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Kepler and the Odds

The Kepler launch is coming up on March 5, marking the first time we will have the ability to find a true Earth analogue around another star; i.e., a planet of about Earth’s mass in the habitable zone where water can exist in liquid form on the surface. Which is not to say that COROT may not come close, though Kepler’s enormous star-field (100,000 targets in the Cygnus-Lyra region) and incredibly sensitive camera — a 95-megapixel array of charged coupled devices (CCDs) — is optimized for planets down to Earth size rather than larger ‘super-Earths.’


Image (click to enlarge): Kepler’s target region, the Milky Way ni the Cygnus region, with the instrument’s field of view superimposed. Each rectangle indicates the specific region of the sky covered by each CCD element of the Kepler photometer. There are a total of 42 CCD elements in pairs, each pair comprising a square. Credit: NASA/Carter Roberts (1946-2008).

We just looked at Alan Boss’ remarkable statement that there could be 100 billion trillion Earth-like planets in the visible universe. It’s startling to think that a mission to be launched within weeks could so quickly give us a chance to size up the idea. The thinking is that dozens of planets like ours in the habitable zone should be visible to Kepler if such worlds are common, but if it comes up short, with few or none, we’re going to quickly re-evaluate how unusual a world we live on. A null result would be striking indeed.

William Borucki, who is science principal investigator on the mission, has this to say:

“Finding that most stars have Earths implies that the conditions that support the development of life could be common throughout our galaxy. Finding few or no Earths indicates that we might be alone.”

But let’s get more specific about terrestrial, as opposed to gas giant planets. The Kepler site posts its own numbers, assuming only orbits with four transits in 3.5 years (the mission duration), and assuming that such planets are common around other stars. Kepler should find fifty terrestrial planets in one-year orbits if most are roughly the size of the Earth, and about 185 planets if most are roughly 1.3 the size of Earth’s radius. The number goes up to 640 if most such planets have a size of 2.2 Earth radii, and goes substantially higher still if orbits ranging from a few days to more than a year are considered.

Kepler is looking for transits in those systems where planets cross in front of their stars as seen from Earth, using an instrument that can detect brightness changes of twenty parts per million. After launch, the spacecraft will undergo a two-month checkout as it achieves its Earth-trailing orbit around the Sun. Eventually, Kepler’s science data will be sent to NASA’s Deep Space Network on a monthly basis, with data analysis performed at Ames Research Center in California. We’re obviously going to have much to say about Kepler in coming months.

Comments on this entry are closed.

  • tacitus February 20, 2009, 12:24

    It’s worth noting that there likely won’t be any announcements of Earth-like planets for at least two more years, and probably three, even under the best of circumstances. At the press conference yesterday, the team was saying that they are going to wait until they have detected three transits of the same planet before they publish a positive result.

    All being well, we’ll get to hear about lots of planets like the hot Jupiters we already know about, after the first few months, but we’re going to have to wait till at least 2011 until we get announcements of habitable zone planets, the first of which will likely be around red dwarfs, because the habitable zone is closer in, and cooler stars make for more easily detectable planets.

    Hearing the scientists talking about “yearly updates” of the results wasn’t exactly thrilling (and you could tell the journalists weren’t terribly enthused by the idea) but that’s the nature of the mission and there’s not much anyone can do about that.

  • Adam February 20, 2009, 17:31

    A habitable zone planet around a red dwarf like Proxima Centauri would do three orbits of its star in 25 days, so a positive detection might be quick indeed.

  • andy February 20, 2009, 20:16

    While the potential for discovering habitable planets is definitely one of the major points with these transit search missions, there are other interesting possibilities which may be harder to dig out of the datasets: planetary transits across eclipsing binary stars for example, and maybe the first hints of exomoons or perhaps even ring systems.

    Given that the main limitation on the rate of COROT discoveries appears to be resources for radial velocity followup, what’s the situation with Kepler and RV confirmation?

  • Administrator February 20, 2009, 21:08

    A bit more from the Kepler site discusses the question of confirmation:

    Radial velocity data also serve to explore or delineate the structure of the planetary systems by detecting giant planets not seen in transit or by reflected light. Normally, such data only yield the quantity, M sini, where M is the planetary mass and i the orbital inclination. The presence of transits implies that i ~90°, which establishes the mass of the planet. By sampling the Doppler shift a few times during an orbit, the orbital parameters of those planets showing transits are completely determined and the planet mass is established to within 3%. The mass (from spectroscopy) and planet radius (from photometry) yield the density of the planet.

    Kepler team member Marcy has developed the technique to measure velocities to 3 m/s on Keck (Marcy, et. al. 2000). Kepler team members also have institutional access to the proven Hamilton Echelle (Basri) and to new spectrometers on the Hobby-Eberly Telescope (Cochran) and the MMTO (Latham) all with <10 m/s capability.

    More here:


  • Eric SECT February 20, 2009, 22:55

    I am kind of disappointed in the COROT results so far, I expected more exo-planets by now… or are they just now starting to mine their data?

  • Maciej February 21, 2009, 7:03


    it is understandable that we must wait 3 years before transit events of earth like planets are going to be officially confirmed. However, taking into account human factor we can predict that in case of massive double detections rumors will spread that earths are common at least one year before official confirmation. I can hardly imagine that after detecting 500 candidates everybody in the Kepler team will be patient enough to keep secret. Moreover we can draw some conclusions on earths frequency from those data that are going to be revealed earlier. If it comes out that hot jupiters are common probably not much space for earth will remain. if opposite however…

    just speculating. we will see

  • neurogalactus February 21, 2009, 16:29

    Hi all,

    Sorry for straying off the topic a bit, but do you have any idea where I could find information on the axial relation between Alpha Centauri orbital disks and our Solar System disk?? 3D pics would be best in this case. In other words, I’d like to know how the orbital planes of the Solar System and A-Cen binary are positioned in relation to each other. Is there any data on that?

