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KOI-872: Timing is Everything

It’s no surprise that the techniques we’re using to look for moons around exoplanets should start turning up new planets on their own. We’re still looking for that first exomoon, but a team of researchers working with the Hunt for Exomoons with Kepler (HEK) project has found transit variations that have revealed a second planet around a star already known to have one transiting planet. The star is the intriguing KOI-872 (KOI stands for Kepler Object of Interest), the data on which were recently released by the Kepler team and analyzed swiftly by HEK.

Kepler’s transit methods examine the change in starlight when an exoplanet passes in front of the star being observed. This lightcurve, however, can tell more than a single tale. David Kipping (Harvard-Smithsonian Center for Astrophysics) is head of HEK and second author of the paper on the new work, which was published online today in the journal Science:

“For a planet following a strictly Keplerian orbit around its host star, the spacing, timing and other properties of the observed transit light curve should be unchanging in time. Several effects, however, can produce deviations from the Keplerian case so that the spacing of the transits is not strictly periodic.”

Those effects are clear cut, changes in how long the transit takes (Transit Duration Variation, or TDV), and changes to when it actually occurs (Transit Timing Variation, or TTV). These deviations are sensitive to objects as small as 0.2 Earth masses, which shows their utility in hunting for large exomoons. In the case of the Sun-like star KOI-872, transits of the known planet show time variations of two hours. The TTVs are large and precise enough to allow scientists to calculate the orbit of the second planet. According to Kipping, both the transiting planet and the unseen planet perturbing its orbit are Saturn-class worlds. Moreover, they orbit close to a 5:3 mean motion resonance, accounting for the size of the timing variations.

Image: Using Kepler Telescope transit data of planet “b”, scientists predicted that a second planet “c” about the mass of Saturn orbits the distant star KOI-872. This research, led by Southwest Research Institute and the Harvard-Smithsonian Center for Astrophysics, is providing evidence of an orderly arrangement of planets orbiting KOI-872, not unlike our own solar system. Credit: Southwest Research Institute.

You may recall from last fall the story of Kepler-19b (discussed here in On Planets and What We Can See), a probable mini-Neptune some 650 light years away in the constellation of Lyra. Researchers at the Harvard-Smithsonian Center for Astrophysics were able to uncover a second world in the same system, Kepler-19c, by studying the transit timing variations of Kepler-19b. In this case, the timing variations were about five minutes long, but other than revealing the planet’s existence, they allowed no further deductions about its size or composition. In fact, the prospects — all consistent with the limited data available — ranged from a rocky planet on a circular five-day orbit to a gas giant on an oblong 100-day orbit.

But the new planet around KOI-872 is another matter entirely, and in Kipping’s words represents the transit timing variation model coming into full maturity, allowing us to measure the mass of the unseen planet and to determine the 5:3 orbital resonance. Moreover, we get tight constraints on the inclination, semi-major axis and eccentricity of both planets, which indicate they follow circular, co-planar orbits much like planets in our own Solar System. We now know that the unseen planet orbits its star every 57 days. Further Kepler observations will allow us to refine these details and tune up TTV and TDV methods for future planet and exomoon discoveries.

Image: Scientists analyzed Kepler Telescope data and identified KOI-872 as a stellar system where measured transits of a planet orbiting the star show large time variations (the shifting bumps in the data) indicative of a hidden companion. A team led by Southwest Research Institute and the Harvard-Smithsonian Center for Astrophysics determined that the observed variations can be best explained by an unseen planet about the mass of Saturn orbiting the host star every 57 days. Credit: Southwest Research Institute.

When the HEK team went to work on KOI-872, it was motivated in part by the fact that the transiting planet was large enough to hold on to a large moon of the size that current TTV techniques could uncover. The large TTVs yielded a planet rather than a moon in this case, but future finds of large moons around gas giants in the habitable zone will give us some idea as to whether single, rocky planets are the only likely venue for life. Are there as many habitable moons in the galaxy as habitable planets, or even more? HEK is the first step toward helping us find out.

The paper is Nesvorný, Kipping et al., “The Detection and Characterization of a Nontransiting Planet by Transit Timing Variations,” published online in Science May 10 2012 (abstract).


Comments on this entry are closed.

  • philw1776 May 10, 2012, 15:59

    Once again excellent science from Kepler, one of the best if not the best science missions ever flown. At last statistically significant data on real extra-solar planetary systems. Sadly ALL Kepler follow on NASA missions, TPF et. al. no longer exist. Not one. A sad commentary on NASA’s priorities. Gotta fund that SLS boondoggle which will soon go the way of Al Gore’s touted X-33 hangar Queen and the last cancelled NASA launch vehicle.

  • Interstellar Bill May 10, 2012, 21:18

    Hopes for habitable moons are even bleaker than hopes for habitable planets, if by that you mean Earth-normal oxygen, water, and metacellular life. Earth’s attainment of that state is a result of a long series of low-probability events and outright bottlenecks, the concatenation of which leads to hopelessly low, near-vanishing liklihood-levels. Not as low as SETI, of course, but you truly should just leave habitablity out of the exomoon question. In contrast to over a thousand explanets, we haven’t a single exomoon. When we have even a few dozen of them, then you can bring up habitability.

  • Daniel May 11, 2012, 13:45

    By the way It’s not about exoplanets specifically but still very interesting Article About More Brown Dwarfs Y and other types Discovery by WISE and updates of the entire stellar and substellar constinuency within 8 parsecs of the Sun:

    Further Defining Spectral Type “Y” and Exploring the Low-mass End of the Field Brown Dwarf Mass Function


  • philw1776 May 11, 2012, 15:28

    @Bill I am also a ‘Copernican multicellular life is common’ skeptic/heretic who feels that what little we do know best says that complex life is the outcome of “rare” events. But exomoons, habitable or not, aren’t known because the means we have of detecting planets far exceeds by orders of magnitude the detection sensitivity for exomoons.

    The same can be said of Earth sized objects in stars HZs. It’s far easier in SNR and in observation time to detect much larger objects closer to stars than any objects relatively far away in HZs, particularly small Earth sized ones. The extended Kepler mission will enable a higher SNR for HZ objects by allowing > 3 transit observations.

  • FrankH May 11, 2012, 16:36

    Agree with most of what you said, except for the X-33 story. The failure was almost exclusively that of NASA and the project managers, unwilling to listen to the engineers.

  • Joy May 12, 2012, 4:21

    Note that the unseen planet is coplanar with the transiting one and in a 57 day orbit, but does not itself transit. Likewise, our Venus transit next month is a relatively uncommon event, even though Earth and Venus are pretty close to coplanar. I know the geometry is straightforward and taken into account by all the people who do analysis of the Kepler data, but looking for transits of planets way out in the HZ of a Sol-like star is somewhat of a long shot, why Kepler has to look at 10^5 stars

  • philw1776 May 12, 2012, 8:21

    X-33 was a flawed engineering concept. You don’t see anyone even proposing to build H2 powered SSTO rockets, no margin for error or payload. The final killer was that they were unable to fabricate the composite H2 tanks.

  • A.B. Atwater May 12, 2012, 20:33

    Where in the galaxy is KOI 872?

  • Paul Gilster May 12, 2012, 20:45

    Kepler is focusing on a field in the northern constellations of Cygnus, Lyra and Draco.