A pair of astronomers combing through data from the Kepler spacecraft have discovered the first exomoon. The moon is in the Kepler 1625 system about 8,000 light years away, in the constellation Cygnus. It orbits the gas giant Kepler 1625b, and, unlike all the moons in our Solar System, this one is a “gas moon.”
It was only a matter of time before we found an exomoon. We’ve found thousands of exoplanets, thanks mostly to the Kepler spacecraft. And where there are planets, we can expect moons. But even though it seemed inevitable, the first confirmed exomoon is still exciting.
The Kepler space telescope has had a relatively brief but distinguished career of service with NASA. Having launched in 2009, the space telescope has spent the past nine years observing distant stars for signs of planetary transits (i.e. the Transit Method). In that time, it has been responsible for the detection of 2,650 confirmed exoplanets, which constitutes the majority of the more than 38oo planets discovered so far.
Earlier this week, the Kepler team was notified that the space telescope’s fuel tank is running very low. NASA responded by placing the spacecraft in hibernation in preparation for a download of its scientific data, which it collected during its latest observation campaign. Once the data is downloaded, the team expects to start its last observation campaign using whatever fuel it has left.
In order to send the data back home, the spacecraft will point is large antenna back towards Earth and transmit it via the Deep Space Network. However, the DSN is responsible for transmitting data from multiple missions and time needs to be allotted in advance. Kepler is scheduled to send data from its 18th campaign back in August, and will remain in a stable orbit and safe mode in order to conserve fuel until then.
On August 2nd, the Kepler team will command the spacecraft to awaken and will maneuver the craft to the correct orientation to transmit the data. If all goes well, they will begin Kepler’s 19th observation campaign on August 6th with what fuel the spacecraft still has. At present, NASA expects that the spacecraft will run out of fuel in the next few months.
However, even after the Kepler mission ends, scientists and engineers will continue to mine the data that has already been sent back for discoveries. According to a recent study by an international team of scientists, 24 new exoplanets were discovered using data from the 10th observation campaign, which has brought the total number of Kepler discoveries to 2,650 confirmed exoplanets.
In the coming years, many more exoplanet discoveries are anticipated as the next-generation of space telescopes begin collecting their first light or are deployed to space. These include the Transiting Exoplanet Survey Satellite (TESS), which launched this past April, and the James Webb Space Telescope (JWST) – which is currently scheduled to launch sometime in 2021.
However, it will be many years before any mission can rival the accomplishments and contributions made by Kepler! Long after she is retired, her legacy will live on in the form of her discoveries.
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 Kepler space telescope is surely the gift that keeps on giving. After being deployed in 2009, it went on to detect a total of 2,335 confirmed exoplanets and 582 multi-planet systems. Even after two of its reaction wheels failed, it carried on with its K2 mission, which has discovered an additional 520 candidates, 148 of which have been confirmed. And with yet another extension, which will last beyond 2018, it shows no signs of stopping!
In the most recent catalog to be released by the Kepler mission, an additional 219 new planet candidates have been added to its database. More significantly, 10 of these planets were found to be terrestrial (i.e. rocky), of comparable in size to Earth and orbited within their star’s habitable zone – the distance where surface temperatures would be warm enough to support liquid water.
These findings were presented at a news conference on Monday, June 19th, at NASA’s Ames Research Center. Of all the catalogs of Kepler candidates that have been released to date, this one is the most comprehensive and detailed. The eighth in a series of Kepler exoplanet catalogs, this one is based on data that was obtained from the first four years of the mission and is the final catalog that covers the spacecraft’s observations of the Cygnus constellation.
Since 2014, Kepler has ceased looking at a set starfield in the Cygnus constellation and has been collecting data on its second mission – observing fields on the plane of the ecliptic of the Milky Way Galaxy. With the release of this catalog, there are now 4,034 planet candidates that have been identified by Kepler – of which 2,335 have been verified.
An important aspect of this catalog were the methods that were used for producing it, which were the most sophisticated to date. As with all planets detected by Kepler, the latest finds were all made using the transit method. This consists of monitoring stars for occasional dips in brightness, which is used to confirm the presence of planets transiting between the star and the observer.
