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Earth 2.0: Still Looking

I’ve come to dislike the term ‘Earth 2.0.’ It’s not so much the idea of a second Earth as the use of 2.0, which in our technological era invariably recalls software updates. Windows 2.0 was better than Windows 1.0, but Windows 3.0 was the one that really took off — the idea here is that progressive iterations improve the product. I’d rather see us use ‘Earth 2’ than ‘Earth 2.0,’ for the latter implies a new and improved Earth, and I’m not sure just what that would be. Speculating about that is, I suppose, a key activity of philosophers.

But Earth 2.0 has stuck as a way of designating a planet much like our own. Here too we have to be careful. A planet with liquid water on at least parts of its surface might exist around a red dwarf, packed into a tidally-locked orbit and divided between a frigid night side and a day side with, perhaps, only a few zones where life might flourish. It’s not Earth 2.0 because it has a star that never moves in its sky and its susceptibility to solar flares offers evolutionary challenges much different from those life has experienced around our G-class Sun.

So we can reserve Earth 2.0 for planets that orbit around their star in roughly the same way we do, meaning a star much like the Sun and a planet of Earth size in a more or less circular orbit at about 1 AU. To really hammer home the comparison, we should ask for a star of a certain age. We might find a planet meeting all these characteristics circling a star so young that life is unlikely to have taken hold, assuming that life takes the same kind of path it did on Earth (obviously, nothing more than an assumption). But the Earth 2.0 that seizes the popular imagination will so closely mirror our own in age, orbit, and size as to suggest a living world.

We’re getting close.


Image: The sweep of NASA Kepler mission’s search for small, habitable planets in the last six years. The first planet smaller than Earth, Kepler-20e, was discovered in December 2011 orbiting a Sun-like star slightly cooler and smaller than our sun every six days. But it is scorching hot and unable to maintain an atmosphere or a liquid water ocean. Kepler-22b was announced in the same month, as the first planet in the habitable zone of a sun-like star, but is more than twice the size of Earth and therefore unlikely to have a solid surface. Kepler-186f was discovered in April 2014 and is the first Earth-size planet found in the habitable zone of a small, cool M dwarf about half the size and mass of our sun. Kepler-452b is the first near-Earth-Size planet in the habitable zone of a star very similar to the sun. Credit: NASA Ames/W. Stenzel.

Enter Kepler (again)

The beauty of the Kepler mission is that it just keeps on giving. The Kepler team has already identified over 4,000 planet candidates, and now we have a new catalog of more than 500. All planets are subjects of interest in their own right, but life-bearing ones still have considerable cachet, as witness Jeffrey Coughlin (SETI Institute), who comments thus:

“This catalog contains our first analysis of all Kepler data, as well as an automated assessment of these results. Improved analysis will allow astronomers to better determine the number of small, cool planets that are the best candidates for hosting life.”

Twelve planet candidates in the new catalog are less than twice Earth’s diameter and orbit in the habitable zone of their star, meaning that region where liquid water can exist on the surface. Among these, Kepler-452b, about 1400 light years from us, has now been confirmed as a planet, and it’s an interesting world, one that orbits a star much like the Sun, being about 5 percent more massive and 10 percent brighter. The planet itself is about 5 times the mass of the Earth, with a radius 50 to 60 percent larger. Moreover, Kepler-452b orbits only 5 percent farther from its parent star than Earth orbits the Sun, with a 385-day year. Jon Jenkins (NASA Ames) is lead author on the paper on this work. He pointed out at the NASA news briefing today that gravity on this world would be about 50 percent larger than that of Earth, on a world with a thicker atmosphere and a larger degree of cloud cover. The star is also older than our Sun, which has predictable consequences:

“Kepler-452b receives 10 percent more energy than the Earth. Bear in mind that stars in their youth are smaller and dimmer, but they get brighter with age. Kepler-452’s star is more than 6 billion years old, and should leave its habitable zone at about the 9 or 10 billion year mark. Earth will receive the same energy as Kepler-452 does today in about one and a half billion years.”


