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Closing on Earth 2.0?

The eight ‘habitable zone’ planets we discussed yesterday appear today in a much broader context. The Kepler mission has verified its 1000th planet, and with the detection of 554 more planet candidates, the total candidate count has now reached 4175. According to this NASA news release, six of the new planet candidates are near-Earth size and orbit in the habitable zone of stars similar to the Sun. These all require follow-up observation to confirm their status as planets, but with confirmed planets like Kepler-438b and Kepler-442b, along with these further candidates in the habitable zone, the numbers keep inching us closer to an Earth 2.0.

“Kepler collected data for four years — long enough that we can now tease out the Earth-size candidates in one Earth-year orbits,” says Fergal Mullally, a SETI Institute Kepler scientist at Ames who led the analysis of a new candidate catalog. “We’re closer than we’ve ever been to finding Earth twins around other sun-like stars. These are the planets we’re looking for.”

The next catalog release of the Kepler data set is now in the works, an analysis that will include the final month of mission data put through a new and more sensitive software iteration, one that should be able to tease out still further signatures of Earth-size planets.


Image: NASA Kepler’s Hall of Fame: Of the more than 1,000 verified planets found by NASA’s Kepler Space Telescope, eight are less than twice Earth-size and in their stars’ habitable zone. All eight orbit stars cooler and smaller than our Sun. The search continues for Earth-size habitable zone worlds around Sun-like stars. Credit: NASA/Kepler team.

The paper on the recently validated ‘habzone’ planets has now reached arXiv, where you can find the preprint of Torres et al., “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone.” I want to pull a paragraph out of this paper with regard to the BLENDER analysis I referred to yesterday. BLENDER is a powerful tool using software analysis of false-positive scenarios that is particularly useful when planets are so far away that radial velocity confirmation is difficult. A planetary transit can be mimicked by other phenomena, such as background or foreground eclipsing binary stars, whose light can create a ‘blend’ of signals that has to be untangled. The paper describes how BLENDER can rule false positives out:

BLENDER makes full use of the detailed shape of the transits to limit the pool of viable blends. It does this by simulating large numbers of blend scenarios and comparing each of them with the Kepler photometry in a χ2 sense. Fits that give the wrong shape for the transit are considered to be ruled out. This enables us to place useful constraints on the properties of the objects that make up the blend, including their sizes and masses, overall color and brightness, the linear distance between the background/foreground eclipsing pair and the target, and even the eccentricities of the orbits. Those constraints are then used to estimate the frequencies of blends of different kinds.


Image: False positive scenarios of the kind BLENDER can help resolve. Credit: Calar Alto Observatory/J. Lillo-Box.

BLENDER’s simulated lightcurves, along with other tools like high-resolution spectroscopy, allow tricky catches like Kepler-438b and Kepler-442b to be validated. The process is exhaustive and highlights just how far we’re progressing in our ability to find these small worlds. The paper presents the current state of the art in action, a fascinating and encouraging account that has been accepted for publication in The Astrophysical Journal.

Also apropos of this week of planetary announcements keyed to the American Astronomical Society meeting in Seattle is work that Courtney Dressing (Harvard-Smithsonian Center for Astrophysics) presented on Monday. Using the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands, Dressing and Harvard astronomer David Charbonneau have been examining the ten known exoplanets with diameter less than 2.7 times that of Earth that have accurately measured masses. The result: The five planets with diameters smaller than 1.6 that of Earth showed a tight relationship between mass and size.

“To find a truly Earth-like world,” says Dressing, “we should focus on planets less than 1.6 times the size of Earth, because those are the rocky worlds.”

In this study, larger and more massive exoplanets showed significantly lower densities, an indication that they include a large amount of water or other volatiles like hydrogen or helium. This CfA news release notes that Dressing and Charbonneau do not believe that all planets less than six times the mass of Earth are necessarily rocky — planets like those in the Kepler-11 system show both low mass and low density. But the assumption here is that the average low-mass planet orbiting near its star has a high chance of having a rocky composition like the Earth.

