How can two planets so similar in some respects have such different densities? According to a new study, a catastrophic collision may be to blame.
In our Solar System, all the inner planets are small rocky worlds with similar densities, while the outer planets are gas giants with their own similar densities. But not all solar systems are like ours.
In September of 2015, the star KIC 8462852 (aka. Tabby’s Star) captured the world’s attention when it was found to be experiencing a mysterious drop in brightness. In the years since then, multiple studies have been conducted that have tried to offer a natural explanation for this behavior – and even an unnatural one (i.e. the “alien megastructure” theory). At the same time, multiple observatories have been tracking the star regularly for further dimming.
Well, it seems that Tabby’s Star is at it again! On Friday, March 16th, Tabetha Boyajian (the astronomer who was responsible for discovering the star’s variations in flux) and her colleagues reported that the star was dimming yet again. As they indicated recently their blog – Where’s the Flux? – the star experienced its greatest dip since it was observed by the Kepler mission in 2013.
However, back in January, Tabetha Boyajian and a team of over 100 astronomers conducted a new study which demonstrated that KIC 8462852 (aka. “Tabby’s Star”) was likely being partially obscured by dust. This study effectively put to rest speculation that the dimming could be caused by an alien megastructure and offered conclusive evidence that the flux was the result of a natural phenomenon.
Nevertheless, on March 19th, Tabetha and her team began reporting how the star’s brightness was once again dropping. Using data obtained by the Las Cumbras Observatory‘s Teide, McDonald and Haleakala Observatories (in Spain, Texas and Hawaii, respectively), they began posting regular updates on its light curve. As they wrote on their blog at the time:
“On Friday (2018 March 16) we noted the last data taken were significantly down compared to normal. Due to poor weather conditions at all 3 sites we weren’t able to observe the star again until last night… This is the deepest dip we have observed since the Kepler Mission in 2013! WOW!!”
On March 22nd, the team provided an updated light curve which indicated that the star was rapidly returning to its normal brightness. As they indicated, “The profile of the new dip having a slow decline with a more rapid increase is again reminiscent to that of a backwards-comet.” On March 23rd, observations from the Catalonia Institute for Space Studies‘ (IEEC) Montsec Astronomical Observatory were also included, which indicated the same.
An update from March 26th indicated that the star’s flux had dropped by a total of 5%, a finding which was confirmed by John Hall – an observer with the American Association of Variable Star Observers. This constituted the greatest dip since the 22% reported in 2015. As Boyajian declared at the time, “Looks like we beat the record set just last week on the deepest dip observed since Kepler!”
The latest update, from March 27th, indicates that despite bad weather at two of their sites, new data had been obtained which indicated that the star’s flux was going back up again, but was still ~2% below normal. In short, it seems that this latest dimming event – the largest since the team first noticed a change in the star’s flux – has peaked and the star is returning to normal.
While this latest dip in light does not cast the obscuring dust conclusion into doubt, it does show that the mystery of Tabby’s Star may not be completely resolved yet. Based on this and future dimming events, scientists may be forced to refine their theories further. In the end, its all about the process of continuous discovery. And Tabby’s Star is proving to be a very interesting case!
However, one can almost certainly guarantee that fans of the “alien megastructure” theory are going to see this as good news!
Since its deployment in March of 2009, the Kepler space telescope has been a boon for exoplanet-hunters. As of March 8th, 2018, a total of 3,743 exoplanets have been confirmed, 2,649 of which were discovered by Kepler alone. At the same time, the telescope has suffered its share of technical challenges. These include the failure of two reaction wheels, which severely hampered the telescope’s ability to conduct its original mission.
Nevertheless, the Kepler team was able to return the telescope to a stable configuration by using small amounts of thruster fuel to compensate for the failed reaction wheels. Unfortunately, after almost four years conducting its K2 observation campaign, the Kepler telescope is now running out fuel. Based on its remaining fuel and rate of consumption, NASA estimates that the telescope’s mission will end in a few months.