Image: This artist’s concept depicts one possible appearance of the planet Kepler-452b, the first near-Earth-size world to be found in the habitable zone of star that is similar to our sun. The habitable zone is a region around a star where temperatures are right for water — an essential ingredient for life as we know it — to pool on the surface. Scientists do not know if Kepler-452b can support life or not. What is known about the planet is that it is about 60 percent larger than Earth, placing it in a class of planets dubbed “super-Earths.” While its mass and composition are not yet determined, previous research suggests that planets the size of Kepler-452b have a better than even chance of being rocky. Kepler-452b orbits its star every 385 days. The planet’s star is about 1,400 light-years away in the constellation Cygnus. It is a G2-type star like our sun, with nearly the same temperature and mass. This star is 6 billion years old, 1.5 billion years older than our sun. As stars age, they grow in size and give out more energy, warming up their planets over time. Credits: NASA Ames/JPL-Caltech/T. Pyle

This is a planet that has been in its star’s habitable zone for longer than the age of the Earth, ample time, as Jenkins noted, for life to begin. Although the size of the world — intermediate between Earth and Neptune — makes it too large to be a true Earth analogue, Jenkins believes that it has a “better than even chance of being rocky.” Thus we could be looking at a world that models changes our planet will be making in the remote future.

We’ll get more habitable zone planets out of the Kepler data, according to Jeff Coughlin (SETI Institute), because we’re getting much better in our planet extraction techniques, but Coughlin noted at the news conference that for every planet we’ve detected, there are at least fifty we cannot see because they are not oriented so as to make transits possible. “Earth-like planets,” Coughlin said, “are common throughout the galaxy.”


Image: Twelve Exoplanet discoveries from Kepler that are less than twice the size of Earth and reside in the habitable zone of their host star. The sizes of the exoplanets are represented by the size of each sphere. These are arranged by size from left to right, and by the type of star they orbit, from the M stars that are significantly cooler and smaller than the sun, to the K stars that are somewhat cooler and smaller than the sun, to the G stars that include the sun. The sizes of the planets are enlarged by 25X compared to the stars. The Earth is shown for reference. Credits: NASA/JPL-CalTech/R. Hurt

Earth 2.0? Not if we’re dealing with a super-Earth. But what an interesting world Kepler-452b seems to be. We have the example of planets like Kepler-438b and Kepler-442b to remind us of worlds that might be rocky like the Earth, but orbiting different kinds of stars, in this case red dwarfs. No Earth 2.0 among that lot either, but it’s clear we’re moving in the right direction.


Image: Since Kepler launched in 2009, twelve planets less than twice the size of Earth have been discovered in the habitable zones of their stars. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. The light and dark shaded regions indicate the conservative and optimistic habitable zone. The sizes of the blue disks indicate the sizes of these exoplanets relative to one another and to the image of Earth, Venus and Mars, placed on this diagram for reference. Note that all the exoplanets discovered up until now are orbiting stars which are somewhat to significantly cooler and smaller than the sun. Kepler-452b is the first planet less than twice the size of Earth discovered in the habitable zone of a G-type star. Credit: NASA Ames/N. Batalha and W. Stenzel.


Comments on this entry are closed.

  • Brett July 23, 2015, 13:21

    It’s good, although I think it’s less likely to be a habitable world than Kepler-62f. The size is a problem – if it has a thick atmosphere plus the higher light, there’s a good chance the planet is a super-Venus. I’d bet that super-Earths are more likely to be habitable when they’re receiving less sunlight than Earth gets on average, since their thick atmospheres can make up for less sunlight (although that also lowers the energy available for photosynthesis).

  • Rangel July 23, 2015, 14:21

    As Brett said, 62f plus 186f and 667Cc, in my opinion, still more likely to be habitable. I don’t like how the media and astronomers keep selling the idea that a planet to be habitable need to be “Earth-like”, our planet suffers from periodic long and harsh ice ages for example and have too cold polar regions and too hot and dry deserts.

