Exoplanet-Hunters Detect Two New “Warm Jupiters”

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

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

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

We Have More Details on the Outermost Trappist-1 Planet!

The announcement of a seven-planet system around the star TRAPPIST-1 earlier this year set off a flurry of scientific interest. Not only was this one of the largest batches of planets to be discovered around a single star, the fact that all seven were shown to be terrestrial (rocky) in nature was highly encouraging. Even more encouraging was the fact that three of these planets were found to be orbiting with the star’s habitable zone.

Since that time, astronomers have been seeking to learn all they can about this system of planets. Aside from whether or not they have atmospheres, astronomers are also looking to learn more about their orbits and surface conditions. Thanks to the efforts of a University of Washington-led international team of astronomers, we now have an accurate idea of what conditions might be like on its outermost planet – TRAPPIST-1h.

According to the team’s study – “A seven-planet resonant chain in TRAPPIST-1“, which was recently published in the journal Nature Astronomy – they relied on data from the Kepler mission to determine the planet’s orbital period. Specifically, they consulted data obtained during Campaign 12 of the K2 mission, a 79-day observation period that ran from December 15th, 2016 to March 4th, 2017.

This artist’s concept shows what each of the TRAPPIST-1 planets may look like, based on available data about their sizes, masses and orbital distances. Credits: NASA/JPL-Caltech

Led by Rodrigo Luger, a graduate student at the University of Washington, the team was already aware of pattern in the orbits of the system’s six inner planets. This was based on prior data provided by the Spitzer Space Telescope, which indicated that these planets are in an orbital resonance – i.e. their respective orbital periods are mathematically related and influence one other.

From this data, the team had already calculated that TRAPPIST-1h would have an orbital period of just less than 19 days. Once they consulted the K2 data, they noticed that during the 79-day observation period, TRAPPIST-1h made four transit of the star – which worked out to an orbital period of 18.77 days. In other words, the team found that their observations were consistent with their calculations.

This finding was a welcome relief to Luger and his colleagues. As he stated in a UW press release:

“TRAPPIST-1h was exactly where our team predicted it to be. It had me worried for a while that we were seeing what we wanted to see. Things are almost never exactly as you expect in this field – there are usually surprises around every corner, but theory and observation matched perfectly in this case.”

The discovery of this resonance means that TRAPPIST-1 has set another record. For starters, it is already renowned from being one of only two star systems to host seven extra-solar planets – the other being the HR 8832 star system, a main-sequence K3V-type variable star located 21 light years away. Second, it has the most confirmed terrestrial planets to be discovered in a single star system to date.

Three of the TRAPPIST-1 planets – TRAPPIST-1e, f and g – dwell in their star’s so-called “habitable zone. CreditL NASA/JPL

But with this latest data, TRAPPIST-1 now holds the record for having the most planets in an orbital resonance as well. The previous place holders were Kepler-80 and Kepler-223, both of which have four planets in an orbital resonance. According to Luger, this resonance was likely established when the TRAPPIST-1 system was still young and the planets were still in the process of formation. As Luger explained:

“The resonant structure is no coincidence, and points to an interesting dynamical history in which the planets likely migrated inward in lock-step. This makes the system a great testbed for planet formation and migration theories. We could therefore be looking at a planet that was once habitable and has since frozen over, which is amazing to contemplate and great for follow-up studies.”

The possibility that the planets achieved their current orbital dance early in the system’s history could also mean that TRAPPIST-1h was once habitable. While three planets orbit with the star’s habitable zone (TRAPPIST-1 d, e, and f), TRAPPIST-1h orbits the star at a distance of about 10 million km (6 million mi), which places it well beyond the reach of the star’s habitable zone.

In fact, at this distance, TRAPPIST-1h gets about as much energy from the Sun as the dwarf planet Ceres (located in our Solar System in Main Asteroid Belt, between Mars and Jupiter), which results in an average surface temperature of 173 K (-100 °C; -148 °F). But in the past, when its star was brighter and hotter, the planet may have received enough energy that its surface would have been warm enough to support liquid water.

Artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. Credit: NASA/JPL

“We could therefore be looking at a planet that was once habitable and has since frozen over, which is amazing to contemplate and great for follow-up studies,” said Luger. TRAPPIST-1 is also a prime candidate for follow-up study given its proximity. Located just 39.5 light years from Earth, this star and its system of planets present some exceptional opportunities for the study of exoplanets and M-type star habitability.

Beyond that, this study also demonstrated that despite the failure of two reaction wheels, the Kepler mission is still extremely useful when it comes to the study of exoplanets. Despite the fact that maintaining a steady eye on the TRAPPIST-1 system presented instrumental challenges, Kepler still managed to produce reliable information that was consistent with the team’s calculations.