For years, the Kepler space telescope has been locating planets around distant stars using the Transit Method (aka. Transit Photometry). This consists of monitors stars for periodic dips in brightness, which are caused by a planet passing in front of the star (i.e. transiting). Of all the methods used to hunt for exoplanets, the Transit Method is considered the most reliable, accounting for a total of 2900 discoveries.
Naturally, this news comes as a disappointment to astronomers and exoplanet enthusiasts. But before anyone starts lamenting the situation, they should keep some things in mind. For one, the Kepler mission has managed to last longer than anyone expected. Ever since the K2 campaign began, the telescope has been required to shift its field of view about every three months to conduct a new observation campaign.
Based on their original estimates, the Kepler team believed they had enough fuel to conduct 10 more campaigns. However, the mission has already completed 16 campaigns and the team just began their 17th. As Charlie Sobeck, a system engineer for the Kepler space telescope mission, explained in a recent NASA press statement:
“Our current estimates are that Kepler’s tank will run dry within several months – but we’ve been surprised by its performance before! So, while we anticipate flight operations ending soon, we are prepared to continue as long as the fuel allows. The Kepler team is planning to collect as much science data as possible in its remaining time and beam it back to Earth before the loss of the fuel-powered thrusters means that we can’t aim the spacecraft for data transfer. We even have plans to take some final calibration data with the last bit of fuel, if the opportunity presents itself.”
So while the mission is due to end soon, the science team hopes to gather as much scientific data as possible and beam it back to Earth before then. They also hope to gather some final calibration data using the telescope’s last bit of fuel, should the opportunity present itself. And since they cannot refuel the spacecraft, they hope to stop collecting data so they can use their last bit of fuel to point the spacecraft back towards Earth and bring it home.
“Without a gas gauge, we have been monitoring the spacecraft for warning signs of low fuel— such as a drop in the fuel tank’s pressure and changes in the performance of the thrusters,” said Sobeck. “But in the end, we only have an estimate – not precise knowledge. Taking these measurements helps us decide how long we can comfortably keep collecting scientific data.”
This has been standard practice for many NASA missions, where enough fuel has been reserved to conduct one last maneuver. For example, the Cassini mission had to reserve fuel in order to descend into Saturn’s atmosphere so it would avoid colliding with one of its moons and contaminating a potentially life-bearing environment. Satellites also regularly conduct final maneuvers to ensure they don’t crash into other satellites or fall to Earth.
While deep-space missions like Kepler are in no danger of crashing to Earth or contaminating a sensitive environment, this final maneuver is designed to ensure that the science team can squeeze every last drop of data from the spacecraft. So before the mission wraps up, we can expect that this venerated planet-hunter will have some final surprises for us!
In the coming years, next-generation telescopes will be taking to space to pick up where Kepler and other space telescopes left off. These include the Transiting Exoplanet Survey Satellite(TESS), which will be conducting Transit surveys shortly after it launches in April of 2018. By 2019, the James Webb Space Telescope (JWST) will also take to space and use its powerful infrared instruments to aid in the hunt for exoplanets.
So while we will soon be saying goodbye to the Kepler mission, its legacy will live on. In truth, the days of exoplanet discovery are just getting started!
As of March 1st, 2018, 3,741 exoplanets have been confirmed in 2,794 systems, with 622 systems having more than one planet. Most of the credit for these discoveries goes to the Kepler space telescope, which has discovered roughly 3500 planets and 4500 planetary candidates. In the wake of all these discoveries, the focus has shifted from pure discovery to research and characterization.
In this respect, planets detected using the Transit Method are especially valuable since they allow for the study of these planets in detail. For example, a team of astronomers recently discovered three Super-Earths orbiting a star known GJ 9827, which is located just 100 light years (30 parsecs) from Earth. The proximity of the star, and the fact that it is orbited by multiple Super-Earths, makes this system ideal for detailed exoplanet studies.