To ensure that the detections in this latest catalog were real, the team relied on two approaches to eliminate false positives. This consisted of introducing simulated transits into the dataset to make sure the dips that Kepler detected were consistent with planets. Then, they added false signals to see how often the analysis mistook these for planet transits. From this, they were able to tell which planets were overcounted and which were undercounted.
This led to another exciting find, which was the indication that for all of the smaller exoplanets discovered by Kepler, most fell within one of two distinct groupings. Essentially, half the planets that we know of in the galaxy are either rocky in nature and larger than Earth (i.e. Super-Earth’s), or are gas giants that are comparable in size to Neptune (i.e. smaller gas giants).
This conclusion was reached by a team of researchers who used the W.M. Keck Observatory to measure the sizes of 1,300 stars in the Kepler field of view. From this, they were able to determine the radii of 2,000 Kepler planets with extreme precision, and found that there was a clear division between rocky, Earth-sized planets and gaseous planets smaller than Neptune – with few in between.
As Benjamin Fulton, a doctoral candidate at the University of Hawaii in Manoa and the lead author of this study, explained:
“We like to think of this study as classifying planets in the same way that biologists identify new species of animals. Finding two distinct groups of exoplanets is like discovering mammals and lizards make up distinct branches of a family tree.”
These results are sure to have drastic implications when it comes to knowing the frequency of different types of planets in our galaxy, as well as the study of planet formation. For instance, they noted that most rocky planets discovered by Kepler are up to 75% larger than Earth. And for reasons that are not yet clear, about half of them take on hydrogen and helium, which swells their size to the point that they become almost Neptune-sized.
These findings could similarly have significant implications in the search for habitable planets and extra-terrestrial life. As Mario Perez, Kepler program scientist in the Astrophysics Division of NASA’s Science Mission Directorate, said during the presentation:
“The Kepler data set is unique, as it is the only one containing a population of these near Earth-analogs – planets with roughly the same size and orbit as Earth. Understanding their frequency in the galaxy will help inform the design of future NASA missions to directly image another Earth.”
From this information, scientists will be able to know with a greater degree of certainty just how many “Earth-like” planets exist within our galaxy. The most recent estimates place the number of planets in the Milky Way at about 100 billion. And based on this data, it would seem that many of these are similar in composition to Earth, albeit larger.
Combined with a statistical models of how many of these can be found within a circumstellar habitable zone, we should have a better idea of just how many potentially-life-bearing worlds are out there. If nothing else, this should simplify some of the math in the Drake Equation!
In the meantime, the Kepler space telescope will continue to make observations of nearby star systems in order to learn more about their exoplanets. This includes the TRAPPIST-1 system and its seven Earth-sized, rocky planets. Its a safe bet that before it is finally retired after 2018, it will have some more surprises in store for us!
It has been an exciting time for exoplanet research of late! Back in February, the world was astounded when astronomers from the European Southern Observatory (ESO) announced the discovery of seven planets in the TRAPPIST-1 system, all of which were comparable in size to Earth, and three of which were found to orbit within the star’s habitable zone.
And now, a team of international astronomers has announced the discovery of an extra-solar body that is similar to another terrestrial planet in our own Solar System. It’s known as Kepler-1649b, a planet that appears to be similar in size and density to Earth and is located in a star system just 219 light-years away. But in terms of its atmosphere, this planet appears to be decidedly more “Venus-like” (i.e. insanely hot!)
Needless to say, this discovery is a significant one, and the implications of it go beyond exoplanet research. For some time, astronomers have wondered how – given their similar sizes, densities, and the fact that they both orbit within the Sun’s habitable zone – that Earth could develop conditions favorable to life while Venus would become so hostile. As such, having a “Venus-like” planet that is close enough to study presents some exciting opportunities.
