Dr. Fujii explained in a NASA press release:

“Using a model that more realistically simulates atmospheric conditions, we discovered a new process that controls the habitability of exoplanets and will guide us in identifying candidates for further study… We found an important role for the type of radiation a star emits and the effect it has on the atmospheric circulation of an exoplanet in making the moist greenhouse state.”

In the end, the team’s new model demonstrated that since low-mass star emit the bulk of their light at NIR wavelengths, a moist greenhouse state will result for planets orbiting closely to them. This would result in conditions on their surfaces that comparable to what Earth experiences in the tropics, where conditions are hot and moist, instead of hot and dry.

Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO

What’s more, their model indicated that NIR-driven processes increased moisture in the stratosphere gradually, to the point that exoplanets orbiting closer to their stars could remain habitable. This new approach to assessing potential habitability will allow astronomers to simulate circulation of planetary atmospheres and the special features of that circulation, which is something one-dimensional models cannot do.

In the future, the team plans to assess how variations in planetary characteristics -such as gravity, size, atmospheric composition, and surface pressure – could affect water vapor circulation and habitability. This will, along with their 3-dimensional model that takes planetary circulation patterns into account, allow astronomers to determine the potential habitability of distant planets with greater accuracy. As Anthony Del Genio indicated:

“As long as we know the temperature of the star, we can estimate whether planets close to their stars have the potential to be in the moist greenhouse state. Current technology will be pushed to the limit to detect small amounts of water vapor in an exoplanet’s atmosphere. If there is enough water to be detected, it probably means that planet is in the moist greenhouse state.”

Beyond offering astronomers a more comprehensive method for determining exoplanet habitability, this study is also good news for exoplanet-hunters hoping to find habitable planets around M-type stars. Low-mass, ultra-cool, M-type stars are the most common star in the Universe, accounting for roughly 75% of all stars in the Milky Way. Knowing that they could support habitable exoplanets greatly increases the odds of find one.

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

In addition, this study is VERY good news given the recent spate of research that has cast serious doubt on the ability of M-type stars to host habitable planets. This research was conducted in response to the many terrestrial planets that have been discovered around nearby red dwarfs in recent years. What they revealed was that, in general, red dwarf stars experience too much flare and could strip their respective planets of their atmospheres.

These include the 7-planet TRAPPIST-1 system (three of which are located in the star’s habitable zone) and the closest exoplanet to the Solar System, Proxima b. The sheer number of Earth-like planets discovered around M-type stars, coupled with this class of star’s natural longevity, has led many in the astrophysical community to venture that red dwarf stars might be the most likely place to find habitable exoplanets.

With this latest study, which indicates that these planets could be habitable after all, it would seem that the ball is effectively back in their court!

Further Reading: NASA, The Astrophysical Journal

 

Posted on by Matt Williams

Debris Disks Around Stars Could Point the Way to Giant Exoplanets

This artist's rendering shows a large exoplanet causing small bodies to collide in a disk of dust. Credit: NASA/JPL-Caltech

According to current estimates, there could be as many as 100 billion planets in the Milky Way Galaxy alone. Unfortunately, finding evidence of these planets is tough, time-consuming work. For the most part, astronomers are forced to rely on indirect methods that measure dips in a star’s brightness (the Transit Method) of Doppler measurements of the star’s own motion (the Radial Velocity Method).

Direct imaging is very difficult because of the cancelling effect stars have, where their brightness makes it difficult to spot planets orbiting them. Luckily a new study led by the Infrared Processing and Analysis Center (IPAC) at Caltech has determined that there may be a shortcut to finding exoplanets using direct imaging. The solution, they claim, is to look for systems with a circumstellar debris disk, for they are sure to have at least one giant planet.

The study, titled “A Direct Imaging Survey of Spitzer Detected Debris Disks: Occurrence of Giant Planets in Dusty Systems“, recently appeared in The Astronomical Journal. Tiffany Meshkat, an assistant research scientist at IPAC/Caltech, was the lead author on the study, which she performed while working at NASA’s Jet Propulsion Laboratory as a postdoctoral researcher.

A circumstellar disk of debris around a mature stellar system could indicate the presence of Earth-like planets. Credit: NASA/JPL
Artist’s impression of circumstellar disk of debris around a distant star. Credit: NASA/JPL

For the sake of this study, Dr. Meshkat and her colleagues examined data on 130 different single-star systems with debris disks, which they then compared to 277 stars that do not appear to host disks. These stars were all observed by NASA’s Spitzer Space Telescope and were all relatively young in age (less than 1 billion years). Of these 130 systems, 100 had previously been studied for the sake of finding exoplanets.

Dr. Meshkat and her team then followed up on the remaining 30 systems using data from the W.M. Keck Observatory in Hawaii and the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile. While they did not detect any new planets in these systems, their examinations helped characterize the abundance of planets in systems that had disks.