As with all Kepler discoveries, these planets were discovered using the Transit Method (aka. Transit Photometry), where stars are monitored for periodic dips of brightness. These dips are the result of exoplanets passing in front of the star (i.e. transiting) relative to the observer. While this method is ideal for placing constraints on the size and orbital periods of a planet, it can also allow for exoplanet characterization.
Basically, scientists are able to learn things about their atmospheres by measuring the spectra produced by the star’s light as it passes through the planet’s atmosphere. Combined with radial velocity measurements of the star, scientists can also place constraints on the planet’s mass and radius and can determine things about the planet’s interior structure.
For the sake of their study, the team analyzed data obtained by the K2 mission, which showed the presence of three Super-Earths around the star GJ 9827 (GJ 9827 b, c, and d). Since they initially submitted their research paper back in September of 2017, the presence of these planets has been confirmed by another team of astronomers. As Dr. Rodriguez told Universe Today via email:
“We detected three super-Earth sized planets orbiting in a very compact configuration. Specifically, the three planets have radii of 1.6, 1.2, and 2.1 times the radius of Earth and all orbit their host star within 6.2 days. We note that this system was independently discovered (simultaneously) by another team from Wesleyan University (Niraula et al. 2017).”
These three exoplanets are especially interesting because the larger of the two have radii that place them in the range between being rocky or gaseous. Few such exoplanets have been discovered so far, which makes these three a prime target for research. As Dr. Rodriguez explained:
“Super Earth sized planets are the most common type of planet we know of but we do not have one in our own solar system, limiting our ability to understand them. They are especially important because their radii span the rock to gas transition (as I discuss below in one of the other responses). Essentially, planets larger then 1.6 times the radius of the Earth are less dense and have thick hydrogen/helium atmospheres while planets smaller are very dense with little to no atmosphere.”
Another interesting thing about these super-Earths is how their short orbital periods – which are 1.2, 3.6 and 6.2 days, respectively – would result in fairly hot temperatures. In short, the team estimates that the three super-Earths experience surface temperatures of 1172 K (899 °C; 1650 °F), 811 K (538 °C; 1000 °F), and 680 K (407 °C; 764 °F), respectively.
By comparison, Venus – the hottest planet in the Solar System – experiences surface temperatures of 735 K (462 °C; 863 °F). So while temperatures on Venus are hot enough to melt lead, conditions on GJ 9827 b are almost hot enough to melt bronze.
However, the most significant thing about this discovery is the opportunities it could provide for exoplanet characterization. At just 100 light-years from Earth, it will be relatively easy for the next-generation telescopes (such as the James Webb Space Telescope) to conduct studies of their atmospheres and provide a more detailed picture of this system of planets.
In addition, these three strange planets are all in the same system, which makes conducting observation campaigns that much easier. As Rodriguez concluded:
“The GJ 9827 system is unique because one planet is smaller than this cutoff, one planet is larger, and the third planet has a radius of ~1.6 times the radius of the Earth, right on that border. So in one system, we have planets that span this rock to gas transition. This is important because we can study the atmosphere’s of these planets, look for differences in the composition of their atmospheres and begin to understand why this transition occurs at 1.6 times the radius of the Earth. Since all three planets orbit the same star, the effect of the host star is kept constant in this “experiment”. Therefore, if these three planets in GJ 9827 were instead orbiting three separate stars, we would have to worry about how the host star is influencing or affecting the planet’s atmosphere. In the GJ 9827 system, we do not have to worry about this since they orbit the same star.”
When it comes to looking for life on extra-solar planets, scientists rely on what is known as the “low-hanging fruit” approach. In lieu of being able to observe these planets directly or up close, they are forced to look for “biosignatures” – substances that indicate that life could exist there. Given that Earth is the only planet (that we know of) that can support life, these include carbon, oxygen, nitrogen and water.
However, while the presence of these elements are a good way of gauging “habitability”, they are not necessarily indications that extra-terrestrial civilizations exist. Hence why scientists engaged in the Search for Extra-Terrestrial Intelligence (SETI) also keep their eyes peeled for “technosignatures”. Targeting the Kepler field, a team of scientists recently conducted a study that examined 14 planetary systems for indications of intelligent life.