In the past, the Kepler mission has located several extra-solar planets that were similar in some ways to Venus. For instance, a few years ago, astronomers detected a Super-Earth – Kepler-69b, which appeared to measure 2.24 times the diameter of Earth – that was in a Venus-like orbit around its host the star. And then there was GJ 1132b, a Venus-like exoplanet candidate that is about 1.5 times the mass of Earth, and located just 39 light-years away.
In addition, dozens of smaller planet candidates have been discovered that astronomers think could have atmospheres similar to that of Venus. But in the case of Kepler-1649b, the team behind the discovery were able to determine that the planet had a sub-Earth radius (similar in size to Venus) and receives a similar amount of light (aka. incident flux) from its star as Venus does from Earth.
However, they also noted that the planet also differs from Venus in a few key ways – not the least of which are its orbital period and the type of star it orbits. As Dr. Angelo told Universe Today via email:
“The planet is similar to Venus in terms of it’s size and the amount of light it receives from it’s host star. This means it could potentially have surface temperatures similar to Venus as well. It differs from Venus because it orbits a star that is much smaller, cooler, and redder than our sun. It completes its orbit in just 9 days, which places it close to its host star and subjects it to potential factors that Venus does not experience, including exposure to magnetic radiation and tidal locking. Also, since it orbits a cooler star, it receives more lower-energy radiation from its host star than Earth receives from the Sun.”
In other words, while the planet appears to receive a comparable amount of light/heat from its host star, it is also subject to far more low-energy radiation. And as a potentially tidally-locked planet, the surface’s exposure to this radiation would be entirely disproportionate. And last, its proximity to its star means it would be subject to greater tidal forces than Venus – all of which has drastic implications for the planet’s geological activity and seasonal variations.
Despite these differences, Kepler-1649b remains the most Venus-like planet discovered to date. Looking to the future, it is hoped that next-generations instruments – like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Telescope and the Gaia spacecraft – will allow for more detailed studies. From these, astronomers hope to more accurately determine the size and distance of the planet, as well as the temperature of its host star.
This information will, in turn, help us learn a great deal more about what goes into making a planet “habitable”. As Angelo explained:
“Understanding how hotter planets develop thick, Venus-like atmospheres that make them inhabitable will be important in constraining our definition of a ‘habitable zone’. This may become possible in the future when we develop instruments sensitive enough to determine chemical compositions of planet atmospheres (around dim stars) using a method called ‘transit spectroscopy’, which looks at the light from the host star that has passed through the planet’s atmosphere during transit.”
The development of such instruments will be especially useful given joust how many exoplanets are being detected around neighboring red dwarf stars. Given that they account for roughly 85% of stars in the Milky Way, knowing whether or not they can have habitable planets will certainly be of interest!
Last fall, astronomers were surprised when the Kepler mission reported some anomalous readings from KIC 8462852 (aka. Tabby’s Star). After noticing a strange and sudden drop in brightness, speculation began as to what could be causing it – with some going so far as to suggest that it was an alien megastructure. Naturally, the speculation didn’t last long, as further observations revealed no signs of intelligent life or artificial structures.
But the mystery of the strange dimming has not gone away. What’s more, in a paper posted this past Friday to arXiv, Benjamin T. Montet and Joshua D. Simon (astronomers from the Cahill Center for Astronomy and Astrophysics at Caltech and the Carnegie Institute of Science, respectively) have shown how an analysis of the star’s long-term behavior has only deepened the mystery further.
To recap, dips in brightness are quite common when observing distant stars. In fact, this is one of the primary techniques employed by the Kepler mission and other telescopes to determine if planets are orbiting a star (known as Transit Method). However, the “light curve” of Tabby’s Star – named after the lead author of the study that first detailed the phenomena (Tabetha S. Boyajian) – was particularly pronounced and unusual.
According to the study, the star would experience a ~20% dip in brightness, which would last for between 5 and 80 days. This was not consistent with a transitting planet, and Boyajian and her colleagues hypothesized that it was due to a swarm of cold, dusty comet fragments in a highly eccentric orbit accounted for the dimming.
However, others speculated that it could be the result of an alien megastructure known as Dyson Sphere (or Swarm), a series of structures that encompass a star in whole or in part. However, the SETI Institute quickly weighed in and indicated that radio reconnaissance of KIC 8462852 found no evidence of technology-related radio signals from the star.