What they found was that young stars with debris disks are more likely to also have giant exoplanets with wide orbits than those that do not. These planets were also likely to have five times the mass of Jupiter, thus making them “Super-Jupiters”. As Dr. Meshkat explained in a recent NASA press release, this study will be of assistance when it comes time for exoplanet-hunters to select their targets:

“Our research is important for how future missions will plan which stars to observe. Many planets that have been found through direct imaging have been in systems that had debris disks, and now we know the dust could be indicators of undiscovered worlds.”

This artist’s conception shows how collisions between planetesimals can create additional debris. Credit: NASA/JPL-Caltech

This study, which was the largest examination of stars with dusty debris disks, also provided the best evidence to date that giant planets are responsible for keeping debris disks in check. While the research did not directly resolve why the presence of a giant planet would cause debris disks to form, the authors indicate that their results are consistent with predictions that debris disks are the products of giant planets stirring up and causing dust collisions.

In other words, they believe that the gravity of a giant planet would cause planestimals to collide, thus preventing them from forming additional planets. As study co-author Dimitri Mawet, who is also a JPL senior research scientist, explained:

“It’s possible we don’t find small planets in these systems because, early on, these massive bodies destroyed the building blocks of rocky planets, sending them smashing into each other at high speeds instead of gently combining.”

Within the Solar System, the giant planets create debris belts of sorts. For example, between Mars and Jupiter, you have the Main Asteroid Belt, while beyond Neptune lies the Kuiper Belt. Many of the systems examined in this study also have two belts, though they are significantly younger than the Solar System’s own belts – roughly 1 billion years old compared to 4.5 billion years old.

Artist’s impression of Beta Pictoris b. Credit: ESO L. Calçada/N. Risinger (skysurvey.org)

One of the systems examined in the study was Beta Pictoris, a system that has a debris disk, comets, and one confirmed exoplanet. This planet, designated Beta Pictoris b, which has 7 Jupiter masses and orbits the star at a distance of 9 AUs – i.e. nine times the distance between the Earth and the Sun. This system has been directly imaged by astronomers in the past using ground-based telescopes.

Interestingly enough, astronomers predicted the existence of this exoplanet well before it was confirmed, based on the presence and structure of the system’s debris disk. Another system that was studied was HR8799, a system with a debris disk that has two prominent dust belts. In these sorts of systems, the presence of more giant planets is inferred based on the need for these dust belts to be maintained.

This is believed to be case for our own Solar System, where 4 billion years ago, the giant planets diverted passing comets towards the Sun. This resulted in the Late Heavy Bombardment, where the inner planets were subject to countless impacts that are still visible today. Scientists also believe that it was during this period that the migrations of Jupiter, Saturn, Uranus and Neptune deflected dust and small bodies to form the Kuiper Belt and Asteroid Belt.

Dr. Meshkat and her team also noted that the systems they examined contained much more dust than our Solar System, which could be attributable to their differences in age. In the case of systems that are around 1 billion years old, the increased presence of dust could be the result of small bodies that have not yet formed larger bodies colliding. From this, it can be inferred that our Solar System was once much dustier as well.

Artist’s concept of the multi-planet system around HR 8799, initially discovered with Gemini North adaptive optics images. Credit: Gemini Observatory/Lynette Cook”

However, the authors note is also possible that the systems they observed – which have one giant planet and a debris disk – may contain more planets that simply have not been discovered yet. In the end, they concede that more data is needed before these results can be considered conclusive. But in the meantime, this study could serve as an guide as to where exoplanets might be found.

As Karl Stapelfeldt, the chief scientist of NASA’s Exoplanet Exploration Program Office and a co-author on the study, stated:

“By showing astronomers where future missions such as NASA’s James Webb Space Telescope have their best chance to find giant exoplanets, this research paves the way to future discoveries.”

In addition, this study could help inform our own understanding of how the Solar System evolved over the course of billions of years. For some time, astronomers have been debating whether or not planets like Jupiter migrated to their current positions, and how this affected the Solar System’s evolution. And there continues to be debate about how the Main Belt formed (i.e. empty of full).

Last, but not least, it could inform future surveys, letting astronomers know which star systems are developing along the same lines as our own did, billions of years ago. Wherever star systems have debris disks, they an infer the presence of a particularly massive gas giant. And where they have a disk with two prominent dust belts, they can infer that it too will become a system containing many planets and and two belts.

