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Kepler: Thoughts on K2

As we start thinking ahead to the TESS mission (Transiting Exoplanet Survey Satellite), currently scheduled for launch in 2017, the exoplanet focus sharpens on stars closer to home. The Kepler mission was designed to look at a whole field of stars, 156,000 of them extending over portions of the constellations Cygnus, Lyra and Draco. Most of the Kepler stars are from 600 to 3000 light years away. In fact, fewer than one percent of these stars are closer than 600 light years, while stars beyond 3000 light years are too faint for effective transit signatures.

Kepler has proven enormously useful in helping us develop statistical models on how common planets are, with the ultimate goal, still quite a way off, of calculating the value of ηEarth (Eta_Earth) — the fraction of stars orbited by planets like our own. Looking closer to home will be the mandate of TESS, which will be performing an all-sky survey rather than the ‘long stare’ Kepler has used so effectively. We should wind up with a growing catalog of nearby main-sequence stars hosting planets, a catalog that future missions will exploit.

But what of Kepler itself? It’s heartening to see a mission rescued when onboard problems have threatened its survival, and in the case of this doughty space telescope, the revival is almost as interesting as the latest results. The Kepler primary mission ended with the failure of the second of four reaction wheels that were used to stabilize the spacecraft. Kepler needed three reaction wheels for effective pointing accuracy, but ground controllers found a way to use the pressure of sunlight as a kind of ‘virtual’ reaction wheel to control the instrument.

Thus we get K2, the extended Kepler mission. Software to correct for drift in the field of view allows the telescope to achieve about half the photometric precision of the original Kepler mission, according to this University of Hawaii news release. A new exoplanet find — HIP 116454 b — demonstrates that the method works and that Kepler is, at least to some extent, back in business. Unlike most of the stars in the original Kepler field of view, HIP 116454 is relatively nearby, at 180 light years from Earth in the constellation Pisces.


Image: This artist’s conception portrays the first planet discovered by the Kepler spacecraft during its K2 mission. A transit of the planet was teased out of K2’s noisier data using ingenious computer algorithms developed by a researcher at the Harvard-Smithsonian Center for Astrophysics (CfA). The newfound planet, HIP 116454 b, has a diameter of 32,000 kilometers. It orbits its star once every 9.1 days. Credit: CfA.

Working with K2 is exceedingly tricky business, with NASA likening the use of solar photons to stabilize the instrument to the act of balancing a pencil on a finger, a manageable but demanding task. Nor was HIP 116454 b an easy catch. For one thing, only a single transit was detected in the engineering data that was being used to prepare the spacecraft for the full K2 mission. That called for confirmation from ground-based observatories, which came from the Robo-AO instrument mounted on the Palomar 1.5-meter telescope and additional follow-up from the Keck II adaptive optics system on Mauna Kea.

We learn that HIP 116454 b is about two and a half times the size of Earth, with a diameter of about 32000 kilometers and a mass twelve times that of our planet. The paper on this work speculates that we are either dealing with a water world or a mini-Neptune with a gaseous envelope. The planet’s orbital period around the K-class host is 9.1 days at a distance of 13.5 million kilometers. As a ‘super-Earth,’ it will be a high-value target for future observation:

HIP 116454 b could be important in the era of the James Webb Space Telescope (JWST) to probe the transition between ice giants and rocky planets. In the Solar system, there are no planets with radii between 1–3 R while population studies with Kepler data have shown these planets to be nearly ubiquitous… Atmospheric studies with transit transmission spectroscopy can help determine whether these planets are in fact solid or have a gaseous envelope, and give a better understanding on how these planets form and why they are so common in the Galaxy.

The planet will also be useful in relation to another similar world:

Also of interest is the fact that HIP 116454 b is very similar to HD 97658 b, in terms of its orbital characteristics (both are in ∼ 10 day low-eccentricity orbits), mass and radius (within 10% in radius, and within 25% in mass), and stellar hosts (both orbit K–dwarfs). Comparative studies of these two super–Earths will be valuable for understanding the diversity and possible origins of close–in Super–Earths around Sun–like stars.

We know from this work that K2 can find transiting planets despite the loss of precision the telescope has suffered, and the paper points out that the engineering test field in which HIP 116454 b was found as well as other potential fields of view will enable observatories in both hemispheres to view the stars for follow-up, something that was not possible in the original Kepler mission. So with K2 capability, we can see HIP 116454 b, relatively nearby, as a kind of bridge between the original Kepler statistical studies and the nearby targets of the TESS mission to follow.

