Astronomers Find a Planet That Orbits its Star in Just 16 HOURS!

The newly discovered planet, designated TOI-2109b, is relatively close to its star, at a distance of only about 1.5 million miles out. Credits: Image: NASA, ESA and G. Bacon

Mercury is the speed champion in our Solar System. It orbits the Sun every 88 days, and its average speed is 47 km/s. Its average distance from the Sun is 58 million km (36 million mi), and it’s so fast it’s named after Mercury, the wing-footed God.

But what if instead of Mercury, Jupiter was closest to the Sun? And what if Jupiter was even closer to the Sun than Mercury and far hotter?

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Astronomers Measure the Atmosphere on a Planet Hundreds of Light-Years Away

The NASA/ESA Hubble Space Telescope’s data was used to analyze the atmosphere of the super-hot exoplanet WASP-79b, located 780 light-years away in the constellation Eridanus. Among exoplanets, WASP-79b is among the largest ever observed. The planet is larger than Jupiter, and its very deep, hazy atmosphere sizzles at 1,650 degrees Celsius – the temperature of molten glass. The Hubble Space Telescope and other observatories measured how starlight is filtered through the planet's atmosphere, allowing for its chemical composition to be analyzed. Hubble has detected the presence of water vapor. The surprise in recently published results, is that the planet's sky doesn't have any evidence for an atmospheric phenomenon called Rayleigh scattering, where certain colors of light are dispersed by very fine dust particles in the upper atmosphere. Rayleigh scattering is what makes Earth's skies blue by scattering the shorter (bluer) wavelengths of sunlight. Because WASP-79b doesn't seem to have this phenomenon, the daytime sky would likely be yellowish, researchers say. Link: NASA Press Release

The field of extrasolar planet research has advanced by leaps and bounds over the past fifteen years. To date, astronomers have relied on space-based and ground-based telescopes to confirm the existence of 4,566 exoplanets in 3,385 systems, with another 7,913 candidates awaiting confirmation. More importantly, in the past few years, the focus of exoplanet studies has slowly shifted from the process of discovery towards characterization.

In particular, astronomers are making great strides when it comes to the characterization of exoplanet atmospheres. Using the Gemini South Telescope (GST) in Chile, an international team led by Arizona State University (ASU) was able to characterize the atmosphere of a “hot Jupiter” located 340 light-years away. This makes them the first team to directly measure the chemical composition of a distant exoplanet’s atmosphere, a significant milestone in the hunt for habitable planets beyond our Solar System.

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In About 3 Million Years, WASP-12b Will Spiral into its Star and be Consumed

Artist's impression of the searing-hot gas planet WASP-12b and its star. A Princeton-led team of astrophysicists has shown that this exoplanet is spiraling in toward its host star, heading toward certain destruction in about 3 million years. Credit: NASA/JPL-Caltech

Astronomers estimate that in about four billion years, our Sun will exit the main sequence phase of its existence and become a red giant. This will consist of the Sun running out of hydrogen and expanding to several times its current size. This will cause Earth to become uninhabitable since this Red Giant Sun will either blow away Earth’s atmosphere (rendering the surface uninhabitable) or expand to consume Earth entirely.

In a lot of ways, Earth is getting off easy with these predicted scenarios. Other planets, such as WASP-12b, don’t have the luxury of waiting billions of years for their star to reach the end of its lifespan before eating them up. According to a recent study by a team of Princeton-led astrophysicists, this extrasolar planet is spiraling in towards its star and will be consumed in a fiery death just 3 million years from now.

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Exoplanet Orbits its Star Every 18 Hours. The Quickest Hot-Jupiter Ever Found

Using data obtained by Kepler and numerous observatories around the world, an international team has found a Super-Earth that orbits its orange dwarf star in just 14 hours. Credit: M. Weiss/CfA

In the past decade, thousands of planets have been discovered beyond our Solar System. These planets have provided astronomers with the opportunity to study planetary systems that have defied our preconcieved notions. This includes particularly massive gas giants that are many times the size of Jupiter (aka. “super-Jupiters”). And then there are those that orbit particularly close to their suns, otherwise known as “hot-Jupiters”.

Conventional wisdom indicates that gas giants should exist far from their suns and have long orbital periods that can last for a decade or longer. However, in a recent study, an international team of astronomers announced the detection of a “hot-Jupiter” with the shortest orbital period to date. Located 1,060 light-years away from Earth, this planet (NGTS-10b) takes just 18 hours to complete a full orbit of its sun.