The paper on this work, which has been accepted for publication at The Astrophysical Journal, isn’t yet up on arXiv, but I’ll publish the reference when it becomes available. Meanwhile, the paper by Laura Schaefer and Dimitar Sasselov on ‘super-Earth’ oceans, examined here on Monday, has now appeared. It’s “The persistence of oceans on Earth-like planets: insights from the deep-water cycle,” likewise accepted for publication at The Astrophysical Journal (preprint).


Comments on this entry are closed.

  • Harry R Ray January 7, 2015, 10:30

    The illustration above is already out of date, according to HEC with respect to Kepler 296e and Kepler 296f (1.7 Re in the illustration). That’s because late last year, HST detected a wide binary (80 au separation) near the primary star, origionally thought to be a 61 CygniB analog. The revised parameters for the two stars are a Gliese 581 analog (primary) and a Gliese 1214 analog (secondary). If e and f BOTH orbit the primary, their radii are roughlt 1Re, or if they both orbit the secondary, BOTH radii are GREATER than 2Re. There was some question as to wich star they do orbit, but since BOTH are now considered in the habitable zone (as in HEC), we can now assume that the 99,7(not QUITE as good as Kepler 186f, which is at 99.9) confidence level is for both orbiting the primary.

  • Harry R Ray January 7, 2015, 11:25

    Correction (MAYBE) ! There is a current discrepancy in the HEC Confirmed planets section vis-a-vis the candidates section. The Confirmed planets section lists Kepler 296e as 1.5Re while the candidates section lists KOI 1244.05 as 1.0Re. Kepler 296e IS KOI1422.05, so it appears that ONLY the Confirmed planets section has been revised. We’ll have to wait a few days and see.

  • Andrew LePage January 7, 2015, 11:35

    I’ve had a chance to scan through the discovery paper preprint and I can honestly say that I am pretty impressed. As I have often stated in my articles and comments here on Centauri Dreams and as well as in articles on my own web site and elsewhere, claims about the the potential habitability of new extrasolar planets are all too often overstated (or in extreme circumstances, simply not warranted). The authors of this paper are quite honest about the uncertainties associated with their new finds and seem refreshingly honest (and cautious) about any habitability claims.

    In the discovery paper, the authors take into account the uncertainties of the derived orbit and star parameters to actually calculate the probability the planet in question is inside the HZ of its system. Another issue I have had is that too many of the planets dubbed “potentially habitable” are probably not rocky planets. Analysis of Kepler data has shown that planets larger than ~1.5 the radius of the Earth or with 4 to 6 times its mass are most likely mini-Neptunes instead.


    What I find impressive is that the authors of this discovery paper use that latest work on the mass-radius relationship of planets derived from earlier Kepler data analyses to give the probability of the planet even being a rocky planet instead of a mini-Neptune. Hopefully the media and others parties will respect the authors’ caution and not overhype the potential habitability of these new discoveries especially the more marginal cases (I’m not holding my breath on that, however).

  • william January 7, 2015, 12:53

    Forgive me here, but I don’t know what really all the excitement is about finding another Earth. After all, if we found an exact duplicate tomorrow of our home world (or perhaps even a better one). This discovery would not be of any significance at all because simply the facts are that we cannot go to any distant worlds in any reasonable length of time. So why the excitement of this when nothing can be accomplished on a practical basis?

  • Ashley Baldwin January 7, 2015, 17:10

    The picture could not be better set than for the arrival of the TESS data set.

  • RobFlores January 7, 2015, 17:49

    No, W,
    If we discovered an Earth Analog with even with 98% N2 plus H20 vapor and C02 and not much 02, in the HZ around a of a hotter K or G type star it would electrify most thinking people with an modicum of curiosity and a willingness to undertake a quest even rather long one. Even one at 100LY distant it would stir the passions, give even spring in most people’s step.
    It would only be a matter of time before it was a destination, and would be an impetus toward attaining fast interstellar speeds and probably hibernation technology. Of course it would be a project for the next century, but Young children alive today would certainly have chance to see the 1st human survey crew to start the journey.

  • VEW January 7, 2015, 18:14

    William: Getting some idea of how many rocky planets orbit in the liquid water zones of their star systems is very significant with respect to the question of whether or not life is commonplace in the observable universe.

    If independently originating life is in turn found elsewhere in our own solar system then it would be possible to get a decent order-of-magnitude estimate of the number of life-bearing planets out there.

  • Brett January 7, 2015, 19:00

    But we can learn interesting things about it, or at least about planets closer to Earth if or when we build the necessary telescopes.

    If you’re not really interested in the frequency and nature of possibly earth-like exoplanets, that’s fine. Many of us are, though.

  • P January 7, 2015, 19:26

    William. The universe is 13.8 billion years old, a discovery which took a lot of effort for ‘no obvious practical advantage’. Would you prefer humans had not discovered this too?

    Luckily, most humans are curious, and it will be a sad day when they stop being so. I can go to each and every one of these new planets in my imagination, imaginings leveraged and refined by better physical data as it comes in. The fact that I’ll never visit them ‘in real life’ is meaningless as long as I believe that someday, no matter how far in the future, someone just might.


  • Bill Thornton January 7, 2015, 22:10

    In reference to William’s comment “Forgive me here, but I don’t know what really all the excitement is about finding another Earth.” IN short I agree with Rob Flores response, though his time lines are too optimistic. Today’s infants, will NOT see a human crew on an interstellar mission to another star system; see http://arxiv.org/ftp/arxiv/papers/1101/1101.1066.pdf – for a paper by Marc Millis (Tau Zero Foundation). See also: see also the “incessant obsolescence postulate” (again from Marc Millis, Tau Zero Foundation) http://www.tauzero.aero/discoveries-log/getting-there/motives/ . We will, However be colonizing our solar system in the 22nd century, as a “Stepping Stone” to travel to other star systems. Interstellar travel with people will likely happen, but it is likely over 400 years from now; and FTL, if possible, will likely be millenniums from now.

  • Lionel January 8, 2015, 7:44


    Because being on the breaking wave of discovery is what bring many people most delight

    Once space is industrialised I see no reason why we wouldn’t eventually build space telescopes as big as the LHC or build a whole lot of Prof Maccone’s FOCAL probes to observe the hell out of these places. A while ago we knew of no planets around other stars, now we know of thousands.

    Currently, we know of no life around other stars. But it may happen during our lifetimes, or of our childrens’, that we have observed strong indications of life in thousands of places in our galaxy. Maybe we will never visit them, but my guess is that once strong evidence of life elsewhere has been observed there’ll be a pretty electrifying effect upon humanity

  • ljk January 8, 2015, 9:21

    william said on January 7, 2015 at 12:53:

    “Forgive me here, but I don’t know what really all the excitement is about finding another Earth. After all, if we found an exact duplicate tomorrow of our home world (or perhaps even a better one). This discovery would not be of any significance at all because simply the facts are that we cannot go to any distant worlds in any reasonable length of time. So why the excitement of this when nothing can be accomplished on a practical basis?”

    Isn’t it almost adorable how a species that barely emerged from the trees just moments ago cosmically speaking wants to know what good the vast, ancient Cosmos is for them? Maybe this is a common reaction from most beings that just became aware of their surroundings, but we won’t know until we find them, right?

    Time for a little adjustment in scale about what and who is the most prominent and important thing in existence:



    Let us face it, if this were an attitude unique to just a few people then we would have much more viable space and science programs. Instead we have a world that focuses on vapid actors and sports jocks with absurd salaries and a military that has recently spent over 1.5 trillion dollars on a fighter jet (the F-35) that won’t have the capability of firing its guns until 2019 and cannot be fueled by a gas truck that has sat in the sun. And I won’t even go into the archaic and often insane attitudes so many have towards social and religious issues that should have been resolved ages ago, assuming we are supposed to be a technological civilization.

  • ljk January 8, 2015, 9:30

    Here is another “false positive” the BLENDER astronomers have probably not considered yet, at least not out loud:


    When professional scientists remains squeamish and dismissive when it comes to extraterrestrial life and intelligence, how can the general public not feel otherwise? Or take it to the other extreme into the realm of pseudoscience and cult behavior as we have seen for decades with UFOs and that ilk.

  • Andrew LePage January 8, 2015, 11:58

    A more detailed look at the discovery paper shows that most of the 8 new found planets are likely to be mini-Neptunes and are unlikely to be potentially habitable. Also, the discovery team has taken liberties with the definition of the HZ and have made it so optimistic that even Venus (which is NOT habitable by any stretch) would be considered to be comfortably inside the HZ of our solar system. Despite these shortcomings, I am impressed with the team’s work and the fairly cautious tone of their paper. In addition, they seem to have discovered one planet that is probably the best potentially habitable planet candidate found to date.


  • ljk January 8, 2015, 13:07

    Kepler Confirms Over 1,000 Exoplanets and Finds More Potentially Habitable Worlds

    By Paul Scott Anderson

    Even after problems threatened to end the Kepler space telescope’s mission for good last year, the planet-hunting observatory has continued to help astronomers discover thousands of exoplanets orbiting other stars, including ones that are potentially habitable. As reported yesterday at the American Astronomical Society (AAS) meeting, Kepler has now confirmed just over 1,000 exoplanets, with thousands more awaiting confirmation. A growing number are also potentially habitable, at least by Earthly standards.

    To date, Kepler has now confirmed 1,004 exoplanets and has found an additional 4,175 candidates. Since the last update, this is an increase of eight more confirmed planets and 554 more candidates. Of the newly-validated planets, three are in their stars’ habitable zones (where temperatures could allow liquid water to exist on the surface of rocky planets), and of those three, two are likely rocky in composition, like Earth. Of the 554 new candidates, six are close in size to Earth and also orbit in the habitable zone.

    “Each result from the planet-hunting Kepler mission’s treasure trove of data takes us another step closer to answering the question of whether we are alone in the Universe,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate at the agency’s headquarters in Washington. “The Kepler team and its science community continue to produce impressive results with the data from this venerable explorer.

    Full article here:


  • RobFlores January 8, 2015, 13:08

    Bill T, I think it is a fair assumption that today’s very young children will
    live to 120 Years at least. You make the assumption that colonization of
    our solar system would be the 1st priority, It would be for some Organizations but; The hostility of the surface almost all terrestrial bodies would be in the psyche of colonists there, Such colonies may not be the
    happiest place off-earth, HOWEVER:
    Travelling to a potential 2nd Earth, where you could be outside with only a modest piece of equipment (even one embeded inside the Body), Under blue skies, Clouds and Oceans, will call like the comeliest siren. The enterprise is very likely to attract some very wealthy people to pony up $ simply because they are by nature are bent to risk taking and adventure, (think Branson). And even the less adventurous plutocrats will pause, and consider their place in history, and alter their mindset. It will come down to how expensive the endeavor is likely to be. IMO Once it costs less than 20% of a foundation’s or individual’s wealth the enterprise will be given a green light. I am counting of nanotech and advanced Robots for making a ship the least costly part of the enterprise and obviously the building the Engines the most expensive part.

  • Mark Zambelli January 9, 2015, 7:07

    Bill said, “Today’s infants, will NOT see a human crew on an interstellar mission to another star system…”

    My own take on that is a more optimistic one given advances in modern medicine. I have heard it mentioned from time to time that the first person to live to see their 200th birthday is most likely alive today, with good odds that by the time that person gets there the advancements might prolong lifetimes even longer. I know this is a grey area of opinion but I’m hopeful that someone from the early C21st will still be around at the start of the C23rd (as mentioned as the earliest timeframe from the .pdf link you provided). It’s a very open question, I know, but it seems a relatively safe-ish (ha ha) bet to me (at least safer than some other topics with timeframes that we consider).

  • Andrew LePage January 9, 2015, 14:36

    While the news has been dominated by the discovery of 8 new planets orbiting inside the habitable zones of a group of dim K- and M-dwarf stars, there was also the announcement of a pair of Kepler planet candidates orbiting inside the HZ of two more Sun-like stars. While their planetary nature still requires confirmation, KOI 2194.03 and KOI 5737.01 provide a foretaste of the Kepler discoveries yet to come.


  • ljk January 20, 2015, 19:40

    Debating the future of exoplanet missions concepts and community

    While astronomers are discovering ever more exoplanets, including some that may be like Earth, there’s a perception that the scientific community can’t agree on future goals and missions.

    Jeff Foust reports on efforts by astronomers to develop greater consensus on the direction of exoplanet research, and what some of the missions to achieve those goals might be.

    Monday, January 19, 2015