The paper is Vanderburg et al., “Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454,” accepted for publication in The Astrophysical Journal (preprint).


Comments on this entry are closed.

  • Mark Zambelli December 29, 2014, 12:13

    I was overjoyed to hear about the revival plans and am amazed that the K2 mission is well underway. I can’t help but wonder… can this system of using solar light-pressure as a virtual gyro help us learn how to attitude-control any future light/mag sails? (a perfect case of ‘on-the-job-training’). Or is this special case too disimilar to benefit much from?
    Either way, it’s good to have Kepler back with us again.

  • Alex Tolley December 29, 2014, 13:55

    So the raw data was for just one transit. Follow up observations from the ground were used to confirm the planet. Does this mean that we could apply the same sort of analysis to the original Kepler data to find candidates with larger orbits, or does this work only for closer stars with close orbiting planets?

  • Ashley Baldwin December 29, 2014, 16:03

    They did say they would accept two rather than the usual three transits for proof of existence given the targets were nearer in this viewing field and as a result in reach of Doppler confirmation . We should probably all accept a higher false positive rate from the great enemy of transit astronomy , the contact binary. Better once uber sensitive ESPRESSO spectrograph up and running in 18 months . The “two transit” confirmation also allows for longer orbital periods for discovered planets to partially compensate for the shorter 75 day maximum “stare” at the field of view. So some might just be in the HBZ of smaller stars and the sample will be more representative of the planetary population as Jim says, rather than all being deep transit “roasters”. Be interesting when we see the first Gaia astrometry planets. Promised for so long , hopefully this method will finally come good and we will be inundated with exoplanets over the next ten years. If TESS can last just over 5 years it will roll nicely into WFIRST ( which got a Mission scientist today-good sign) and PLATO.

  • kamal ali December 30, 2014, 1:59

    this is why i’m interested in astronomy (& other science): with all the things going on in our world, the ingenuity of scientists in delivering this kind of unambiguous advance is necessary for all our spirits.
    well done K2 team!
    cant wait for TESS, JWST…
    pixel-images of near earth-size planets?!

  • Ashley Baldwin December 30, 2014, 19:16

    Using data from a smaller number of transits is only possible for planets sufficiently near to have their existence confirmed by the alternative RV spectroscopy method. The K2 field is relatively nearby stars .This was the big issue with the original Kepler target stars which were much further away than this in the current fields of view so could only be confirmed by transit photometry thus the more stringent three confirmed transits for proof of existence. It also meant their masses couldn’t be constrained and consequently their bulk densities too. The draw back is that the telescope has to be realigned every 75 days to avoid sun damage so the maximum number of transits in that time is two, with a consequent orbital period of 37.5 days , ruling out habitable zone planets for all but late M dwarf stars( M3 and above).
    This led the thinking behind the TESS targets ,all of which will be near enough to be confirmed by RV and thus will also only require two transits , possibly even one for very “deep” transit signals . They know the discovery will be followed up so can accept the risk of a false positive.

  • ljk December 31, 2014, 11:21

    Planet Hunters Plot Course for Inhabited Worlds

    Researchers aim to set aside differences in search for life on distant worlds

    December 29, 2014 |By Alexandra Witze and Nature magazine

    Scott Gaudi is tired of the fighting. An astronomer at Ohio State University in Columbus, he specializes in the notoriously fractious field of exoplanet research, in which battles have included bitter fights over data access and epic rifts between teams searching for planets outside our Solar System.

    On January 4 in Seattle, Washington, Gaudi will take a tentative first step towards corralling this rowdy bunch. As chair of NASA’s Exoplanet Exploration Program Analysis Group, he will try to nudge a roomful of US exoplanet scientists into generating a coherent, specific vision for where the field should go.

    The time is right. Researchers have almost finished combing through the thousands of leads that were produced by NASA’s planet-hunting Kepler spacecraft between 2009 and 2013, and are squeezing some more data out of the craft’s limited ‘K2’ mission extension (see Nature 514, 414–415; 2014). By the mid-2020s, budgets permitting, astronomers expect to have a satellite called the Wide-Field Infrared Survey Telescope (WFIRST) busy cataloguing planets that are too far away from their host stars for Kepler to have spotted them.

    Together, Kepler and WFIRST will produce a rough census of how many planets there are in our Galaxy. But NASA has yet to work out how to tackle the next, more crucial questions: could anything actually live on any of these planets? And what will it take to understand a given world’s chances of being habitable?

    “The big thing we’re wondering now is: what is it that we want to do after WFIRST?” says Gaudi.

