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.
The study of extra-solar planets has revealed some fantastic and fascinating things. For instance, of the thousands of planets discovered so far, many have been much larger than their Solar counterparts. For instance, most of the gas giants that have been observed orbiting closely to their stars (aka. “Hot Jupiters”) have been similar in mass to Jupiter or Saturn, but have also been significantly larger in size.
Ever since astronomers first placed constraints on the size of a extra-solar gas giant seven years ago, the mystery of why these planets are so massive has endured. Thanks to the recent discovery of twin planets in the K2-132 and K2-97 system – made by a team from the University of Hawaii’s Institute for Astronomy using data from the Kepler mission – scientists believe we are getting closer to the answer.
Because of the “hot” nature of these planets, their unusual sizes are believed to be related to heat flowing in and out of their atmospheres. Several theories have been developed to explain this process, but no means of testing them have been available. As Grunblatt explained, “since we don’t have millions of years to see how a particular planetary system evolves, planet inflation theories have been difficult to prove or disprove.”
To address this, Grunblatt and his colleagues searched through the data collected by NASA’s Kepler mission (specifically from its K2 mission) to look for “Hot Jupiters” orbiting red giant stars. These are stars that have exited the main sequence of their lifespans and entered the Red Giant Branch (RGB) phase, which is characterized by massive expansion and a decrease in surface temperature.
As a result, red giants may overtake planets that orbit closely to them while planets that were once distant will begin to orbit closely. In accordance with a theory put forth by Eric Lopez – a member of NASA Goddard’s Science and Exploration Directorate – hot Jupiter’s that orbit red giants should become inflated if direct energy output from their host star is the dominant process inflating planets.
So far, their search has turned up two planets – K2-132b and K2-97 b – which were almost identical in terms of their orbital periods (9 days), radii and masses. Based on their observations, the team was able to precisely calculate the radii of both planets and determine that they were 30% larger than Jupiter. Follow-up observations from the W.M. Keck Observatory at Maunakea, Hawaii, also showed that the planets were only half as massive as Jupiter.
The team then used models to track the evolution of the planets and their stars over time, which allowed them to calculate how much heat the planets absorbed from their stars. As this heat was transferring from their outer layers to their deep interiors, the planets increased in size and decreased in density. Their results indicated that while the planets likely needed the increased radiation to inflate, the amount they got was lower than expected.
While the study is limited in scope, Grunblatt and his team’s study is consistent with the theory that huge gas giants are inflated by the heat of their host stars. It is bolstered by other lines of evidence that hint that stellar radiation is all a gas giant needs to dramatically alter its size and density. This is certainly significant, given that our own Sun will exit its main sequence someday, which will have a drastic effect on our system of planets.
As such, studying distant red giant stars and what their planets are going through will help astronomers to predict what our Solar System will experience, albeit in a few billion years. As Grunblatt explained in a IfA press statement:
“Studying how stellar evolution affects planets is a new frontier, both in other solar systems as well as our own. With a better idea of how planets respond to these changes, we can start to determine how the Sun’s evolution will affect the atmosphere, oceans, and life here on Earth.”
It is hoped that future surveys which are dedicated to the study of gas giants around red giant stars will help settle the debate between competing planet inflation theories. For their efforts, Grunblatt and his team were recently awarded time with NASA’s Spitzer Space Telescope, which they plan to use to conduct further observations of K2-132 and K2-97, and their respective gas giants.
The search for planets around red giant stars is also expected to intensify in the coming years with he deployment of NASA’s Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST). These missions will be launching in 2018 and 2019, respectively, while the K2 mission is expected to last for at least another year.
Since it was launched in 2009, NASA’s Kepler mission has continued to make important exoplanet discoveries. Even after the failure of two reaction wheels, the space observatory has found new life in the form of its K2 mission. All told, this space observatory has detected 5,017 candidates and confirmed the existence of 2,494 exoplanets using the Transit Method during its past eight years in service.
The most recent discovery was made by an international team of astronomers around Gliese 9827 (GJ 9827), a late K-type dwarf star located about 100 light-years from Earth. Using data provided by the K2 mission, they detected the presence of three Super-Earths. This star system is the closest exoplanet-hosting star discovered by K2 to date, which makes these planets well-suited for follow-up studies.
The Transit Method, which remains one of the most trusted means for exoplanet detection, consists of monitoring stars for periodic dips in brightness. These dips correspond to planets passing (aka. transiting) in front of the star causing a measurable drop in the light coming from it. This method also offers unique opportunities to examine light passing through an exoplanet’s atmosphere. As Dr. Rodriguez told Universe Today via email:
“The success of Kepler combined with ground based radial velocity and transit surveys has now led to the discovery of over 4000 planetary system. Since we now know that planets appear to be quite common, the field has shifted its focus to understand architectures, interior structures, and atmospheres. These key properties of planetary systems help us understand some fundamental questions: how do planets form and evolve? What are the terrestrial planets around other stars like, are they similar to Earth in composition and atmosphere?”