Together, the team selected 14 systems from the Kepler catalog and examined them for technosignatures. While radio waves are a common occurrence in the cosmos, not all sources can be easily attributed to natural causes. Where and when this is the case, scientists conduct additional studies to try and rule out the possibility that they are a technosignature. As Professor Margot told Universe Today via email:
“In our article, we define a “technosignature” as any measurable property or effect that provides scientific evidence of past or present technology, by analogy with “biosignatures,” which provide evidence of past or present life.”
For the sake of their study, the team conducted an L-band radio survey of these 14 planetary systems. Specifically, they looked for signs of radio waves in the 1.15 to 1.73 gigahertz (GHz) range. At those frequencies, their study is sensitive to Arecibo-class transmitters located within 450 light-years of Earth. So if any of these systems have civilizations capable of building radio observatories comparable to Arecibo, the team hoped to find out!
“We searched for signals that are narrow (< 10 Hz) in the frequency domain,” said Margot. “Such signals are technosignatures because natural sources do not emit such narrowband signals… We identified approximately 850,000 candidate signals, of which 19 were of particular interest. Ultimately, none of these signals were attributable to an extraterrestrial source.”
What they found was that of the 850,000 candidate signals, about 99% of them were automatically ruled out because they were quickly determined to be the result of human-generated radio-frequency interference (RFI). Of the remaining candidates, another 99% were also flagged as anthropogenic because their frequencies overlapped with other known sources of RFI – such as GPS systems, satellites, etc.
The 19 candidate signals that remained were heavily scrutinized, but none could be attributed to an extraterrestrial source. This is key when attempting to distinguish potential signs of intelligence from radio signals that come from the only intelligence we know of (i.e. us!) Hence why astronomers have historically been intrigued by strong narrowband signals (like the WOW! Signal, detected in 1977) and the Lorimer Burst detected in 2007.
In these cases, the sources appeared to be coming from the Messier 55 globular cluster and the Large Magellanic Cloud, respectively. The latter was especially fascinating since it was the first time that astronomers had observered what are now known as Fast Radio Bursts (FRBs). Such bursts, especially when they are repeating in nature, are considered to be one of the best candidates in the search for intelligent, technologically-advanced life.
Unfortunately, these sources are still being investigated and scientists cannot attribute them to unnatural causes just yet. And as Professor Margot indicated, this study (which covered only 14 of the many thousand exoplanets discovered by Kepler) is just the tip of the iceberg:
“Our study encompassed only a small fraction of the search volume. For instance, we covered less than five-millionths of the entire sky. We are eager to scale the effort to sample a larger fraction of the search volume. We are currently seeking funds to expand our search.”
It would therefore be no exaggeration to say that the hunt for ETI is still in its infancy, and our efforts are definitely beginning to pick up speed. There is literally a Universe of possibilities out there and to think that there are no other civilizations that are also looking for us seems downright unfathomable. To quote the late and great Carl Sagan: “The Universe is a pretty big place. If it’s just us, seems like an awful waste of space.”
And be sure to check out this video of the 2017 UCLA SETI Group, courtesy of the UCLA EPSS department:
Finding planets beyond our Solar System is already tough, laborious work. But when it comes to confirmed exoplanets, an even more challenging task is determining whether or not these worlds have their own satellites – aka. “exomoons”. Nevertheless, much like the study of exoplanets themselves, the study of exomoons presents some incredible opportunities to learn more about our Universe.
Of all possible candidates, the most recent (and arguably, most likely) one was announced back in July 2017. This moon, known as Kepler-1625 b-i, orbits a gas giant roughly 4,000 light years from Earth. But according to a new study, this exomoon may actually be a Neptune-sized gas giant itself. If true, this will constitute the first instance where a gas giant has been found orbiting another gas giant.