Further Reading: NASA, The Astrophysical Journal

Posted on by Matt Williams

Hubble Spots Pitch Black Hot Jupiter that “Eats Light”

Illustration showing one of the darkest known exoplanets - a hot Jupiter as black as fresh asphalt - orbiting a star like our Sun. The day side of the planet, called WASP-12b, eats light rather than reflects it into space. Something is pulling this planet into its star. Credit: NASA, ESA, and G. Bacon (STScI)
Illustration showing one of the darkest known exoplanets - a hot Jupiter as black as fresh asphalt - orbiting a star like our Sun. The day side of the planet, called WASP-12b, eats light rather than reflects it into space. Something is pulling this planet into its star. Credit: NASA, ESA, and G. Bacon (STScI)

The study of extra-solar planets has revealed discoveries that have confounded expectations and boggled the mind! Whether it’s Super-Earths that become diamond planets, multiple rocky planets orbiting closely together, or “Hot Jupiters” with traces of gaseous metal in their atmospheres, there’s been no shortage of planets out there for which there is no comparison here in the Solar System.

In this respect, WASP-12b is in good company. This Hot-Jupiter, located in a star system 1400 light years from Earth in the direction of the Auriga constellation, was recently studied by a team of astronomers using the Hubble Space Telescope. Due to the particular nature of its atmosphere, which absorbs the vast majority of light it receives instead of reflecting it, this planet appeared pitch black when observed by the Hubble team.

The study which details their findings, “The Very Low Albedo of WASP-12b from Spectral Eclipse Observations with Hubble“, was recently published in The Astrophysical Journal. Led by Taylor Bell, a researcher at the Institute for Research on Exoplanets (IREx) at McGill University, the team consulted data from the Hubble’s Space Telescope Imaging Spectrograph (STIS) to observe WASP-12b during an optical eclipse.

WASP-12b orbits so close to its star that it is heated to a record-breaking 2500°C. Credit: ESA/C Carreau

Like all Hot Jupiters, WASP-12b is similar in mass to Jupiter (1.35 to 1.43 Jupiter masses) and orbits very close to its star. At a distance of just 3.4 million km (2.115 million mi), or 0.0229 AU, it takes a little over a day to complete a single orbit. Because of its proximity, one side of the planet is constantly facing towards it’s sun – i.e. it is tidally locked with its star.

Because of its orbit, temperatures on the day side of the planet are estimated to reach as high as 2811 K (2538 °C; 4600 °F). It is because of these extreme temperatures that most molecules are unable to survive on the day side of the planet, so clouds cannot form to reflect light back into space. As a result, most incoming light penetrates deep into the planet’s atmosphere, where it is absorbed by hydrogen atoms and converted into heat energy.

This was what Bell and his team noticed as they observed the planet passing behind its star (aka. an optical eclipse). Using the STIS, they monitored the system for any dips in starlight, which would indicate how much reflected light was being given off by the planet. However, their observations did not detect reflected light, which indicated that the sun-facing side was absorbing most of the light it was receiving.

As Bell explained in a NASA press statement, this was quite the unusual find: “We did not expect to find such a dark exoplanet,” he said. “Most hot Jupiters reflect about 40 percent of starlight.” However, observations conducted of the night side of the planet show that things are quite different there. On this side, temperatures are about 1366 K (1093 °C; 2000 °F) cooler, which allows water vapor and clouds to form.

An artist’s impression of WASP 12-b being slowly consumed as a result of its ridiculously tight orbit around its star. Credit: NASA.

Back in 2013, scientists working with the HST detected traces of water vapor in the atmosphere (and possible traces of clouds as well) while studying the day/night boundary. As Bell indicated, this new research just goes to show just how diverse this type of gas giant can be:

“This new Hubble research further demonstrates the vast diversity among the strange population of hot Jupiters. You can have planets like WASP-12b that are 4,600 degrees Fahrenheit and some that are 2,200 degrees Fahrenheit, and they’re both called hot Jupiters. Past observations of hot Jupiters indicate that the temperature difference between the day and night sides of the planet increases with hotter day sides. This previous research suggests that more heat is being pumped into the day side of the planet, but the processes, such as winds, that carry the heat to the night side of the planet don’t keep up the pace.”

Since its discovery in 2008, several telescopes have studied WASP-12b, including Hubble, NASA’s Spitzer Space Telescope, and NASA’s Chandra X-ray Observatory. Previous observations by Hubble’s Cosmic Origins Spectrograph (COS) also revealed that the planet may be losing size and mass due to super-heated material from its atmosphere slowly being accreted onto the star.

This is just the latest find in a slew that has confounded scientists expectations about exoplanets. The more we come to learn about the nature and diversity of these distant worlds, the more tantalizing they seem and the more appealing the prospect of exploring them directly someday becomes!

Further Reading: NASA, IREx, Astrophysical Journal Letters

Posted on by Matt Williams

Astronomers Spot Hellish World with Titanium in its Atmosphere

Artist's impression showing the exoplanet WASP-19b, in which atmosphere astronomers detected titanium oxide for the first time. Credit: ESO

The hunt for exoplanets has turned up many fascinating case studies. For example, surveys have turned up many “Hot Jupiters”, gas giants that are similar in size to Jupiter but orbit very close to their suns. This particular type of exoplanet has been a source of interest to astronomers, mainly because their existence challenges conventional thinking about where gas giants can exist in a star system.