These questions were central to the team’s study, which relied on data obtained during Campaign 12 of the K2 mission – from December 2016 to March 2017. After consulting this data, the team noted the presence of three super-Earth sized planets orbiting in a very compact configuration. This system, as they note in their study, was independently and simultaneously discovered by another team from Wesleyan University.
These three planetary objects, designated as GJ 9827 b, c, and d, are located at a distance of about 0.02, 0.04 and 0.06 AU from their host star (respectively). Owing to their sizes and radii, these planets are classified as “Super-Earths”, and have radii of 1.6, 1.2, and 2.1 times the radius of Earth. They are also located very close to their host star, completing orbits within 6.2 days.
Specifically, GJ 9827 b measures 1.64 Earth radii, has a mass of up to 4.25 Earth masses, a 1.2 day orbital period, and a temperature of 1,119 K (846 °C; 1555 °F). Meanwhile, GJ 9827 c measures 1.29 Earth radii, has a mass of 2.62 Earth masses, an orbital period of 3.6 days, and a temperature of 774 K (500 °C; 934°F). Lastly, GJ 9827 d measures 2.08 Earth radii, has a mass of 5.3 Earth masses, a 6.2 day period, and a temperature of 648 K (375 °C; 707 °F).
In short, all three planets are very hot, with temperatures that are hot as Venus and Mercury or (in the case of GJ 9827b) is even hotter! Interestingly, these radii and mass estimates place these planets within the transition boundary between terrestrial (i.e. rocky) planets and gas giants. In fact, the team found that GJ 9827 b and c fall in or close to the known gap in radius distribution for planets that are in between these two populations.
In other words, these planets could be rocky or gaseous, and the team won’t know for sure until they can place more accurate constraints on their masses. What’s more, none of these planets are likely to be capable of supporting life, certainly not as we know it! So if you were hoping that this latest find would produce an Earth-analog or potentially habitable planet, you’re sadly mistaken.
Nevertheless, the fact that these planets straddle the radius and mass boundary between terrestrial and gaseous planets – and the fact that this system is the closest planetary system to be identified by the K2 mission – makes the system well-situated for studies designed to probe the interior structure and atmosphere of exoplanets.
The reason for this has much to do with the brightness of the host star. In addition to being relatively close to our Sun (~100 light-years), this K-type star is very bright and also relatively small – about 60% the size of our Sun. As a result, any planet passing in front of it would be able to block out more light than if the star were larger. But as noted, there’s also the curious nature of the planets themselves. As Dr. Rodriguez indicated:
“Recently, we have found planets around other stars that have no analogue to a planet in our own system. These are known as “super Earths” and they have radii of 1-3 times the radius of the Earth. To add to the complexity of these planets, their is a clear dichotomy in their composition within this radius range. The larger super Earths (>1.6 x radius of the Earth) appear to be less dense, consistent with a puffy Hydrogen/Helium atmosphere. However, the smaller super Earths are more dense, consistent with an Earth-like composition (rock).
“As mentioned above, the GJ 9827 system hosts three super Earth sized planets. Interestingly, planet c has a radius consistent with it being rocky, planet d is consistent with being puffy, and planet b has a radius that is right on what we believe to be the transition boundary between rock and gas. Therefore, by studying the atmospheres of super-Earths, we may better understand the transition from dense rocky planets to puffier planets with very thick atmospheres (like Neptune).”
Looking ahead, the team hopes to conduct further studies to determine the masses of these planets more precisely. From this, they will be able to place better constraints on their compositions and determine if they are Super-Earths, mini gas giants, or some of each. Beyond that, they are to conduct more detailed studies of this system with next-generation instruments like the James Webb Space Telescope (JWST), which is scheduled to launch in 2018.
“I am really interested in studying the atmosphere of GJ 9827 b, whether it is rocky or puffy,” said Dr. Rodriguez. “This planet has a radius at the rock/gas transition but it is very close to its host star. Therefore, by studying the chemical composition of its atmosphere we may better understand the impact of the host star’s proximity has on the evolution of its atmosphere.To do this we would use JWST to take spectroscopic observations during the transit of GJ 9827b (known as “Transmission Spectroscopy”). From this observations we will gather information on the chemical composition and extent of the planet’s atmosphere.“
Now that we have thousands of extra-solar planet discoveries under our belt, its only natural that research would be shifting towards trying to understand these planets better. In the coming years and decades, we are likely to learn volumes about the respective structures, compositions, atmospheres, and surface features of many distant worlds. One can only imagine what kind of things these studies will turn up!