Within the Solar System, moons tell us much about their host planet’s formation and evolution. In the same way, the study of exomoons is likely to provide insight into extra-solar planetary systems. As Dr. Heller explained to Universe Today via email, these studies could also shed light on whether or not these systems have habitable planets:
“Moons have proven to be extremely helpful to study the formation and evolution of the planets in the solar system. The Earth’s Moon, for example, was key to set the initial astrophysical conditions, such as the total mass of the Earth and the Earth’s primordial spin state, for what has become our habitable environment. As another example, the Galilean moons around Jupiter have been used to study the conditions of the primordial accretion disk around Jupiter from which the planet pulled its mass 4.5 billion years ago. This accretion disk has long gone, but the moons that formed within the disk are still there. And so we can use the moons, in particular their contemporary composition and water contents, to study planet formation in the far past.”
When it comes to the Kepler-1625 star system, previous studies were able to produce estimates of the radii of both Kepler-1625 b and its possible moon, based on three observed transits it made in front of its star. The light curves produced by these three observed transits are what led to the theory that Kepler-1625 had a Neptune-size exomoon orbiting it, and at a distance of about 20 times the planet’s radius.
But as Dr. Heller indicated in his study, radial velocity measurements of the host star (Kepler-1625) were not considered, which would have produced mass estimates for both bodies. To address this, Dr. Heller considered various mass regimes in addition to the planet and moon’s apparent sizes based on their observed signatures. Beyond that, he also attempted to place the planet and moon into the context of moon formation in the Solar System.
The first step, accroding to Dr. Heller, was to conduct estimates of the possible mass of the exomoon candidate and its host planet based on the properties that were shown in the transit lightcurves observed by Kepler.
“A dynamical interpretation of the data suggests that the host planet is a roughly Jupiter-sized (“size” in terms of radius) brown dwarf with a mass of almost 18 Jupiter masses,” he said. “The uncertainties, however, are very large mostly due to the noisiness of the Kepler data and due to the low number of transits (three). In fact, the host object could be a Jupiter-like planet or even be a moderate-sized brown dwarf of up to 37 Jupiter masses. The mass of the moon candidate ranges somewhere between a super-Earth of a few Earth masses and Neptune’s mass.”
Next, Dr. Heller compared the relative mass of the exomoon candidate and Kepler-1625 b and compared this value to various planets and moons of the Solar System. This step was necessary because the moons of the Solar System show two distinct populations, based the mass of the planets compared to their moon-to-planet mass ratios. These comparisons indicate that a moon’s mass is closely related to how it formed.
For instance, moons that formed through impacts – such as Earth’s Moon, and Pluto’s moon Charon – are relatively heavy, whereas moons that formed from a planet’s accretion disk are relatively light. While Jupiter’s moon Ganymede is the most massive moon in the Solar System, it is rather diminutive and tiny compared to Jupiter itself – the largest and most massive body in the Solar System.
In the end, the results Dr. Heller obtained proved to be rather interesting. Basically, they indicated that Kepler-1625 b-i cannot be definitively placed in either of these families (heavy, impact moons vs. lighter, accretion moons). As Dr. Heller explained:
“[T]]he most reasonable scenarios suggest that the moon candidate is more of the heavy kind, which suggests it should have formed through an impact. However, this exomoon, if real, is most likely gaseous. The solar system moons are all rocky/icy bodies without a significant gas envelope (Titan has a thick atmosphere but its mass is negligible). So how would a gas giant moon have formed through an impact? I don’t know. I don’t know if anybody knows.
“Alternatively, in a third scenario, Kepler-1625 b-i could have formed through capture, but this implies a very unlikely progenitor planetary binary system, from which it was pulled into a bound orbit around Kepler-1625 b, while its former planetary companion was ejected from the system.”
What was equally interesting were the mass estimates for Keple-1625 b, which Dr. Heller averaged to be 19 Jupiter masses, but could be as high as 112 Jupiter Masses. This means that the host planet could be anything from a gas giant that is just slightly larger than Saturn to a Brown Dwarf or even a Very-Low-Mass-Star (VLMS). So rather than a gas giant moon orbiting a gas giant, we could be dealing with a gas giant moon orbiting a small star, which together orbit a larger star!