Hence why an international team led by researchers from the European Southern Observatory (ESO) used the Very Large Telescope (VLT) to get a better look at WASP-19b, a Hot Jupiter located 815 light-years from Earth. In the course of these observations, they noticed that the planet’s atmosphere contained traces of titanium oxide, making this the first time that this compound has been detected in the atmosphere of a gas giant.

The study which describes their findings, titled “Detection of titanium oxide in the atmosphere of a hot Jupiter“, recently appeared in the science journal Nature. Led by Elyar Sedaghati – a recent graduate from the Technical University of Berlin and a fellow at the European Southern Observatory – the team used data collected by the VLT array over the course of a year to study WASP-19b.

Like all Hot Jupiters, WASP-19b has about the same mass as Jupiter and orbits very close to its sun. In fact, its orbital period is so short  – just 19 hours – that temperatures in its atmosphere are estimated to reach as high as 2273 K (2000 °C; 3632 °F). That’s over four times as hot as Venus, where temperatures are hot enough to melt lead! In fact, temperatures on WASP-19b are hot enough to melt silicate minerals and platinum!

The study relied on the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument on the VLT, a multi-mode optical instrument capable of conducting imaging, spectroscopy and the study of polarized light (polarimetry). Using FORS2, the team observing the planet as it passed in front of its star (aka. made a transit), which revealed valuable spectra from its atmosphere.

After carefully analyzing the light that passed through its hazy clouds, the team was surprised to find trace amounts of titanium oxide (as well as sodium and water). As Elyar Sedaghati, who spent 2 years as a student with the ESO to work on this project, said of the discovery in an ES press release:

“Detecting such molecules is, however, no simple feat. Not only do we need data of exceptional quality, but we also need to perform a sophisticated analysis. We used an algorithm that explores many millions of spectra spanning a wide range of chemical compositions, temperatures, and cloud or haze properties in order to draw our conclusions.”

Titanium oxide is a very rare compound which is known to exist in the atmospheres of cool stars. In small quantities, it acts as a heat absorber, and is therefore likely to be partly responsible for WASP-19b experiencing such high temperatures. In large enough quantities, it can prevent heat from entering or escaping an atmosphere, causing what is known as thermal inversion.

This is a phenomena where temperatures are higher in the upper atmosphere and lower further down. On Earth, ozone plays a similar role, causing an inversion of temperatures in the stratosphere. But on gas giants, this is the opposite of what usually happens. Whereas Jupiter, Saturn, Uranus and Neptune experience colder temperatures in their upper atmospheres, temperatures are much hotter closer to the core due to increases in pressure.

The team believes that the presence of this compound could have a substantial effect on the atmosphere’s temperature, structure and circulation. What’s more, the fact that the team was able to detect this compound (a first for exoplanet researchers) is an indication of how exoplanet studies are achieving new levels of detail. All of this is likely to have a profound impact on future studies of exoplanet atmospheres.

The study would also have not been possible were it not for the FORS2 instrument, which was added to the VLT array in recent years. As Henri Boffin, the instrument scientist who led the refurbishment project, commented:

“This important discovery is the outcome of a refurbishment of the FORS2 instrument that was done exactly for this purpose. Since then, FORS2 has become the best instrument to perform this kind of study from the ground.”

Looking ahead, it is clear that the detection of metal oxides and other similar substances in exoplanet atmospheres will also allow for the creation of better atmospheric models. With these in hand, astronomers will be able to conduct far more detailed and accurate studies on exoplanet atmospheres, which will allow them to gauge with greater certainty whether or not any of them are habitable.

So while this latest planet has no chance of supporting life – you’d have better luck finding ice cubes in the Gobi desert! – its discovery could help point the way towards habitable exoplanets in the future. On step closer to finding a world that could support life, or possibly that elusive Earth 2.0!

Further Reading: ESO, Nature

Posted on by Matt Williams

Three Possible Super-Earths Discovered Around Nearby Sun-Like Star

Artist’s impression of a Super-Earth planet orbiting a Sun-like star. Credit: ESO/M. Kornmesser

Since it was launched in 2009, NASA’s Kepler mission has continued to make important exoplanet discoveries. Even after the failure of two reaction wheels, the space observatory has found new life in the form of its K2 mission. All told, this space observatory has detected 5,017 candidates and confirmed the existence of 2,494 exoplanets using the Transit Method during its past eight years in service.

The most recent discovery was made by an international team of astronomers around Gliese 9827 (GJ 9827), a late K-type dwarf star located about 100 light-years from Earth. Using data provided by the K2 mission, they detected the presence of three Super-Earths. This star system is the closest exoplanet-hosting star discovered by K2 to date, which makes these planets well-suited for follow-up studies.