Besides determining TRAPPIST-1h’s orbital period, the team used the K2 data to further characterize the orbits of the other six planets, rule out the possibility of there being more planets in the system, and learn more about the star itself (such as its rotation period and level of activity). This information will also be crucial in determining whether or not any of the planets located within the star’s habitable zone could in fact be habitable.

The discovery of the TRAPPIST-1’s system was an event that was many years in the making. But the rate at which new discoveries have turned up has been very impressive. In the coming years, with the deployment of next-generation planet-hunters – like the James Webb Telescope and the Transitting Exoplanet Survey Satellite (TESS) – we will be able to dig deeper and learn even more.

And be sure to enjoy this video of TRAPPIST-1’s orbital resonance, courtesy of Assistant Professor Daniel Fabrycky of the University of Chicago:

Further Reading: UW Today, Nature Astronomy

 

NASA Brings Trappist-1 Into Focus… Kinda Sorta

On February 22nd, 2017, NASA announced the discovery of a seven-planet system around the red dwarf star known as TRAPPIST-1. Since that time, a number of interesting revelations have been made. For starters, the Search for Extra-Terrestrial Intelligence (SETI) recently announced that it was already monitoring this system for signs of advanced life (sadly, the results were not encouraging).

In their latest news release about this nearby star system, NASA announced the release of the first images taken of this system by the Kepler mission. As humanity’s premier planet-hunting mission, Kepler has been observing this system since December 2016, a few months after the existence of the first three of its exoplanets were announced.

These observations took place between December 15th, 2016 and March 4th, 2017, as part of Kepler’s extended mission (known as K2). During this 74-day period, K2 collected data on minuscule changes in the star’s brightness, which were caused by transits made by the star’s exoplanets. And as of Wednesday, March. 8th, this information is now available to the scientific community.

Artist’s impression of of the exoplanets orbiting the ultracool dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

These observations constituted the longest continuous set of observations of the star system to date. But in truth, the initial coordinates designated for this observation (known as Campaign 12) were set back in Oct. 2015, and did not focus on TRAPPIST-1. But as of May 2016, when the system’s first three planets were announced, the science teams adjusted their focus to observe them.

As Michael Haas, the science office director for the Kepler and K2 missions at NASA’s Ames Research Center, explained:

“We were lucky that the K2 mission was able to observe TRAPPIST-1. The observing field for Campaign 12 was set when the discovery of the first planets orbiting TRAPPIST-1 was announced, and the science community had already submitted proposals for specific targets of interest in that field. The unexpected opportunity to further study the TRAPPIST-1 system was quickly reconized and the agility of the K2 team and science community prevailed once again.”

While the data is raw and uncalibrated, it is expected to help astronomers learn more about this system of planets. In particular, it could help astronomers to place constraints on the seventh planet’s orbital period and mass (which are currently unknown). Additional information about the other six planets, particularly their size and mass, could also help astronomers make more accurate assessments about their composition.

Raw data from the K2 Category 12 survey. Credit: NASA/Kepler/K2 Campaign12

The magnetic activity of the host star, which is important in determining if any of its planets could be habitable, is also something that astronomers would like to learn more about. Last, but not least, the new data will help astronomers to prepare proposals for the use Earth-based telescopes next winter to further investigate TRAPPIST-1.

These proposals are due this month, and the timely arrival of this data ought to help research teams to refine their research objectives for next year. Any refinements made using this data will also help astronomer plan for follow-up studies using next-generations telescopes like the James Webb Space Telescope. As Geert Barentsen, a K2 research scientist at NASA’s Ames Research Center, explained:

“Scientists and enthusiasts around the world are invested in learning everything they can about these Earth-size worlds. Providing the K2 raw data as quickly as possible was a priority to give investigators an early look so they could best define their follow-up research plans. We’re thrilled that this will also allow the public to witness the process of discovery.

By the end of May 2017, the data will be fully processed and calibrated, which will also be made available to the public. As you can see from the images above, it was a little on the pixelated side! Still, we can expect some interesting finds to come out of this crowded star system in the coming months. Hopefully, some of that information will help us to determine if there’s any real chance of life forming there.

Further Reading: NASA, Kepler and K2

New Finds From Kepler: 8 New Worlds Discovered in the Habitable Zone

A fascinating set of finds was announced today at the 225th meeting of the American Astronomical Society (AAS), currently underway this week in Seattle, Washington. A team of astronomers announced the discovery of eight new planets potentially orbiting their host stars in their respective habitable zones. Also dubbed the ‘Goldilocks Zone,’ this is the distance where — like the tempting fairytale porridge — it’s not too hot, and not too cold, but juuusst right for liquid water to exist.

And chasing the water is the name of the game when it comes to hunting for life on other worlds. Two of the discoveries announced, Kepler-438b and Kepler-442b, are especially intriguing, as they are the most comparable to the Earth size-wise of any exoplanets yet discovered.