It’s the stuff science fiction is made of! And while this study cannot provide exact mass constraints on Keplder-1625 b and its possible moon, its significance cannot be denied. Beyond providing astrophysicists with the first possible example of a gas giant moon, this study is of immense significance as far as the study of exoplanet systems is concerned. If and when Kepler-1625 b-i is confirmed, it will tell us much about the conditions under which its host formed.
In the meantime, more observations are needed to confirm or rule out the existence of this moon. Fortunately, these observations will be taking place in the very near future. When Kepler-1625 b makes it next transit – on October 29th, 2017 – the Hubble Space Telescope will be watching! Based on the light curves it observes coming from the star, scientist should be able to get a better idea of whether or not this mysterious moon is real and what it looks like.
“If the moon turns out to be a ghost in the data, then most of this study would not be applicable to the Kepler-1625 system,” said Dr. Heller. “The paper would nevertheless present an example study of how to classify future exomoons and how to put them into the context of the solar system. Alternatively, if Kepler-1625 b-i turns out to be a genuine exomoon, then my study suggests that we have found a new kind of moon that has a very different formation history than the moons we know as of today. Certainly an exquisite riddle for astrophysicists to solve.”
The study of exoplanet systems is like pealing an onion, albeit in a dark room with the lights turned off. With every successive layer scientists peel back, the more mysteries they find. And with the deployment of next-generation telescopes in the near future, we are bound to learn a great deal more!
The mystery of KIC 8462852 (aka. Boyajian’s Star or Tabby’s Star) continues to excite and intrigue! Ever since it was first seen to be undergoing strange and sudden dips in brightness (back in October of 2015) astronomers have been speculating as to what could be causing this. Since that time, various explanations have been offered, including large asteroids, a large planet, a debris disc or even an alien megastructure.
Many studies have been produced that have sought to assign some other natural explanation to the star’s behavior. The latest comes from an international team of scientists – which included Tabetha Boyajian, the lead author on the original 2016 paper. According to this latest study, which was recently published in The Astrophysical Journal, the star’s long-term dimming patterns are likely the result of an uneven dust cloud moving around the star.
For the sake of their study, the team consulted data that was obtained by NASA’s Spitzer Space Telescope and the Swift Gamma-Ray Burst mission between January and December of 2016. Whereas Spitzer conducted observations in the infrared band, Swift gathered data in the ultraviolet band. This was then compared to visible light gathered during the same period by AstroLAB IRIS’s 68-cm (27-inch) reflecting telescope.
What they found was that KIC 8462852 experienced less dimming in the infrared band than in the ultraviolet. This, they concluded, was a strong indication that material transiting in front of the star was likely no larger than a few micrometers (about one ten-thousands of an inch) in diameter, since anything larger would cause the light to dim equally across all wavelengths.
This finding contradicts many theories that have been ventured since the mysterious dimming of Tabby’s Star was first noticed. As Dr. Meng indicated in a recent NASA press statement:
“This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming. We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”
Based on the strong dip in the ultraviolet band, the research team also concluded that the particles must be larger than interstellar dust. Otherwise, the pressure caused by KIC 8462852’s solar wind would drive these particles out into space over time. A circumstellar disk of dust particles would not only be able to remain in orbit, it would also explain the uneven changes in the star’s brightness.
So far, this is the best explanation for the mysterious long-term behavior of Tabby’s Star. As with previous observations, much of the credit for this latest study goes to amateur astronomers who assisted in the observations. It was participants in the Planet Hunters project, which provides open to access Kepler mission data, that first noticed that light coming from KIC 8462852 was experiencing strange dips.
In addition, it was the work of amateur astronomers – who provided the necessary technical and software support to AstroLAB – allowed for this study to take place. After the Astrolab group posted the data they had obtained on Tabby’s star in a public astronomy archive, George Rieke (one of the co-authors on this latest study) contacted them and proposed combining their results.