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“Monster Planet” Discovered, Makes Scientists Rethink Theories of Planetary Formation

Artist’s impression of the cool red star and gas-giant planet NGTS-1b against the Milky Way. Credit: University of Warwick/Mark Garlick.

When it comes to how and where planetary systems form, astronomers thought they had a pretty good handle on things. The predominant theory, known as the Nebular Hypothesis, states that stars and planets form from massive clouds of dust and gas (i.e. nebulae). Once this cloud experiences gravitational collapse at the center, its remaining dust and gas forms a protoplanetary disk that eventually accretes to form planets.

However, when studying the distant star NGTS-1 – an M-type (red dwarf) located about 600 light-years away – an international team led by astronomers from the University of Warwick discovered a massive “hot Jupiter” that appeared far too large to be orbiting such a small star. The discovery of this “monster planet” has naturally challenged some previously-held notions about planetary formation.

The study, titled “NGTS-1b: A hot Jupiter transiting an M-dwarf“, recently appeared in the Monthly Notices of the Royal Astronomical Society. The team was led by Dr Daniel Bayliss and Professor Peter Wheatley from the University of Warwick and included members from the of the Geneva Observatory, the Cavendish Laboratory, the German Aerospace Center, the Leicester Institute of Space and Earth Observation, the TU Berlin Center for Astronomy and Astrophysics, and multiple universities and research institutes.

Artist’s impression of the cool red star above NGTS-1b. Credit: University of Warwick/Mark Garlick.

The discovery was made using data obtained by the ESO’s Next-Generation Transit Survey (NGTS) facility, which is located at the Paranal Observatory in Chile. This facility is run by an international consortium of astronomers who come from the Universities of Warwick, Leicester, Cambridge, Queen’s University Belfast, the Geneva Observatory, the German Aerospace Center, and the University of Chile.

Using a full array of fully-robotic compact telescopes, this photometric survey is one of several projects meant to compliment the Kepler Space Telescope. Like Kepler, it monitors distant stars for signs of sudden dips in brightness, which are an indication of a planet passing in front of (aka. “transiting”) the star, relative to the observer.  When examining data obtained from NGTS-1, the first star to be found by the survey, they made a surprising discovery.

Based on the signal produced by its exoplanet (NGTS-1b), they determined that it was a gas giant roughly the same size as Jupiter and almost as massive (0.812 Jupiter masses). Its orbital period of 2.6 days also indicated that it orbits very close to its star – about 0.0326 AU – which makes it a “hot Jupiter”. Based on these parameters, the team also estimated that NGTS-1b experiences temperatures of approximately 800 K (530°C; 986 °F).

The discovery threw the team for a loop, as it was believed to be impossible for planets of this size to form around small, M-type stars. In accordance with current theories about planet formation, red dwarf stars are believed to be able to form rocky planets – as evidenced by the many that have been discovered around red dwarfs of late – but are unable to gather enough material to create Jupiter-sized planets.

Artist’s concept of Jupiter-sized exoplanet that orbits relatively close to its star (aka. a “hot Jupiter”). Credit: NASA/JPL-Caltech)

As Dr. Daniel Bayliss, an astronomer with the University of Geneva and the lead-author on the paper, commented in University of Warwick press release:

“The discovery of NGTS-1b was a complete surprise to us – such massive planets were not thought to exist around such small stars. This is the first exoplanet we have found with our new NGTS facility and we are already challenging the received wisdom of how planets form. Our challenge is to now find out how common these types of planets are in the Galaxy, and with the new NGTS facility we are well-placed to do just that.”

What is also impressive is the fact that the astronomers noticed the transit at all. Compared to other classes of stars, M-type stars are the smallest, coolest and dimmest. In the past, rocky bodies have been detected around them by measuring shifts in their position relative to Earth (aka. the Radial Velocity Method). These shifts are caused by the gravitational tug of one or more planets that cause the planet to “wobble” back and forth.

In short, the low light of an M-type star has made monitoring them for dips in brightness (aka. the Transit Method) highly impractical. However, using the NGTS’s red-sensitive cameras, the team was able to monitored patches of the night sky for many months. Over time, they noticed dips coming from NGTS-1 every 2.6 days, which indicated that a planet with a short orbital period was periodically passing in front of it.