Other suggestions were made as well, but as Dr. Simon of the Carnegie Institute of Science explained via email, they fell short. “Because the brief dimming events identified by Boyajian et al. were unprecedented, they sparked a wide range of ideas to explain them,” he said. “So far, none of the proposals have been very compelling – in general, they can explain some of the behavior of KIC 8462852, but not all of it.”
To put the observations made last Fall into a larger context, Montet and Simon decided to examine the full-frame photometeric images of KIC 8462852 obtained by Kepler over the last four years. What they found was that the total brightness of the star had been diminishing quite astonishingly during that time, a fact which only deepens the mystery of the star’s light curve.
As Dr. Montet told Universe Today via email:
“Every 30 minutes, Kepler measures the brightness of 160,000 stars in its field of view (100 square degrees, or approximately as big as your hand at arm’s length). The Kepler data processing pipeline intentionally removes long-term trends, because they are hard to separate from instrumental effects and they make the search for planets harder. Once a month though, they download the full frame, so the brightness of every object in the field can be measured. From this data, we can separate the instrumental effects from astrophysical effects by seeing how the brightness of any particular star changes relative to all its neighboring stars.”
Specifically, they found that over the course of the first 1000 days of observation, the star experienced a relatively consistent drop in brightness of 0.341% ± 0.041%, which worked out to a total dimming of 0.9%. However, during the next 200 days, the star dimmed much more rapidly, with its total stellar flux dropping by more than 2%.
For the final 200 days, the star’s magnitude once again consistent and similar to what it was during the first 1000 – roughly equivalent to 0.341%. What is impressive about this is the highly anomalous nature of it, and how it only makes the star seem stranger. As Simon put it:
“Our results show that over the four years KIC 8462852 was observed by Kepler, it steadily dimmed. For the first 2.7 years of the Kepler mission the star faded by about 0.9%. Its brightness then decreased much faster for the next six months, declining by almost 2.5% more, for a total brightness change of around 3%. We haven’t yet found any other Kepler stars that faded by that much over the four-year mission, or that decreased by 2.5% in six months.”
Of the over 150,000 stars monitored by the Kepler mission, Tabby’s Starr is the only one known to exhibit this type of behavior. In addition, Monetet and Cahill compared the results they obtained to data from 193 nearby stars that had been observed by Kepler, as well as data obtained on 355 stars with similar stellar parameters.
From this rather large sampling, they found that a 0.6% change in luminosity over a four year period – which worked out to about 0.341% per year – was quite common. But none ever experienced the rapid decline of more than 2% that KIC 8462852 experienced during that 200 days interval, or the cumulative fading of 3% that it experienced overall.
Montet and Cahill looked for possible explanations, considering whether the rapid decline could be caused by a cloud of transiting circumstellar material. But whereas some phenomena can explain the long-term trend, and other the short-term trend, no one explanation can account for it all. As Montet explained:
“We propose in our paper that a cloud of gas and dust from the remnants of a planetesimal after a collision in the outer solar system of this star could explain the 2.5% dip of the star (as it passes along our line of sight). Additionally, if some clumps of matter from this collision were collided into high-eccentricity comet-like orbits, they could explain the flickering from Boyajian et al., but this model doesn’t do a nice job of explaining the long-term dimming. Other researchers are working to develop different models to explain what we see, but they’re still working on these models and haven’t submitted them for publication yet. Broadly speaking, all three effects we observe cannot be explained by any known stellar phenomenon, so it’s almost certainly the result of some material along our line of sight passing between us and the star. We just have to figure out what!”
So the question remains, what accounts for this strange dimming effect around this star? Is there yet some singular stellar phenomena that could account for it all? Or is this just the result of good timing, with astronomers being fortunate enough to see a combination of a things at work in the same period? Hard to say, and the only way we will know for sure is to keep our eye on this strangely dimming star.
And in the meantime, will the alien enthusiasts not see this as a possible resolution to the Fermi Paradox? Most likely!