The AstroLAB group consists of Franky Dubois, who operated the telescope during the Tabby’s Star observations, Ludwig Logie, who helps with technical issues on the telescope, and Steve Rau, who processes observations of star brightness, is a trainer at a Belgian railway company. Together, they began monitoring Tabby’s Star after they read Dr. Boyajian 2016 study.
Naturally, more observations and research is needed to confirm this latest study. While it does fit the long-term observations, there is still the matter of shorter-term dimming events. These include the three-day spurts that were noticed in 2017, as well as the major 20-percent dips that were observed during Kepler’s primary mission. The theory that these could have been the result of a swarm of comets is also still a possible explanation.
This theory, which was based on data collected by the Wide-field Infrared Survey Explorer (WISE) mission, could explain both the short period dips and the longer-term dips. Whereas the comets passing in front of the star could explain the former, dust produced by the sublimation of material from the comets as they draw nearer the star (or through collisions) could explain the latter. As Vanaverbeke said:
“Tabby’s Star could have something like a solar activity cycle. This is something that needs further investigation and will continue to interest scientists for many years to come.”
So for those hoping that Tabby’s Star was the first indication of an alien megastructure, there’s still hope (albeit a faint one)! As Professor Loeb of the Harvard Smithsonian Center for Astrophysics (CfA) told Universe Today recently (with regards to a new study he co-authored), there’s always the possibility that dimming patterns are due to massive structures – like a magnetic shield – passing in front of a host star:
“The imprint of a shield built by another civilization could involve the changes it induces in the brightness of the host star due to occultation (similar behavior to Tabby’s star) if the structure is big enough,” he said. “The situation could be similar to Dyson’s spheres, but instead of harvesting the energy of the star the purpose of the infrastructure is to protect a technological civilization on a planet from the flares of its host star.”
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”.
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.
“We have discovered two transiting warm Jupiter exoplanets initially identified as transiting candidates in K2 photometry… Both 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.”
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.”
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.
Ever since it was deployed in March of 2009, the Kepler mission has detected thousands of extra-solar planet candidates. In fact, between 2009 and 2012, it detected a total of 4,496 candidates, and confirmed the existence of 2,337 exoplanets. Even after two of its reaction wheels failed, the spacecraft still managed to turn up distant planets as part of its K2 mission, accounting for another 521 candidates and confirming 157.
However, according to a new study conducted by a pair of researches from Columbia University and a citizen scientist, Kepler may also have also found evidence of an extra-solar moon. After sifting through data from hundreds of transits detected by the Kepler mission, the researchers found one instance where a transiting planet showed signs of having a satellite.
For years, Dr. Kipping has been searching the Kepler database for evidence of exomoons, as part of the HEK. This is not surprising, considering the kinds of opportunities that exomoons present for scientific research. Within our Solar System, the study of natural satellites has revealed important things about the mechanisms that drive early and late planet formation, and moons possess interesting geological features that are commonly found on other bodies.
It is for this reason that extending that research to the hunt for exoplanets is seen as necessary. Already, exoplanet-hunting missions like Kepler have turned up a wealth of planets that challenge conventional ideas about how planet formation and what kinds of planets are possible. The most noteworthy example are gas giants that have observed orbiting very close to their stars (aka. “Hot Jupiters”).
As such, the study of exomoons could yield valuable information about what kinds of satellites are possible, and whether or not our own moons are typical. As Teachey told Universe Today via email:
“Exomoons could tell us a lot about the formation of our Solar System, and other star systems. We see moons in our Solar System, but are they common elsewhere? We tend to think so, but we can’t know for sure until we actually see them. But it’s an important question because, if we find out there aren’t very many moons out there, it suggests maybe something unusual was going on in our Solar System in the early days, and that could have major implications for how life arose on the Earth. In other words, is the history of our Solar System common across the galaxy, or do we have a very unusual origin story? And what does that say about the chances of life arising here? Exomoons stand to offer us clues to answering these questions.”