The study which describes their findings, titled “A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at Thirty Parsecs“, was recently published online. Led by Dr. Jospeh E. Rodriguez from the Harvard-Smithsonian Center for Astrophysics (CfA), the team includes researchers from the University of Austin, the Massachusetts Institute of Technology (MIT), and the NASA Exoplanet Science Institute (NExSci) at Caltech.

The Transit Method, which remains one of the most trusted means for exoplanet detection, consists of monitoring stars for periodic dips in brightness. These dips correspond to planets passing (aka. transiting) in front of the star causing a measurable drop in the light coming from it. This method also offers unique opportunities to examine light passing through an exoplanet’s atmosphere. As Dr. Rodriguez told Universe Today via email:

“The success of Kepler combined with ground based radial velocity and transit surveys has now led to the discovery of over 4000 planetary system. Since we now know that planets appear to be quite common, the field has shifted its focus to understand architectures, interior structures, and atmospheres. These key properties of planetary systems help us understand some fundamental questions: how do planets form and evolve? What are the terrestrial planets around other stars like, are they similar to Earth in composition and atmosphere?”

These questions were central to the team’s study, which relied on data obtained during Campaign 12 of the K2 mission – from December 2016 to March 2017. After consulting this data, the team noted the presence of three super-Earth sized planets orbiting in a very compact configuration. This system, as they note in their study, was independently and simultaneously discovered by another team from Wesleyan University.

These three planetary objects, designated as GJ 9827 b, c, and d, are located at a distance of about 0.02, 0.04 and 0.06 AU from their host star (respectively). Owing to their sizes and radii, these planets are classified as “Super-Earths”, and have radii of 1.6, 1.2, and 2.1 times the radius of Earth. They are also located very close to their host star, completing orbits within 6.2 days.

The light curve obtained during Campaign 12 of the K2 mission of the GJ 9827 system. Credit: Rodriguez et al., 2017

Specifically, GJ 9827 b measures 1.64 Earth radii, has a mass of up to 4.25 Earth masses, a 1.2 day orbital period, and a temperature of 1,119 K (846 °C; 1555 °F). Meanwhile, GJ 9827 c measures 1.29 Earth radii, has a mass of 2.62 Earth masses, an orbital period of 3.6 days, and a temperature of 774 K (500 °C; 934°F). Lastly, GJ 9827 d measures 2.08 Earth radii, has a mass of 5.3 Earth masses, a 6.2 day period, and a temperature of 648 K (375 °C; 707 °F).

In short, all three planets are very hot, with temperatures that are hot as Venus and Mercury or (in the case of GJ 9827b) is even hotter! Interestingly, these radii and mass estimates place these planets within the transition boundary between terrestrial (i.e. rocky) planets and gas giants. In fact, the team found that GJ 9827 b and c fall in or close to the known gap in radius distribution for planets that are in between these two populations.

In other words, these planets could be rocky or gaseous, and the team won’t know for sure until they can place more accurate constraints on their masses. What’s more, none of these planets are likely to be capable of supporting life, certainly not as we know it! So if you were hoping that this latest find would produce an Earth-analog or potentially habitable planet, you’re sadly mistaken.

Nevertheless, the fact that these planets straddle the radius and mass boundary between terrestrial and gaseous planets – and the fact that this system is the closest planetary system to be identified by the K2 mission – makes the system well-situated for studies designed to probe the interior structure and atmosphere of exoplanets.

Artistic design of the super-Earth orbiting a Sun-like star. Credit: Gabriel Pérez/SMM (IAC)

The reason for this has much to do with the brightness of the host star. In addition to being relatively close to our Sun (~100 light-years), this K-type star is very bright and also relatively small – about 60% the size of our Sun. As a result, any planet passing in front of it would be able to block out more light than if the star were larger. But as noted, there’s also the curious nature of the planets themselves. As Dr. Rodriguez indicated:

“Recently, we have found planets around other stars that have no analogue to a planet in our own system. These are known as “super Earths” and they have radii of 1-3 times the radius of the Earth. To add to the complexity of these planets, their is a clear dichotomy in their composition within this radius range. The larger super Earths (>1.6 x radius of the Earth) appear to be less dense, consistent with a puffy Hydrogen/Helium atmosphere. However, the smaller super Earths are more dense, consistent with an Earth-like composition (rock).

“As mentioned above, the GJ 9827 system hosts three super Earth sized planets. Interestingly, planet c has a radius consistent with it being rocky, planet d is consistent with being puffy, and planet b has a radius that is right on what we believe to be the transition boundary between rock and gas. Therefore, by studying the atmospheres of super-Earths, we may better understand the transition from dense rocky planets to puffier planets with very thick atmospheres (like Neptune).”