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

They then tracked the planet’s orbit around the star and combined the transit data with Radial Velocity measurements to determine its size, position and mass. As Professor Peter Wheatley (who leads NGTS) indicated, finding the planet was painstaking work. But in the end, its discovery could lead to the detection of many more gas giants around low-mass stars:

“NGTS-1b was difficult to find, despite being a monster of a planet, because its parent star is small and faint. Small stars are actually the most common in the universe, so it is possible that there are many of these giant planets waiting to found. Having worked for almost a decade to develop the NGTS telescope array, it is thrilling to see it picking out new and unexpected types of planets. I’m looking forward to seeing what other kinds of exciting new planets we can turn up.”

Within the known Universe, M-type stars are by far the most common, accounting for 75% of all stars in the Milky Way Galaxy alone. In the past, the discovery of rocky bodies around stars like Proxima Centauri, LHS 1140, GJ 625, and the seven rocky planets around TRAPPIST-1, led many in the astronomical community to conclude that red dwarf stars were the best place to look for Earth-like planets.

The discovery of a Hot Jupiter orbiting NGTS-1 is therefore seen as an indication that other red dwarf stars could have orbiting gas giants as well. Above all, this latest find once again demonstrates the importance of exoplanet research. With every find we make beyond our Solar System, the more we learn about the ways in which planets form and evolve.

Every discovery we make also advances our understanding of how likely we may be to discover life out there somewhere. For in the end, what greater scientific goal is there than determining whether or not we are alone in the Universe?

Further Reading: UofWarwick, RAS, MNRAS

Exoplanet-Hunting Survey Discovers Three More Giant Alien Worlds!

Artist's conception of a gas giant orbiting close to its star. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

The discovery of extra-solar planets has certainly heated up in the past few years. With the deployment of the Kepler mission in 2009, several thousands of exoplanet candidates have been discovered and over 2,500 have been confirmed. In many cases, these planets have been gas giants orbiting close to their respective stars (aka. “Hot Jupiters”), which has confounded some commonly-held notions of how and where planets form.

Beyond these massive planets, astronomers also discovered a wide range of planets that range from massive terrestrial planets (“Super-Earths) to Neptune-sized giants. In a recent study, an international team astronomers discovered three new exoplanets orbiting three different stars. These planets are an interesting batch of finds, consisting of two “Hot Saturns” and one Super-Neptune.

This study, titled “The discovery of WASP-151b, WASP-153b, WASP-156b: Insights on giant planet migration and the upper boundary of the Neptunian desert“, recently appeared in the scientific journal Astronomy and Astrophysics. Led by Olivier. D. S. Demangeon, a researcher from the Institute of Astrophysics and Space Science in Portugal, the team used data from the SuperWASP exoplanet-hunting survey to detect signs of three new gas giants.

Artist’s concdption of a Neptune-sized planet with a clear atmosphere, passing across the face of its star. Credit: NASA/JPL-Caltech

The Super Wide Angle Search for Planets (SuperWASP) is an international consortium that uses wide-angle Transit Photometry to monitor the night sky for transit events. The program relies on robotic observatories located on two continents – SuperWASP-North, located at the Roque de los Muchachos Observatory in Canary Island; and SuperWASP South, at the South African Astronomical Observatory, near Sutherland, South Africa.

From the SuperWASP survey data, Dr. Demangeon and her colleagues were able to detect three transit signals coming from three distant stars – WASP-151, WASP-153 and WASP-156. This was then followed by spectroscopic observations performed using the Haute-Provence Observatory in France and the La Silla Observatory in Chile, which allowed the team to confirm the nature of these planets.

From this, they determined that WASP-151b and WASP-153b are two “hot Saturns”, meaning they are low-density gas giants with close orbits. They orbit their respective suns, which are both early G-type stars (aka. yellow dwarfs, like our Sun), with an orbital period of 4.53 and 3.33 days. WASP-156b, meanwhile, is a Super-Neptune that orbits a K-type (orange dwarf) star. As they indicated in their study:

“WASP-151b and WASP-153b are relatively similar. Their masses of 0.31 and 0.39 M Jup and semi-major axes of 0.056 AU and 0.048 AU respectively indicate two Saturn-size objects around early G type stars of V magnitude ~ 12.8. WASP-156b’s radius of 0.51R Jup suggests a Super-Neptune and makes it the smallest planet ever detected by WASP. Its mass of 0.128 M Jup is also the 3rd lightest detected by WASP after WASP-139b and WASP-107b. Also interesting is the fact that WASP-156 is a bright (magV = 11.6) K type star.”