“Most of these planets have a good chance of being rocky, like Earth,” said Guillermo Torres in a recent press release. Guillermo is the lead author in the study for the Harvard-Smithsonian Center for Astrophysics (CfA).

This also doubles the count of suspected terrestrial exo-worlds — planets with less than twice the diameter of the Earth — inferred to orbit in the habitable zone of their host stars.

Fans on exoplanet science will remember the announcement of the first prospective Earth-like world orbiting in the habitable zone of its host star, Kepler-186f announced just last year.

The Kepler Space Telescope looks for planets used a technique known as the transit method. If a planet is orbiting its host star along our line of sight, a small but measurable dip in the star’s brightness occurs. This has advantages over the radial velocity technique because it allows researchers to pin down the hidden planet’s orbit and size much more precisely. The transit method is biased, however, to planets close in to its host which happen to lie along our solar system-bound line of sight. Kepler may miss most exo-worlds inclined out of its view, but it overcomes this by staring at thousands of stars.

Kepler launch
The launch of Kepler from the Cape in 2009. Credit: NASA/Kim Shiflett.

Launched in 2009, Kepler has wrapped up its primary phase of starring at a patch of sky along the plane of the Milky Way in the directions of the constellations of Cygnus, Lyra and Hercules, and is now in its extended K2 mission using the solar wind pressure as a 3rd ‘reaction wheel’ to carry out targeted searches along the ecliptic plane.

Both newly discovered worlds highlighted in today’s announcement orbit distant red dwarf stars. Kepler-438 b is estimated to be 12% larger in diameter than the Earth, and Kepler-442 b is estimated by the team to be 33% larger. These worlds have a 70% and 60% chance of being rocky, respectively. For comparison, Ice giant planet Uranus is 4 times the diameter of the Earth, and over 14 times more massive.

A comparison of the new exoplanet finds between Earth and Jupiter. Credit: NASA/Kepler.
A comparison of the new exoplanet finds between Earth and Jupiter. Credit: NASA/Kepler.

“We don’t know for sure whether any of the planets in our sample are truly habitable,” Said CfA co-researcher in the study David Kipping. All we can say is that they’re promising candidates.”

The idea of habitable worlds around red dwarf stars is a tantalizing one. These stars are fainter and cooler than our Sun, and 7.5% to 50% as massive. They also have two primary factors going for them: they’re the most common type of stars in the universe, and they have life spans measured in trillions of years, much longer than the current age of the universe. If life could go from muck to making microwave dinners here on Earth in just a few billion years, it’s had lots longer to do the same on worlds orbiting red dwarf stars.

There is, however, one catch: the habitable zone surrounding a red dwarf is much closer in to its host star, and any would-be planet is subject to frequent surface-sterilizing flares. Perhaps a world with a synchronous rotation might be spared this fate and feature a habitable hemisphere well inside the snow line permanently turned away from its host.

The team made these discoveries by sifting though Kepler’s preliminary finds that are termed KOI’s, or Kepler Objects of Interest. Though these potential discoveries were far too small to pin down their masses using the traditional method, the team employed a program named BLENDER to statically validate the finds. BLENDER has been employed before in concert with backup observations for extremely tiny exoplanet discoveries. Torres and Francois Fressin developed the BLENDER program, and it is currently run on the massive Pleiades supercomputer at NASA Ames.

It was also noted in today’s press conference that two KOIs awaiting validation — 5737.01 and 2194.03 — may also prove to be terrestrial worlds  orbiting Sun-like stars that are possibly similar in size to the Earth.

The proposed target regions for the Kepler K2 mission. Credit: NASA/Kepler.
The proposed target regions for the Kepler K2 mission. Credit: NASA/Kepler.

But don’t plan on building an interstellar ark and heading off to these newly found worlds just yet. Kepler-438b sits 470 light years from Earth, and Kepler-442b is even farther away at 1,100 light years. And we’ll also add our usual caveat and caution that, from a distance, the planet Venus in our own solar system might look like a tempting vacation spot. (Spoiler alert: it’s not).

Still, these discoveries are fascinating finds and add to the growing menagerie of exoplanet systems. These will also serve as great follow up targets for TESS, Gaia and LSST survey, all set to add to our exoplanet knowledge in the coming decade.

The LSST mirror in the Tuscon Mirror Lab. (Photo by author).
The LSST mirror in the Tuscon Mirror Lab. (Photo by author).

And to think, I remember growing up as a child of the 1970s reading that exoplanet detections were soooo difficult that they might never occur in our lifetime… now, fast-forward to 2015, and we’re beginning to classify and characterize other brave new solar systems in the modern Age of Exoplanet Science.

-Looking to observe red dwarf stars with your backyard scope? Check out our handy list.