Artist’s impression of the super-Earth orbiting closely to its parent star. Credit: ESA/NASA

Looking ahead, the team hopes to conduct further studies to determine the masses of these planets more precisely. From this, they will be able to place better constraints on their compositions and determine if they are Super-Earths, mini gas giants, or some of each. Beyond that, they are to conduct more detailed studies of this system with next-generation instruments like the James Webb Space Telescope (JWST), which is scheduled to launch in 2018.

“I am really interested in studying the atmosphere of GJ 9827 b, whether it is rocky or puffy,” said Dr. Rodriguez. “This planet has a radius at the rock/gas transition but it is very close to its host star. Therefore, by studying the chemical composition of its atmosphere we may better understand the impact of the host star’s proximity has on the evolution of its atmosphere.  To do this we would use JWST to take spectroscopic observations during the transit of GJ 9827b (known as “Transmission Spectroscopy”). From this observations we will gather information on the chemical composition and extent of the planet’s atmosphere.“

Now that we have thousands of extra-solar planet discoveries under our belt, its only natural that research would be shifting towards trying to understand these planets better. In the coming years and decades, we are likely to learn volumes about the respective structures, compositions, atmospheres, and surface features of many distant worlds. One can only imagine what kind of things these studies will turn up!

Further Reading: arXiv

Posted on by Matt Williams

New Study Claims that TRAPPIST-1 Could Also Have Gas Giants

Most exoplanets orbit red dwarf stars because they're the most plentiful stars. This is an artist's illustration of what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credits: NASA/JPL-Caltech
Most exoplanets orbit red dwarf stars because they're the most plentiful stars. This is an artist's illustration of what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credits: NASA/JPL-Caltech

In February of 2017, NASA scientists announced the existence of seven terrestrial (i.e. rocky) planets within the TRAPPIST-1 star system. Since that time, the system has been the focal point of intense research to determine whether or not any of these planets could be habitable. At the same time, astronomers have been wondering if all of the system’s planets are actually accounted for.

For instance, could this system have gas giants lurking in its outer reaches, as many other systems with rocky planets (for instance, ours) do? That was the question that a team of scientists, led by researchers from the Carnegie Institute of Science, sought to address in a recent study. According to their findings, TRAPPIST-1 may be orbited by gas giants at a much-greater distance than its seven rocky planets.

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Posted on by Matt Williams

Ultraviolet Light Could Point the Way To Life Throughout the Universe

Artist's impression of how the surface of a planet orbiting a red dwarf star may appear. The planet is in the habitable zone so liquid water exists. However, low levels of ultraviolet radiation from the star have prevented or severely impeded chemical processes thought to be required for life to emerge. This causes the planet to be devoid of life. Credit: M. Weiss/CfA

Ultraviolet light is what you might call a controversial type of radiation. On the one hand, overexposure can lead to sunburn, an increased risk of skin cancer, and damage to a person’s eyesight and immune system. On the other hand, it also has some tremendous health benefits, which includes promoting stress relief and stimulating the body’s natural production of vitamin D, seratonin, and melanin.

And according to a new study from a team from Harvard University and the Harvard-Smithsonian Center for Astrophysics (CfA), ultraviolet radiation may even have played a critical role in the emergence of life here on Earth. As such, determining how much UV radiation is produced by other types of stars could be one of the keys to finding evidence of life any planets that orbit them.

The study, titled “The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up“, recently appeared in The Astrophysical Journal. Led by Sukrit Ranjan, a visiting postdoctoral researcher at the CfA, the team focused on M-type (red dwarf) stars to determine if this class of star produces enough UV radiation to kick-start the biological processes necessary for life to emerge.

Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO

Recent studies have indicated that UV radiation may be necessary for the formation of ribonucleic acid (RNA), which is necessary for all forms of life as we know it. And given the rate at which rocky planets have been discovered around red dwarf stars of late (exampled include Proxima b, LHS 1140b, and the seven planets of the TRAPPIST-1 system), how much UV radiation red dwarfs give off could be central to determining exoplanet habitability.

As Dr. Ranjan explained in a CfA press release:

“It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match. Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite.”

For the sake of their study, the team created radiative transfer models of red dwarf stars. They then sought to determine if the UV environment on prebiotic Earth-analog planets which orbited them would be sufficient to stimulate the photoprocesses that would lead to the formation of RNA. From this, they calculated that planets orbiting M-dwarf stars would have access to 100–1000 times less bioactive UV radiation than a young Earth.

As a result, the chemistry that depends on UV light to turn chemical elements and prebiotic conditions into biological organisms would likely shut down. Alternately, the team estimated that even if this chemistry was able to proceed under a diminished level of UV radiation, it would operate at a much slower rate than it did on Earth billions of years ago.

Artist’s impression of the planet orbiting a red dwarf star. Credit: ESO/M. Kornmesser

As Robin Wordsworth – an assistant professor at the Harvard School of Engineering and Applied Science and a co-author on the study – explained, this is not necessarily bad news as far as questions of habitability go. “It may be a matter of finding the sweet spot,” he said. “There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet’s atmosphere.”

Previous studies have shown that even calm red dwarfs experience dramatic flares that periodically bombard their planets with bursts UV energy. While this was considered to be something hazardous, which could strip orbiting planets of their atmospheres and irradiate life, it is possible that such flares could compensate for the lower levels of UV being steadily produced by the star.

This news also comes on the heels of a study that indicated how the outer planets of the TRAPPIST-1 system (including the three located within its habitable zone) might still have plenty of water of their surfaces. Here too, the key was UV radiation, where the team responsible for the study monitored the TRAPPIST-1 planets for signs of hydrogen loss from their atmospheres (a sign of photodissociation).

This research also calls to mind a recent study led by Professor Avi Loeb, the Chair of the astronomy department at Harvard University, Director of the Institute for Theory and Computation, and also a member of the CfA. Titled, “Relative Likelihood for Life as a Function of Cosmic Time“, Loeb and his team concluded that red dwarf stars are the most likely to give rise to life because of their low mass and extreme longevity.

Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada

Compared to higher-mass stars that have shorter life spans, red dwarf stars are likely to remain in their main sequence for as long as six to twelve trillion years. Hence, red dwarf stars would certainly be around long enough to accommodate even a vastly decelerated rate of organic evolution. In this respect, this latest study might even be considered a possible resolution for the Fermi Paradox – Where are all the aliens? They’re still evolving!

But as Dimitar Sasselov – the Phillips Professor of Astronomy at Harvard, the Director of the Origins of Life Initiative and a co-author on the paper – indicated, there are still many unanswered questions:

“We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life. Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth.”

As always, scientists are forced to work with a limited frame of reference when it comes to assessing the habitability of other planets. To our knowledge, life exists on only on planet (i.e. Earth), which naturally influences our understanding of where and under what conditions life can thrive. And despite ongoing research, the question of how life emerged on Earth is still something of a mystery.

If life should be found on a planet orbiting a red dwarf, or in extreme environments we thought were uninhabitable, it would suggest that life can emerge and evolve in conditions that are very different from those of Earth. In the coming years, next-generation missions like the James Webb Space Telescope are the Giant Magellan Telescope are expected to reveal more about distant stars and their systems of planets.

The payoff of this research is likely to include new insights into where life can emerge and the conditions under which it can thrive.

Further Reading: CfA, The Astrophysical Journal

Posted on by Matt Williams

Hubble Spots First Indications of Water on TRAPPIST-1s Planets

This artist’s impression shows the view from the surface of one of the planets in the TRAPPIST-1 system. A powerful laser beacon using current and near-future technology could send a signal strong enough to be detected by any alien astronomers here. Credit: NASA/ESA/HST
This artist’s impression shows the view from the surface of one of the planets in the TRAPPIST-1 system. A powerful laser beacon using current and near-future technology could send a signal strong enough to be detected by any alien astronomers here. Credit: NASA/ESA/HST

In February of 2017, astronomers from the European Southern Observatory (ESO) announced the discovery of seven rocky planets around the nearby star of TRAPPIST-1. Not only was this the largest number of Earth-like planets discovered in a single star system to date, the news was also bolstered by the fact that three of these planets were found to orbit within the star’s habitable zone.

Since that time, multiple studies have been conducted to ascertain the likelihood that these planets are actually habitable. Thanks to an international team of scientists who used the Hubble Space Telescope to study the system’s planets, we now have the first clues as to whether or not water (a key ingredient

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Posted on by Matt Williams

Another Nearby Red Dwarf Star System, Another Possible Exoplanet Discovered!

Artist's impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

In the past few years, there has been no shortages of extra-solar planets discoveries which orbit red dwarf stars. In 2016 and 2017 alone,  astronomers announced the discovery of a terrestrial (i.e. rocky) planet around Proxima Centauri (Proxima b), a seven-planet system orbiting TRAPPIST-1, and super-Earths orbiting the nearby stars of LHS 1140 (LHS 1140b), and GJ 625 (GJ 625b).

In what could be the latest discovery, physicists at the University of Texas Arlington (UTA) recently announced the possible discovery of an Earth-like planet orbiting Gliese 832, a red dwarf star just 16 light years away. In the past, astronomers detected two exoplanets orbiting Gliese 832. But after conducting a series of computations, the UTA team indicated that an additional Earth-like planet could be orbiting the star.