Number of exoplanets discovered by the Kepler mission as of May 10th, 2016, based on their classification. Credit: W. Stenzel/NASA Ames

Taken together, these planets represent some major opportunities for exoplanet research. As they indicate, “these three planets also lie close to (WASP-151b and WASP-153b) or below (WASP-156b) the upper boundary of the Neptunian desert.” This refers to the boundary astronomers have observed around stars where shot period Neptune-size planets are very unlikely to be found.

Basically, of all  the short period exoplanets (less than 10 days) to be discovered so far, the majority have tended to be in the “Super-Earth” or “Super-Jupiter” category. This deficit of Neptune-like planets has been attributed to different mechanisms when it comes to the formation and evolution for hot Jupiters and short-period super-Earths, as well as it being the result of gas envelop-depletion caused by a star’s ultraviolet radiation.

So far, only nine “Super-Neptunes” have been discovered; so this latest discovery (who’s characteristics are well know) should provide plenty of opportunities for research. Or as Dr. Demangeon and her colleagues explain in the study:

“WASP-156b, being one of the few well characterised Super-Neptunes, will help to constrain the formation of Neptune size planets and the transition between gas and ice giants. The estimates of the age of these three stars confirms the tendency for some stars to have gyrochronological ages significantly lower than their isochronal ages.”

Artist’s impression of two super-Earths in the same system as a Neptune-sized exoplanet in the Kepler-62 system. Credit: David A. Aguilar (CfA)

The team also offered some possible explanations for the existence of a “Neptunian desert” based on their findings. For starters, they proposed that a high-eccentricity migration could be responsible, where Neptune-sized ice giants form in the outer reaches of a star system and migrate inward over time. They also indicate that their discovery offers compelling evidence that ultra-violet radiation and gas envelope-depletion could be a key part of the puzzle.

But of course, Dr. Demangeon and her colleagues indicate that further research will be necessary to confirm their hypothesis, and that further studies are needed to properly constrain the boundaries of the so-called “Neptunian desert”. They also indicate that future missions like NASA’s Transiting Exoplanet Survey Satellite and the ESA’s PLAnetary Transits and Oscillations of stars (PLATO) mission  will be vital to these efforts.

“Obviously, a more thorough analysis is necessary to investigate all the possible implications behind this hypothesis,” they conclude. “Such an analysis is out of the scope of this paper but we think that this hypothesis is worth investigating. In this context, a search for long period companions that might have triggered the high eccentricity migration or an independent age estimate through asterosiesmology with TESS or Plato would be particularly interesting.”

The sheer number of exoplanets discoveries made in recent decades has allowed astronomers to test and revise commonly-held theories about how planetary systems form and evolve. These same discoveries have also helped advance our understanding of how our own Solar System came to be. In the end, being able to study a diverse array of planetary systems, which are different stages in their history, is allowing us to create a sort of timeline for cosmic evolution.

Further Reading: Astronomy and Astrophysics

Hubble Spots Pitch Black Hot Jupiter that “Eats Light”

Illustration showing one of the darkest known exoplanets - an alien world 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. 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

Exoplanet-Hunters Detect Two New “Warm Jupiters”

Artist's concept of Jupiter-sized exoplanet that orbits relatively close to its star (aka. a "hot Jupiter"). Credit: NASA/JPL-Caltech)
Artist's concept of Jupiter-sized exoplanet that orbits relatively close to its star (aka. a "hot Jupiter"). Credit: NASA/JPL-Caltech)

The study of extra-solar planets has turned up some rather interesting candidates in the past few years. As of August 1st, 2017, a total of 3,639 exoplanets have been discovered in 2,729 planetary systems and 612 multiple planetary systems. Many of these discoveries have challenged conventional thinking about planets, especially where their sizes and distances from their suns are concerned.

According to a study by an international team of astronomers, the latest exoplanet discoveries are in keeping with this trend. Known as EPIC 211418729b and EPIC 211442297b, these two gas giants orbit stars that are located about 1569 and 1360 light-years from Earth (respectively) and are similar in size to Jupiter. Combined with their relatively close orbit to their stars, the team has designated them as “Warm Jupiters”.

The study, titled “EPIC 211418729b and EPIC 211442297b: Two Transiting Warm Jupiters“, recently appeared online. Led by Avi Shporer – a postdoctoral scholar with the Geological and Planetary Sciences (GPS) division at the California Institute of Technology (Caltech) – the team relied on data from the Kepler and K2 missions, and follow-up observations with multiple ground-based telescopes, to determine the sizes, masses and orbits of these planets.

Simulation of the turbulent atmosphere of a hot, gaseous planet, based on data from NASA’s Spitzer Space Telescope. Credits: NASA/JPL-Caltech/MIT/Principia College

As they indicate in their study, the two planets were initially identified as transiting planet candidates by the K2 mission. In other words, they were initially detected through the transit method, where astronomers measure dips in a star brightness to confirm that a planet is passing between the observer and the star. These observations took place during K2‘s Campaign 5 observations, which took place between April 27th and July 10th, 2015.

The team then conducted follow-up observations using the Keck II telescope (located at the W.M. Keck Observatory in Hawaii) and the Gemini North Telescope (at the Gemini Observatory, also in Hawaii). These observations, conducted from January 2016 to May 2017, were then combined with spectral data and radial velocity measurements from the High Resolution Echelle Spectrometer (HIRES) the on the Keck I telescope.

Finally, they added photometric data from the Cerro Tololo Inter-American Observatory (CTIO) in Chile, the South African Astronomical Observatory (SAAO), and the Siding Spring Observatory (SSO) in Australia. These follow-up observations confirmed the presence of these two exoplanets. As they wrote in the study:

“We have discovered two transiting warm Jupiter exoplanets initially identified as transiting candidates in K2 photometryBoth planets are among the longest period transiting gas giant planets with a measured mass, and they are orbiting relatively old host stars. Both planets are not inflated as their radii are consistent with theoretical expectations.”

The transit light curve of EPIC 211418729b. Credit: Shporer (et al.)

From their observations, the team was also able to produce estimates on the planets respective sizes, masses and orbital periods. Whereas EPIC 211418729 b measures 0.942 Jupiter radii, has approximately 1.85 Jupiter masses and orbital period of 11.4 days, EPIC 211442297 b measures 1.115 Jupiter radii, has approximately 0.84 Jupiter masses and an orbital period of 20.3 days.

Based on their estimates, these planets experience surface temperatures of up to 719 K (445.85 °C; 834.5 °F) and 682 K (408.85°C; 768 °F), respectively. As such, they classified these planets as “Warm Jupiters”, since they fall short of what is considered typical for “Hot Jupiters” – which have exotic atmosphere’s that experience temperatures as high as several thousand kelvin.

The researchers noted that based on their orbital periods, these two planets have some of the longest orbital periods of any transiting gas giant (i.e. those that have been detected using the transit method) detected to date. Or as they state in their study:

“Both EPIC 211418729b and EPIC 211442297b are among the longest period transiting gas giant planets with a measured mass. In fact, according to the NASA Exoplanet Archive (Akeson et al. 2013) EPIC 211442297b is currently the longest period K2 transiting exoplanet with a well constrained mass.”

Artist’s conception of a “Hot Jupiter” orbiting close to its star. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Another interesting observation was the fact that neither of these exoplanets were inflated, which is something they did not anticipate. In the case of Hot Jupiters, the atmospheres undergo expansion as a result of the amount of solar irradiation they receive, resulting in what the team refers to as a “radius-irradiation correlation” in their paper. In other words, Hot Jupiters are massive, but are also known to have low densities compared to cooler gas giants.

Instead, the team found that both EPIC 211418729b and EPIC 211442297b had radii that were consistent with what theoretical models predict for gas giants of their mass. Their results also led them to make some tentative conclusions about the planets’ structures and compositions. As they wrote:

“Both planets are not inflated compared to theoretical expectations, unlike many other planets in the diagram. Their positions are close to or consistent with theoretical expectations for a planet with little to no rocky core, for EPIC 211442297b, and a planet with a significant rocky core for EPIC 211418729b.”