    Full article here:


  • ljk January 3, 2015, 15:23

    Kepler Candidate Planets Searched for Signs of Intelligent Life

    by Nola Taylor Redd, SPACE.com Contributor | January 02, 2015 07:00 am ET

    A recent Search for Extraterrestrial Intelligence (SETI) effort studied 86 candidates in the Kepler space observatory’s field for radio signals that could potentially indicate the presence of an intelligent civilization.

    Of course, no radio signals were found, but the search did identify the most promising Kepler objects for wide-band observations using the Green Bank Telescope in West Virginia.

    “The 86 target stars were selected because they hosted planets discovered by [the year] 2011 with properties that could be conducive to the development of life,” said Abhimat Gautam, of the University of California, Berkeley.

    Full article here:


    I know I should be grateful that SETI is being conducted at all, but so long as we keep doing only the method and targets that Frank Drake started with in 1960 we will be waiting a very long time for a signal. I have said before that many SETI projects are token efforts and this seems just like another one.

    Does anyone know if the ATA is back to doing real SETI or not? The fact that I have to ask and have not seen any recent news about them, especially as I look for such news items, says a lot.

  • ljk January 5, 2015, 11:31

    January 02, 2015

    New Kepler Discovery: Five Alien Planets in Ancient Star System from the Dawn of the Milky Way

    Kepler’s ultra-precise, long-duration photometry is ideal for detecting systems with multiple transiting planets. These systems provide important data for understanding the dynamics, formation, and evolution of planetary systems. The fascinating YouTube video below presents a detailed analysis of an extremely compact and old five-planet system around a seismic K dwarf (KOI-3158, HIP 94931).

    KOI-3158 is a bright, high-proper motion, main-sequence star of spectral type K0. Its overabundance of alpha elements and peculiar kinematics make it a member of the Galactic thick disk. Interestingly enough, it belongs to the Arcturus stellar stream, a moving group originally thought to be of extragalactic origin, but nowadays interpreted as arising from dynamical perturbations within the Galaxy.

    Full article here:


  • ljk January 8, 2015, 13:48

    No Alien Signals Detected From Kepler’s New Exoplanet

    by Ian O’Neill, Discovery News | January 08, 2015 11:00 am ET

    SETI has pointed a powerful radio telescope array at the Kepler Space Telescope’s most recent exoplanetary discovery in the hope of detecting an artificially-generated radio signal. Alas, the world is not transmitting, and probably isn’t home to an extraterrestrial intelligence.

    SETI, which stands for the Search for Extraterrestrial Intelligence, utilized the Allen Telescope Array (ATA), located in at Hat Creek Radio Observatory, 290 miles northeast of San Francisco, Calif., to study HIP 116454b, an exoplanet 180 light-years away in the constellation Pisces. The exoplanet, that measures 2.5 times the diameter of Earth (and is therefore defined as a ‘super-Earth’), was the first discovery made by Kepler in its rebooted “K2” mission.

    Full article here:


  • ljk January 19, 2015, 16:08


    On the abundance of extraterrestrial life after the Kepler mission

    Amri Wandel

    (Submitted on 3 Dec 2014)

    The data recently accumulated by the Kepler mission have demonstrated that small planets are quite common and that a significant fraction of all stars may have an Earth-like planet within their Habitable Zone. These results are combined with a Drake-equation formalism to derive the space density of biotic planets as a function of the relatively modest uncertainty in the astronomical data and of the (yet unknown) probability for the evolution of biotic life, Fb.

    I suggest that Fb may be estimated by future spectral observations of exoplanet biomarkers. If Fb is in the range 0.001 — 1 then a biotic planet may be expected within 10 — 100 light years from Earth. Extending the biotic results to advanced life I derive expressions for the distance to putative civilizations in terms of two additional Drake parameters – the probability for evolution of a civilization, Fc, and its average longevity.

    For instance, assuming optimistic probability values (Fb Fc 1) and a broadcasting longevity of a few thousand years, the likely distance to the nearest civilizations detectable by SETI is of the order of a few thousand light years. The probability of detecting intelligent signals with present and future radio telescopes is calculated as a function of the Drake parameters. Finally, I describe how the detection of intelligent signals would constrain the Drake parameters.

    Comments: 12 pages, 3 figures, accepted for publication, International Journal of Astrobiology 2015

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1412.1302 [astro-ph.EP]
    (or arXiv:1412.1302v1 [astro-ph.EP] for this version)

    Submission history

    From: Amri Wandel [view email]

    [v1] Wed, 3 Dec 2014 12:40:34 GMT (600kb)