The study which details their findings, titled “Dynamics of a Probable Earth-mass Planet in the GJ 832 System“, recently appeared in The Astrophysical Journal. Led by Dr. Suman Satyal – a physics researcher, lecturer and laboratory supervisor at UTA – the team sought to investigate the stability of planetary orbits around Gliese 832 using a numerical and detailed phase-space analysis.

Artistic representation of the potentially habitable exoplanet Gliese 832c as compared with Earth. Credit: PHL/UPR Arecibo.

As indicated, two other exoplanets had been discovered around Gliese 832 in the past, including a Jupiter-like gas giant (Gliese 832b) in 2008, and the super-Earth (Gliese 832c) in 2014. In many ways, these planets could not be more different. In addition to their disparity in mass, they vary widely in terms of their orbits – with Gliese 832b orbiting at a distance of about 0.16 AU and Gliese 832c orbiting at a distance of 3 to 3.8 AU.

Because of this, the UTA team sought to determine if perhaps there was a third planet with a stable orbit between the two. To this end, they conducted numerical simulations for a three and four body system of planets with elliptical orbits around the star. These simulations took into account a large number of initial conditions, which allowed for  all possible states (aka. s phase-space simulation) of the planet’s orbits to be represented.

They then included the radial velocity measurements of Gliese 832, accounting for them based on the presence of planets with 1 to 15 Earth masses. The Radial Velocity (RV) method, it should be noted, determines the existence of planets around a star based on variations in the star’s velocity. In other words, the fact that a star is moving back and forth indicates that it is being influenced by the presence of a planetary system.

Simulating the star’s RV signal using a hypothetical system of planets also allowed the UTA team to constrain the average distances at which these planets would orbit the star (aka. their semi-major axes) and their upper mass-limits. In the end, their results provided strong indications for the existence of a third planet. As Dr. Satyal explained in a UTA press release:

“We also used the integrated data from the time evolution of orbital parameters to generate the synthetic radial velocity curves of the known and the Earth-like planets in the system. We obtained several radial velocity curves for varying masses and distances indicating a possible new middle planet.”

Diagram showing the possible orbit of a third exoplanet around Gliese 832, a star system located just 16 light years away. Credit: uta.edu/Suman Satyal

Based on their computations, this possible planet of the Gliese 832 system would be between 1 and 15 Earth masses and would orbit the star at a distance ranging from 0.25 to 2.0 AU. They also determined that it would likely have a stable orbit for about 1 billion years. As Dr. Satyal indicated, all signs coming from the Gliese 832 system point towards there being a third planet.

“The existence of this possible planet is supported by long-term orbital stability of the system, orbital dynamics and the synthetic radial velocity signal analysis,” he said. “At the same time, a significantly large number of radial velocity observations, transit method studies, as well as direct imaging are still needed to confirm the presence of possible new planets in the Gliese 832 system.”

Alexander Weiss, the UTA Physics Chair, also lauded the achievement, saying:

“This is an important breakthrough demonstrating the possible existence of a potential new planet orbiting a star close to our own. The fact that Dr. Satyal was able to demonstrate that the planet could maintain a stable orbit in the habitable zone of a red dwarf for more than 1 billion years is extremely impressive and demonstrates the world class capabilities of our department’s astrophysics group.”

Artist’s impression of a Super-Earth orbiting close to a red dwarf star. Credit: M. Weiss/CfA

Another interesting tidbit is that this planet’s orbit would place it beyond or just within Gliese 832’s habitable zone. Whereas the Super-Earth Gliese 832c has an eccentric orbit that places it at the inner edge of this zone, this third planet would skirt its outer edge at the nearest. In this sense, Gliese 832’s two Super-Earths could very well be Venus-like and Mars-like in nature.

Looking ahead, Dr. Satyal and his colleagues will be naturally be looking to confirm the existence of this planet, and other institutions are sure to conduct similar studies. This star system is yet another that is sure to be the subject of follow-up studies in the coming years, most likely from next-generation space telescopes like the James Webb Space Telescope.

Further Reading: University of Texas Arlington, The Astrophysical Journal

Posted on by Matt Williams

Potentially Habitable, Tidally-Locked Exoplanets May be Very Common, say New Study

Artist's impression of a system of exoplanets orbiting a low mass, red dwarf star. Credit: NASA/JPL

Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.

The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.

The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth (left side). Credit: Wikipedia

For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.

Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.

Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.

Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.

Tidally-locked, rocky planets are common around low-mass, M-type (red dwarf) stars, due to their close orbits. Credit: M. Weiss/CfA

As he explained in a University of Washington press statement:

“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”

From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.

In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.

For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.

Artist’s impression of a “Earth-like” planet orbiting a nearby red dwarf star. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.

In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.

Prof. Barnes paper was accepted for publication by the journal Celestial Mechanics and Dynamical Astronomy. The research was funded by a NASA grant through the Virtual Planetary Laboratory.

Further Reading: University of Washington, arXiv