These results suggest that solar irradiation does not play a significant role in determining the radius of Warm Jupiters. It also raises some interesting questions about the correlation between radii and irradiation with other gas giants. In the future, EPIC 211418729b and EPIC 211442297b will be targets of future K2 observations during the mission’s Campaign 18 – which will run from May to August 2018.

These observations are sure to offer some additional insight into these planets and the mysteries this study has raised. Future surveys of transiting exoplanets – conducting by next-generation instruments like the Transiting Exoplanet Survey Satellites (TESS) – and direct-imaging surveys conducted by the James Webb Space Telescope (JWST) are sure to reveal even more about distant, exotic exoplanets.

Further Reading: arXiv

What are Gas Giants?

The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute

Between the planets of the inner and outer Solar System, there are some stark differences. The planets that resides closer to the Sun are terrestrial (i.e. rocky) in nature, meaning that they are composed of silicate minerals and metals. Beyond the Asteroid Belt, however, the planets are predominantly composed of gases, and are much larger than their terrestrial peers.

This is why astronomers use the term “gas giants” when referring to the planets of the outer Solar System. The more we’ve come to know about these four planets, the more we’ve come to understand that no two gas giants are exactly alike. In addition, ongoing studies of planets beyond our Solar System (aka. “extra-solar planets“) has shown that there are many types of gas giants that do not conform to Solar examples. So what exactly is a “gas giant”?

Definition and Classification:

By definition, a gas giant is a planet that is primarily composed of hydrogen and helium. The name was originally coined in 1952 by James Blish, a science fiction writer who used the term to refer to all giant planets. In truth, the term is something of a misnomer, since these elements largely take a liquid and solid form within a gas giant, as a result of the extreme pressure conditions that exist within the interior.

The four gas giants of the Solar System (from right to left): Jupiter, Saturn, Uranus and Neptune. Credit: NASA/JPL

What’s more, gas giants are also thought to have large concentrations of metal and silicate material in their cores. Nevertheless, the term has remained in popular usage for decades and refers to all planets  – be they Solar or extra-solar in nature – that are composed mainly of gases. It is also in keeping with the practice of planetary scientists, who use a shorthand – i.e. “rock”, “gas”, and “ice” – to classify planets based on the most common element within them.

Hence the difference between Jupiter and Saturn on the one and, and Uranus and Neptune on the other. Due to the high concentrations of volatiles (such as water, methane and ammonia) within the latter two – which planetary scientists classify as “ices” – these two giant planets are often called “ice giants”. But since they are composed mainly of hydrogen and helium, they are still considered gas giants alongside Jupiter and Saturn.


Today, Gas giants are divided into five classes, based on the classification scheme proposed by David Sudarki (et al.) in a 2000 study. Titled “Albedo and Reflection Spectra of Extrasolar Giant Planets“, Sudarsky and his colleagues designated five different types of gas giant based on their appearances and albedo, and how this is affected by their respective distances from their star.

Class I: Ammonia Clouds – this class applies to gas giants whose appearances are dominated by ammonia clouds, and which are found in the outer regions of a planetary system. In other words, it applies only to planets that are beyond the “Frost Line”, the distance in a solar nebula from the central protostar where volatile compounds – i.e. water, ammonia, methane, carbon dioxide, carbon monoxide – condense into solid ice grains.

These cutaways illustrate interior models of the giant planets. Jupiter is shown with a rocky core overlaid by a deep layer of metallic hydrogen. Credit: NASA/JPL

Class II: Water Clouds – this applies to planets that have average temperatures typically below 250 K (-23 °C; -9 °F), and are therefore too warm to form ammonia clouds. Instead, these gas giants have clouds that are formed from condensed water vapor. Since water is more reflective than ammonia, Class II gas giants have higher albedos.

Class III: Cloudless – this class applies to gas giants that are generally warmer – 350 K (80 °C; 170 °F) to 800 K ( 530 °C; 980 °F) – and do not form cloud cover because they lack the necessary chemicals. These planets have low albedos since they do not reflect as much light into space. These bodies would also appear like clear blue globes because of the way methane in their atmospheres absorbs light (like Uranus and Neptune).

Class IV: Alkali Metals – this class of planets experience temperatures in excess of 900 K (627 °C; 1160 °F), at which point Carbon Monoxide becomes the dominant carbon-carrying molecule in their atmospheres (rather than methane). The abundance of alkali metals also increases substantially, and cloud decks of silicates and metals form deep in their atmospheres. Planets belonging to Class IV and V are referred to as “Hot Jupiters”.

Class V: Silicate Clouds – this applies to the hottest of gas giants, with temperatures above 1400 K (1100 °C; 2100 °F), or cooler planets with lower gravity than Jupiter. For these gas giants, the silicate and iron cloud decks are believed to be high up in the atmosphere. In the case of the former, such gas giants are likely to glow red from thermal radiation and reflected light.

Artist’s concept of “hot Jupiter” exoplanet, a gas giant that orbits very close to its star. Credit: NASA/JPL-Caltech)


The study of exoplanets has also revealed a wealth of other types of gas giants that are more massive than the Solar counterparts (aka. Super-Jupiters) as well as many that are comparable in size. Other discoveries have been a fraction of the size of their solar counterparts, while some have been so massive that they are just shy of becoming a star. However, given their distance from Earth, their spectra and albedo have cannot always be accurately measured.

As such, exoplanet-hunters tend to designate extra-solar gas giants based on their apparent sizes and distances from their stars. In the case of the former, they are often referred to as “Super-Jupiters”, Jupiter-sized, and Neptune-sized. To date, these types of exoplanet account for the majority of discoveries made by Kepler and other missions, since their larger sizes and greater distances from their stars makes them the easiest to detect.

In terms of their respective distances from their sun, exoplanet-hunters divide extra-solar gas giants into two categories: “cold gas giants” and “hot Jupiters”. Typically, cold hydrogen-rich gas giants are more massive than Jupiter but less than about 1.6 Jupiter masses, and will only be slightly larger in volume than Jupiter. For masses above this, gravity will cause the planets to shrink.

Exoplanet surveys have also turned up a class of planet known as “gas dwarfs”, which applies to hydrogen planets that are not as large as the gas giants of the Solar System. These stars have been observed to orbit close to their respective stars, causing them to lose atmospheric mass faster than planets that orbit at greater distances.

For gas giants that occupy the mass range between 13 to 75-80 Jupiter masses, the term “brown dwarf” is used. This designation is reserved for the largest of planetary/substellar objects; in other words, objects that are incredibly large, but not quite massive enough to undergo nuclear fusion in their core and become a star. Below this range are sub-brown dwarfs, while anything above are known as the lightest red dwarf (M9 V) stars.

An artist’s conception of a T-type brown dwarf. Credit: Tyrogthekreeper/Wikimedia Commons

Like all things astronomical in nature, gas giants are diverse, complex, and immensely fascinating. Between missions that seek to examine the gas giants of our Solar System directly to increasingly sophisticated surveys of distant planets, our knowledge of these mysterious objects continues to grow. And with that, so is our understanding of how star systems form and evolve.

We have written many interesting articles about gas giants here at Universe Today. Here’s The Planet Jupiter, The Planet Saturn, The Planet Uranus, The Planet Neptune, What are the Jovian Planets?, What are the Outer Planets of the Solar System?, What’s Inside a Gas Giant?, and Which Planets Have Rings?

For more information, check out NASA’s Solar System Exploration.

Astronomy Cast also has some great episodes on the subject. Here’s Episode 56: Jupiter to get you started!


Earth-Sized Planet Takes Just Four Hours to Orbit its Star

Using data obtained by Kepler and numerous observatories around the world, an international team has found a Super-Earth that orbits its orange dwarf star in just 14 hours. Credit: M. Weiss/CfA

The Kepler space observatory has made some interesting finds since it began its mission back in March of 2009. Even after the mission suffered the loss of two reaction wheels, it has continued to make discoveries as part of its K2 mission. All told, the Kepler and K2 missions have detected a total of 5,106 planetary candidates, and confirmed the existence of 2,493 planets.

One of the latest finds made using Kepler is EPIC 228813918 b, a terrestrial (i.e. rocky) planet that orbits a red dwarf star some 264 to 355 light years from Earth. This discovery raises some interesting questions, as it is the second time that a planet with an ultra-short orbital period – it completes a single orbit in just 4 hours and 20 minutes – has been found orbiting a red dwarf star.

The study, which was recently published online, was conducted by an international team of scientists who hail from institutions ranging from the Massachusetts Institute of Technology (MIT), the California Institute of Technology (Caltech), the Tokyo Institute of Technology, and the Institute of Astrophysics of the Canary Islands (IAC) to observatories and universities from all around the world.