    Thanx in advance!

  • ljk February 24, 2009, 1:05

    T.I.P.O. (Tesla Interferometric Planetary Observer)

    Authors: Adrian Sabin Popescu

    (Submitted on 23 Feb 2009)

    Abstract: In the last years the Space Science community was confronted to a continuous increasing interest in Martian missions, extra-solar planet search and multi-satellite missions. The presented T.I.P.O. mission is a proposal for a research program dedicated to study, by space borne interferometric methods, the radio emissions generated in the atmospheres and magnetospheres of planets, both solar and extra-solar.

    Comments: 44 pages, 5 figures

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

    Cite as: arXiv:0902.3902v1 [astro-ph.EP]

    Submission history

    From: Adrian Sabin Popescu [view email]

    [v1] Mon, 23 Feb 2009 15:12:17 GMT (557kb)


  • David Carpenter March 7, 2009, 1:26

    Just watched NASA channel Kepler launch. Different than any I have seen, last telemetry info was 1200 miles down range at 105 miles altitude and 15,500 mph, second stage engine cut off & satellite running out to LOS range. Launch crew looked a bit nervous. I assume an engine was again firing during LOS time to attain orbit (~25,400 mph).

    Kepler was re-acquired at 11:40 PM (Outback tracking). No more telemetry given for further 2nd stage & 3rd stage burns. Is Kepler in a 105 mile circular earth orbit at this time?

    Anyone know when Kepler will be parked (orbit adjustment) at Lagrange point? Is that done with LOX & H2 engine fired at perigee? When? Nice launch but ran out of information for me.
    dave carpenter
    Clarksville MD 21029

  • Administrator March 7, 2009, 17:04

    David, Kepler is going to be in an Earth trailing heliocentric orbit slowly drifting away from the Earth. You can see an orbital diagram here:


  • Kevin G March 8, 2009, 15:00

    “Kepler is going to be in an Earth trailing heliocentric orbit slowly drifting away from the Earth.”

    Can anyone explain why things in this orbit don’t ‘fall back to earth’?

    My understanding is that most satellites are placed in orbit around earth so that the ‘in orbit’ part stops them from falling back down.

    If Kepler is just sitting there next to the earth, what stops the earths gravity from pulling it back in?


  • Administrator March 8, 2009, 19:22

    Kevin, it’s an understandable question, but remember that Kepler has been given enough velocity to make a return to the Earth out of the question. It will not be sitting next to the Earth for long, but is moving slowly away from the Earth, on an orbit around the Sun of its own. Putting it into that orbit meant getting past escape velocity from our planet so that it could take up its own orbit.

    In other words, it’s not in orbit around the Earth, but around the Sun.

  • ljk March 15, 2009, 20:06

    Keck Compliments Kepler


    With the successful launch of NASA’s Kepler mission, astronomers will now use the powerful instrument alongside Hawaii’s W. M. Keck telescopes to increase their chances of finding planets beyond our solar system. Together, the two sets of telescopes will examine new planets to determine whether or not they are habitable.

  • ljk March 18, 2009, 13:01


    Jonathan’s Space Report

    No. 608 2009 Mar 18 Somerville, MA


    The Kepler space telescope was launched on Mar 7 into a hyperbolic
    Earth escape trajectory. Kepler is a Schmidt-type telescope with a 1.4-meter
    primary mirror, and will stare at one field in Cygnus, between
    Deneb and Vega, monitoring stars to search for transiting planets.

    The Delta 7925-10L rocket put Kepler in a 185 x 185 km x 28.5 deg
    parking orbit at 0359 UTC; at 0443 second stage restart put it in a 174
    x 2815 km x 28.5 deg intermediate orbit, quickly followed at 0445 by
    third stage burn to escape trajectory. The second stage depletion burn
    then put the empty stage in a 2246 x 7360 km x 28.55 deg disposal orbit.

    Kepler will leave the Earth-Moon system quite slowly on a
    185 x -1372000 km x 28.5 deg hyperbola, passing lunar orbit
    at 0420 UTC on Mar 9, and leaving the sphere of influence at 2130 UTC on
    Mar 13. By Apr 20 range will be 4.5 million km and Kepler will be
    in a 0.967 x 1.041 AU x 0.5 deg heliocentric ecliptic orbit.

    (technical note: the negative apogee I quote above is a formal, unphysical,
    representation that preserves the usual equations for semi-major axis
    and eccentricity; it corresponds to a C3 of 0.60 km^2/s^2).

    Joining Kepler in orbit around the sun are the PAM-D (Star 48) third
    stage motor, two despin weights, and the telescope dust cover which will
    be ejected around Mar 27.

    SpaceTrack has issued element sets for Kepler and the PAM-D (34380
    and 34381) showing the Delta disposal orbit – these are spurious, since
    the Delta (34382) only moved to this orbit after separating from 34380
    and 34381.

  • ljk April 8, 2009, 12:56

    April 8, 2009

    Kepler Flips Its Lid; Soon Ready for Planet Hunt

    Written by Nancy Atkinson

    Engineers successfully ejected the dust cover from NASA’s Kepler telescope last night and the space observatory will soon begin searching for Earth-like planets.

    “The cover released and flew away exactly as we designed it to do,” said Kepler Project Manager James Fanson from JPL.

    “This is a critical step toward answering a question that has come down to us across 100 generations of human history — are there other planets like Earth, or are we alone in the galaxy?”

    Full article here: