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Water Worlds in the Habitable Zone

“The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings.”

Thus Cassius speaking to Brutus in Shakespeare’s Julius Caesar, trying to convince him that what happens to us comes not from some malign fate but from our own actions. I’m sure he’s right, too, but I admit there are days when I wonder. For the stars seem aligned in such a way that whenever there is a significant news conference about exoplanets, I have a schedule conflict. This is true yet again today, so that I’m writing before the NASA-hosted news briefing and will have to set this up to post automatically after the embargo expires.

Here, though, are the main points. We have found Kepler 62f, an interesting world about 1.4 times the size of Earth and most likely rocky. When you add up the other known facts about the planet, the attention builds. Discovered through Kepler data in the constellation Lyra, this world receives about half the heat and radiation that the Earth does, while orbiting its star every 267.3 days. The University of Washington’s Eric Agol lays out the case for this intriguing planet:

“The planets this small that we have found until now have been very close to their stars and much too hot to be possibly habitable. This is the first one Kepler has found in the habitable zone that satisfies this small size. Kepler 62f is the smallest size and the most promising distance from its star, which by these measures makes it the most similar exoplanet to Earth that has been found by Kepler.”

The best guess — and remember that we lack knowledge of Kepler 62f’s mass and density — is that we’re looking at a rocky world with an atmosphere, but Agol doubts it’s a thick gaseous envelope like Neptune’s. We seem to be looking at a planet a bit larger and somewhat cooler than the Earth, and it’s not alone in this system, where four other planets are known to exist. One of these, Kepler 62e, is a fellow traveler in the star’s habitable zone. It’s about 1.61 times the size of Earth and orbits every 122.4 days, giving it 20 percent more stellar flux than the Earth.

new_kepler

Image: The newly discovered planets named Kepler 62e and f are super-Earths in the habitable zone of a distant sun-like star. The largest planet in the image, Kepler 62f, is farthest from its star and covered by ice. Kepler 62e, in the foreground, is nearer to its star and covered by dense clouds. Closer in orbits a Neptune-size ice giant with another small planet transiting its star. Both habitable-zone planets may be capable of supporting life. Credit: David A. Aguilar (CfA)

Two small super-Earths in the habitable zone make for understandable excitement. Modeling at the Harvard-Smithsonian Center for Astrophysics (CfA) suggests that both these planets are water worlds, completely covered with a global ocean. Might they have life? Lisa Kaltenegger (Max Planck Institute for Astronomy/CfA) speculates:

“These planets are unlike anything in our solar system. They have endless oceans. There may be life there, but could it be technology-based like ours? Life on these worlds would be under water with no easy access to metals, to electricity, or fire for metallurgy. Nonetheless, these worlds will still be beautiful blue planets circling an orange star — and maybe life’s inventiveness to get to a technology stage will surprise us.”

Kepler 62 is a class K star somewhat smaller and cooler than the Sun. According to the CfA’s models, Kepler 62e should be cloudier than the Earth, while the cooler Kepler 62f would need to take advantage of the greenhouse effect from carbon dioxide in the atmosphere to keep it warm enough for the ocean to remain liquid. Harvard’s Dimitar Sasselov notes in this CfA news release that discoveries like this raise the prospect of other stars with not one but two planets in the habitable zone. That conjures up what until recently was our view of the Solar System, when not so many decades ago we used to believe Venus and Mars might each be habitable worlds.

I think Sasselov is right to linger over this thought. These may be water worlds and most likely would not develop technological civilizations, but imagine a system where two worlds with continents and oceans, worlds much like Earth, existed close to each other in the habitable zone, each highly visible to the other. Surely the inhabitants of planets like these, with the prospect of a truly colonizable world this close, would be impelled to make the crossing.

The other planets around this star? Kepler 62 b, c and d are 1.31, 0.54 and 1.95 times the size of the Earth, respectively, all orbiting the star too tightly to be in the habitable zone. For more, see this University of Washington news release. Kaltenegger and Sasselov’s work is to be published in The Astrophysical Journal, while Agol is second author of the discovery paper published online in Science today as Borucki et al., “Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone” (abstract).

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Comments on this entry are closed.

  • Daniel April 22, 2013, 9:36

    sorry for my text mistakes:

    *because for a telescope detect an Earth-like planet, we Need a telescope to image this planets (like a Starshade or/and TPF (that been cancelled)

    *that is NOT the Kepler case

  • ljk April 22, 2013, 10:19

    GaryChurch said on April 18, 2013 at 18:33:

    “-simple life is likely to emerge on any wet, rocky planet”

    “Perfect for Earth megafauna and flora to invade. On such a world we can grow a second Earth (if that “simple life” is not lethal to us). We could eventually have thousands (millions?) of versions of Earth. Heinlein described a similar progression in Starship Troopers.”

    This then begs the question – Eniac, I hope you will be proud of me :^) – as to why this has not happened before? Of course we can barely even detect Earthlike (or size) worlds, let alone know what is going on with them, so it is much too soon to be certain what is going on out there.

    Are there indeed many Earthlike worlds but the native life forms are not smart enough to go star travelling or have no interest in such activities? Is humanity a fluke and every other highly intelligent species prefers to stay at home? Note that even among our own kind, the number of individuals and groups truly interested in exploring and colonizing space are quite small compared to the overall population.

    If the majority of humanity wanted to expand into the galaxy we would be doing so by now. Or perhaps I am just being impatient about a society that is just getting its cosmic bearings and ability to do this for real. On a cosmic scale we just figured out we are on a planet circling a star. We have yet to colonize a single world in our Sol system and trying to reach even the nearest star system in a matter of decades is a very tall order.

  • Rob Flores April 22, 2013, 11:48

    Obcourse the Kepler Mission team thinks Earths Twins discoveries
    will happen in the future. But the question is just how many good
    candidates do they have in the present data.

    Here we are in year four of the Kepler’s mision, Let us assume that
    they have in their data stream 3 transit events.
    I have to believe that the Kepler team is pulling out all the stops. and
    using ground based assets to verify a fourth transit. I would
    think Candidates that are >RE 2 and out of the HZ of star have been placed on the backburner (there appear to be lots of those) unless they are peculiar.
    They want that Earth Twin found in the worse way possible, as we all do.

    One of the blind spots I have is that I don’t know if it is actually easier for
    a ground based telescope to comfirm the transits. Is their sensitivity
    better.

  • andy April 22, 2013, 12:32

    @Enzo:

    Excuse the ignorance, but why are earth analogs called “eta-earths” ?

    They aren’t. Eta-Earth is the frequency of Earth-like planets.

  • Enzo April 22, 2013, 20:22

    @andy

    Thanks, but is it the frequency of earth size planets in the HZ of G stars or the frequency of earth size planets in the HZ around any star ?

  • Enzo April 22, 2013, 20:57

    @Daniel, Eniac,

    What can I say, I’m glad you are not troubled at all by the fact that Kepler has already observed 6 periods for low luminosity stars. Maybe the G stars will save the day.
    I guess the significance of this really depends on the % of low luminosity stars that Kepler has observed (this would be different from the % of existing M or K stars, together some 88% of all stars).
    I believe that files containing the list of all stars observed by Kepler exist (not just the KOI list, ALL the stars observed). An analysis of that should give further insight.

    @loggain

    I find hard to believe that Kepler scientists have not noticed that 6 periods have been gone through a lot of stars.
    I had a quick browse through the video and one thing stuck out like a sore thumb : at 18:00 there’s a diagram of the HZ of the solar system with the inner bound well inside Venus!!
    Modern stuff I’ve seen puts it a 0.95 AU or even, more recently, to .99 AU.
    There’s an indication that Venus started as earth twin with ocean etc. but I don’t think anyone who studied it thinks it lasted more than 1 Gyr and only because the sun was less luminous.
    I might be cynical, but pushing that diagram to .7 AU or less tells you how much the team WANTS to find a earth.

    For the record, I’d love the solar systems to be just as familiar as the ones ACRETE used to spit out in the 70s but that does not mean that it really doesn’t look that good.
    Another thing : less likely earths in HZ does not mean no earths in HZ, there are a LOT of stars. It just means that the chances of us studying it in detail are much lower.

  • coolstar April 22, 2013, 21:45

    @Rob Flores It’s IMPOSSIBLE, unfortunately, for ground based telescopes to confirm transits of earth sized planets around solar type stars. That requires a precision of about what Kepler can do, maybe 20 parts per million, while the VERY best ground based photometry (on stars much brighter than those in the Kepler field) has a precision of about 1 part in ten thousand (10x worse than that is still very good photometry). Only the very brightest Kepler candidates could even conceivably be “confirmed” by the next generation of rv spectrographs on the next generation of ground based telescopes. Any earth analogs that Kepler finds will NOT be “confirmed” any time soon, if ever, but everyone knew that going in.
    @ Enzo Thanks, I’d seen the Laughlin post and simply used the M vs. R relationship that’s most commonly used in the field and has the most data behind it. As Laughlin points are, there are OTHER ones (some of which are “better” some worse). It’s refreshing to see a theoretician so pessimistic about their own field, would there were more!

  • Eniac April 22, 2013, 22:18

    See the plot with a large number of jupiters with periods of 1-300 days around various stars :
    http://oklo.org/2012/11/10/the-mmen/
    It includes all jupiters known from Kepler and radial velocity.
    Again, there are many. The same can’t be said of earth size planets in the HZ.”

    An interesting plot, indeed. Consider that the data is censored by the detection limit, an invisible line that runs (roughly) from bottom left to top right. All planets to the right and bottom of that line are simply not seen, but they are very likely there. The cluster of super-Earths is clipped from below, where we would see many more Earth sized and smaller planets if our methods were more sensitive. It is also clipped from the right, where we would find large numbers of Earths and super-Earths further out from the star if our instruments were better. Were it not for the censoring, Earth and Venus would be well within a much larger cloud of “small” planets, in my opinion. At least, as far as I know, and certainly not in this plot, there is no evidence to the contrary.

    As for the Jupiters, I think it is still a matter of debate whether the apparent bimodality in distance is an artifact of combining multiple methods of detection with very different sensitivity profiles, or whether it may be real. Maybe I am wrong and this question has been decided unequivocally. If so, I would much appreciate being set straight.

    What the plot really does seem to show beyond reasonable doubt is the bimodality in mass, i.e. the classification of planets into rocky (?) worlds and gas giants as two distinct classes.

  • Eniac April 22, 2013, 22:36

    @LJK

    This then begs the question – Eniac, I hope you will be proud of me :^) – as to why this has not happened before?

    I am indeed very proud of you :-))

    Note that even among our own kind, the number of individuals and groups truly interested in exploring and colonizing space are quite small compared to the overall population.

    An important observation that should rub your nose in the important fact that a very small fraction is actually all that is needed. A few hundred individuals can make a full-fledged colony, if need be.

    We have yet to colonize a single world in our Sol system and trying to reach even the nearest star system in a matter of decades is a very tall order.

    You are indeed a little impatient, here. For the question of whether or not the galaxy will eventually be colonized (or should have been, already) a few million years more or less hardly make a difference. Certainly not decades or centuries ….

  • GaryChurch April 22, 2013, 23:45

    “If the majority of humanity wanted to expand into the galaxy we would be doing so by now. Or perhaps I am just being impatient-”

    No, you are correct; we should at least be colonizing our own solar system by now. It is a bad sign that indicates we may not be quite smart enough to survive. I am certainly worried about it but find myself a member of a tiny minority almost completely ignored by the rest of our species.

    In the form of H-bombs we have the most powerful devices ever created and capable of being used for propulsion; but we created these bombs to threaten others. By freezing human beings we have a means to travel for centuries through space but have spent no resources to date doing even the most basic research to enable such a procedure.

    As I often state in my posts- we may be too stupid to survive.

  • Enzo April 23, 2013, 1:21

    @loggain

    I did find the diagram you mentioned at 26:00 in the video. And I have to say that it shows much better results for G stars. It also shows a relative vacuum for the K stars. That looks promising but it depends what the KOIs are.
    I wonder if they are in the last public list or they if they are withheld by NASA.

  • Holger April 23, 2013, 13:04

    @Enzo:
    “I guess the significance of this really depends on the % of low luminosity stars that Kepler has observed (this would be different from the % of existing M or K stars, together some 88% of all stars).”
    Indeed: M dwarfs are massively underrepresented in the Kepler targets; only “a few thousand” (i.e. a few percent of the >100,000) stars are M dwarfs, according to an old CD comment (by “Mike”, https://www.centauri-dreams.org/?p=11576). So the current lack of Earth analogs around M dwarfs (in terms of size and temperature; they’d all be tidally locked, presumably) is probably not (yet) statistically significant.

    Also note that all Kepler’s (sub-)Earth-sized (say, <1.1 m_E) discoveries have periods of at most 20 days (acc. to their Discoveries table), thus, judging from the times of their discoveries, they apparently took ~20 transits to discover, not just 6. (The smaller the planet, the more transits you need, since their signature comes much closer to the "background noise"). And Kepler will naturally dedicate most of their manpower to sunlike-star candidates, I expect.

  • andy April 23, 2013, 17:06

    @Enzo:
    On further investigation, apparently the term is defined in section 11.2 of the ExoPlanet Task Force final report (pdf).

    The prospects for finding habitable planets are generally characterized by a parameter η⊕ (pronounced “eta sub earth”), the fraction of stars that have at least one potentially habitable planet. For concreteness the Task Force defines a potentially habitable planet as one that is close to the size of the Earth and that orbits within the stellar habitable zone. Close to Earth-sized means between 1/2 and twice the radius of the Earth or in terms of mass between 0.1-10 times the mass of the Earth. These two definitions are equivalent if a fixed density equal to that of the Earth is adopted.

    It doesn’t appear to restrict the stellar spectral type.

  • loggain April 24, 2013, 0:18

    @Enzo

    regarding the HZ position. There is the narrow and wide HZ on the charts. Wide is “empirical” e.g. basically Venus – Mars distance (correncted for effective insolation for other stars). Narrow is modelled and earth is almost on the edge of it. Lisa Kaltenegger explains there this issue in detail. Basically she says that

    a) Narrow HZ are likely to be a little bit wider – because they do not include in their models the albedo from the clouds

    b) HZ adapts itself to the star rising luminosity as star ages – planet can do this automatically by adjusting the concentration of CO2 and other GHG in the atmosphere

    c) if in the HZ you have a planet that is large enough to keep GHG and H2 but not to large to be overheated – it should have a lot of liquid water and in that sense be habitable

    to my astonishement they sounded pretty optimistic as if they “already knew” but maybe I am exagerating, Nevertheless what counts in the Kepler mission is not a single prey to be hunted down but a a good sample of planets with different sizes, distances around different stars in order to draw nice statistical distributions and be able to design future missions better. Can they achive this 100% with apparently breaking telescope? I am sceptical but…

  • Ronald May 3, 2013, 19:44

    I am late in this very interesting thread, I am sorry, but I have been rather busy lately.
    “Kepler 62e, is a fellow traveler in the star’s habitable zone”
    “discoveries like this raise the prospect of other stars with not one but two planets in the habitable zone”
    Well, I honestly don’t think so: according to the recent update of the Habitable Zone definition by Kopparapu, Ramirez, Kasting, et al. (Habitable Zones Around Main-Sequence Stars: New Estimates; Jan. 2013), the HZ extends from 0.97 – 1.70 AU in our solar system.
    Even if we apply the ‘old’ 0.95 AU as the inner edge, this implies that about 10% more stellar flux is really the limit before runaway greenhouse kicks in. And for a large-mass planet (with a dense atmosphere) like 62e this limit is probably even considerably lower. I expect that 62e is much too hot.

    “imagine a system where two worlds with continents and oceans, worlds much like Earth, existed close to each other in the habitable zone, each highly visible to the other. Surely the inhabitants of planets like these, with the prospect of a truly colonizable world this close, would be impelled to make the crossing.”
    Sure, but the same may also be true in the case of binary (solar) stars, each with a habitable planet. Zeta Reticuli, 16 Cygni and, of course, Alpha Centauri are good examples of this.