NASA’s Kepler space telescope was the first agency mission capable of detecting Earth-size planets. Credit: NASA/Wendy Stenzel

As the team indicated in their study, the detection of this exoplanet was made thanks to data collected by numerous instruments. This included spectrographic data from the 8.2-m Subaru telescope and the 10-m Keck I telescope (both of which are located on Mauna Kea, Hawaii) and the Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory in La Palma, Spain.

This was combined with speckle imaging from the 3.5-m WIYN telescope at the Kitt Peak National Observatory in Arizona, photometry from the NASA’s K2 mission, and archival information of the star that goes back over 60 years. After eliminating any other possible explanations – such as an eclipsing binary (EB) – they not only confirmed the orbital period of the planet, but also provided constrains on its mass and size. As they wrote:

“Using a combination of archival images, AO imaging, RV measurements, and light curve modelling, we show that no plausible eclipsing binary scenario can explain the K2 light curve, and thus confirm the planetary nature of the system. The planet, whose radius we determine to be 0.89 ± 0.09 [Earth radii], and which must have a iron mass fraction greater than 0.45, orbits a star of mass 0.463 ± 0.052 M and radius 0.442 ± 0.044 R.”
This orbital period – four hours and 20 minutes – is the second shortest of any exoplanet discovered to date, being just 4 minutes longer than that of KOI 1843.03, which also orbits an M-type (red dwarf) star. It is also the latest in a long line of recently-discovered exoplanets that complete a single orbit of their stars in less than a day. Planets belonging to this group are known as ultra-short-period (USP) planets, of which Kepler has found a total of 106.

Archival images of the star EPIC 228813918, demonstrating its proper motion over nearly six decades – from (i) 1954, (ii) 1992, and (iii) 2012. Credit: Smith et al.

However, what is perhaps most surprising about this find is just how massive it is. Though they didn’t measure the planet’s mass directly, their constraints indicate that the exoplanet has an upper mass limit of 0.7 Jupiter masses – which works out to over 222 Earth masses. And yet, the planet manages to pack this gas giant-like mass into a radius that is 0.80 to 0.98 times that of Earth.

The reason for this, they indicate, has to do with the planet’s apparent composition, which is particularly metal-rich:

“This leads to a constraint on the composition, assuming an iron core and a silicate mantle. We determine the minimum iron mass fraction to be 0.525 ± 0.075 (cf. 0.7 for KOI 1843.03), which is greater than that of Earth, Venus or Mars, but smaller than that of Mercury (approximately 0.38, 0.35, 0.26, and 0.68, respectively; Reynolds & Summers 1969).”

Ultimately, the discovery of this planet is significant for a number of reasons. On the one hand, the team indicated that the constraints their study placed on the planet’s composition could prove useful in helping to understand how our own Solar planets came to be.

“Discovering and characterizing extreme systems, such as USP planets like EPIC 228813918 b, is important as they offer constraints for planet formation theories,” they conclude. “Furthermore, they allow us to begin to constrain their interior structure – and potentially that of longer-period planets too, if they are shown to be a single population of objects.”

An artist’s depiction of extra-solar planets transiting an M-type (red dwarf) star. Credit: NASA/ESA/STScl

On the other hand, the study raises some interesting questions about USP planets – for instance, why the two shortest-period planets were both found orbiting red dwarf stars. A possible explanations, they claim, is that short-period planets could have longer lifetimes around M-dwarfs since their orbital decay would likely be much slower. However, they are quick to caution against making any tentative conclusions before more research is conducted.

In the future, the team hopes to conduct measurements of the planet’s mass using the radial velocity method. This would likely involve a next-generation high-resolution spectrograph, like the Infrared Doppler (IFD) instrument or the CARMENES instrument – which are currently being built for the Subaru Telescope and the Calar Alto Observatory (respectively) to assist in the hunt for exoplanets around red dwarf stars.

One thing is clear though. This latest find is just another indication that red dwarf stars are where exoplanet-hunters will need to be focusing their efforts in the coming years and decades. These low mass, ultra-cool and low-luminosity stars are where some of the most interesting and extreme finds are being made. And what we stand to learn by studying them promises to be most profound!

Further Reading: arXiv