What’s more, many moons in the Solar System – including Europa, Ganymede, Enceladus and Titan – are thought to be potentially habitable. This is due to the fact that these bodies have steady supplies of volatiles (such as nitrogen, water, carbon dioxide, ammonia, hydrogen, methane and sulfur dioxide) and possess internal heating mechanisms that could provide the necessary energy to power biological processes.
Here too, the study of exomoons presents interesting possibilities, such as whether or not they may be habitable or even Earth-like. For these and other reasons, astronomers want to see if the planets that have been confirmed in distant star systems have systems of moons and what conditions are like on them. But as Teachey indicated, the search for exomoons presents a number of challenges compared to exoplanet-hunting:
“Moons are difficult to find because 1) we expect them to be quite small most of the time, meaning the transit signal will be quite weak to begin with, and 2) every time a planet transits, the moon will show up in a different place. This makes them more difficult to detect in the data, and modeling the transit events is significantly more computationally expensive. But our work leverages the moons showing up in different places by taking the time-averaged signal across many different transit events, and even across many different exoplanetary systems. If the moons are there, they will in effect carve out a signal on either side of the planetary transit over time. Then it’s a matter of modeling this signal and understanding what it means in terms of moon size and occurrence rate.”
To locate signs of exomoons, Teachey and his colleagues searched through the Kepler database and analyzed the transits of 284 exoplanet candidates in front of their respective stars. These planets ranged in size from being Earth-like to Jupiter-like in diameter, and orbited their stars at a distance of between ~0.1 to 1.0 AU. They then modeled the light curve of the stars using the techniques of phase-folding and stacking.
These techniques are commonly used by astronomers who monitor stars for dips in luminosity that are caused by the transits of planets (i.e. the transit method). As Teachey explained, the process is quite similar:
“Basically we cut up the time-series data into equal pieces, each piece having one transit of the planet in the middle. And when we stack these pieces together we’re able to get a clearer picture of what the transit looks like… For the moon search we do essentially the same thing, only now we’re looking at the data outside the main planetary transit. Once we stack the data, we take the average values of all the data points within a certain time window and, if a moon is present, we ought to see some missing starlight there, which allows us to deduce its presence.”
What they found was a single candidate located in the Kepler-1625 system, a yellow star located about 4000 light years from Earth. Designated Kepler-1625B I, this moon orbits the large gas giant that is located within the star’s habitable zone, is 5.9 to 11.67 times the size of Earth, and orbits its star with a period of 287.4 days. This exomoon candidate, if it should be confirmed, will be the first exomoon ever discovered
The team’s results (which await peer review) also demonstrated that large moons to be a rare occurrence in the inner regions of star systems (within 1 AU). This was something of a surprise, though Teachey acknowledges that it is consistent with recent theoretical work. According to what some recent studies suggest, large planets like Jupiter could lose their moons as they migrate inward.
If this should prove to be the case, then what Teachey and his colleagues witnessed could be seen as evidence of that process. It could also be an indication our current exoplanet-hunting missions may not be up to the task of detecting exomoons. In the coming years, next-generations missions are expected to provide more detailed analyses of distant stars and their planetary systems.
However, as Teachey indicated, these too could be limited in terms of what they can detect, and new strategies may ultimately be needed:
“The rarity of moons in the inner regions of these star systems suggests that individual moons will remain difficult to find in the Kepler data, and upcoming missions like TESS, which should find lots of very short period planets, will also have a difficult time finding these moons. It’s likely the moons, which we still expect to be out there somewhere, reside in the outer regions of these star systems, much as they do in our Solar System. But these regions are much more difficult to probe, so we will have to get even more clever about how we look for these worlds with present and near-future datasets.”
In the meantime, we can certainly be exited about the fact that the first exomoon appears to have been discovered. While these results await peer review, confirmation of this moon will mean additional research opportunities for Kepler-1625 system. The fact that this moon orbits within the star’s habitable zone is also an interesting feature, though its not likely the moon itself is habitable.
Still, the possibility of a habitable moon orbiting a gas giant is certainly interesting. Does that sound like something that might have come up in some science fiction movies?
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.
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.
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.”
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!