    While it is uncertain how much land can be habitable on a planet orbiting a red dwarf, i believe some parts can still always be habitable, the star flares may make things harsh, but certain life there evolved to resist it. The problem i see with M-stars is the very fact we humans live on a G-stars, if all star types have the same habitability factor, we are supossed to be on a M-star since they make up the great majority of the stars, the odds to end on a G-star is fairly small… and we we are, somehow this lead me to think M-stars have issues we still don’t know.

    Finally, while G-stars seems pretty good for life development, somehow i still believe K-stars (late K’s to early G’s) are the best for life to thrive, longer life span and usually no tidal-lock issues and no flares.

    Anyway, great news, and i still waiting for a planet on the habitable zone of any star of Centauri system on follow up missions, since Kepler is not designed for that.

  • Ross Turner July 23, 2015, 14:30

    The gravity at the surface of the planet would also be higher if the estimates of it’s size and mass are correct. If it is 60% larger in radius and 5 times as massive, then the surface gravity would be 1.95 times that of earth. Not sure what that means in terms of life, but as noted above, holding on to a thicker atmosphere could cause issues. Looking forward to when they will be able to determine if moons exist around such planets.

  • Luis July 23, 2015, 14:31

    I guess now what we have to answer is can we detect Oxygen in the atmosphere.

  • andy July 23, 2015, 14:58

    Since Kepler launched in 2009, twelve planets less than twice the size of Earth have been discovered in the habitable zones of their stars.

    Great. But why is NASA still promoting the pre-Kepler estimate of where the rocky planet transition is?

  • Andrew LePage July 23, 2015, 15:40

    I was hoping for the announcement of a more Earth-like exoplanet. Back in January, there were two candidate “Earth twins” (Earth-size planets in Earth-like orbits around a Sun-like star) quietly announced – KOI 2194.03 and 5737.01:


    Still, Kepler 452b (also known as KOI 7016.01) is not too bad as far as potentially habitable planets go. Given the uncertainties in its properties (and subject a bit more analysis when I have the time), it has about a 50-50 chance of orbiting inside even a conservative definition of the HZ and given its size, 1.6 times the radius of the Earth, it has about a 50-50 chance of being a terrestrial planet instead of a mini-Neptune:


    Since it orbits a Sun-like star, it is not too bad of a potentially habitable planet candidate. Thankfully, even better ones are yet to come!

  • Andrew_W July 23, 2015, 17:04

    “This star is 6 billion years old, 1.5 billion years older than our sun. ”

    That’s a staggering figure when you stand it against Fermi’s paradox, a people 1.5 billion years ahead of us should have colonized the entire Virgo Supercluster by now, that is, if widely held expectations of interstellar expansion and space travel were even remotely accurate.

  • Andrew Palfreyman July 23, 2015, 18:39

    At 1 gee acceleration/deceleration, ship time for 1200 LY is 13.8 years, perhaps surprisingly. All that remains is to invent the drive that can handle that much energy :)

  • Andrew Palfreyman July 23, 2015, 18:42

    I should add that at 1.5 gee (in preparation for surface forays) it would only take 9.5 years ship time. And of course a bonus trip 2400 years into the future upon return.

  • Eniac July 23, 2015, 19:36

    Sadly, the lower than expected sensitivity of Kepler (because of some unanticipated starlight noise issue, IIRC) has cost us dearly. Kepler was designed to detect Earth-like planets, instead it now falls just short of that goal. This is why we are all strenuously reaching for Earth 2, but are left with a smattering of planets that are bigger, hotter, or around smaller suns. So close, yet so far away!

    Not to detract from Kepler’s value, which is in characterizing much of the other parts of the planet parameter space. Most of all, in my opinion, Kepler has shown us that if anything about our solar system is special, it would be that it has so few planets. Note that even if the solar system were perfectly positioned for Kepler, not a single of our own solar system’s planets would be noticeable. I sure hope our next planet-finding mission will be luckier in the Earth-like department!

  • P July 23, 2015, 21:13

    I’m with Eniac here. From a few years back where just about every planet discovered smaller that 3 earth radii was hailed as an “Earth 2.0”, we now have the kind of disappointed opposite reaction that seems to inevitably follow too much media hype. ‘Are we there yet’? No. Perhaps if Kepler had lasted another two years we might be having a different discussion.

    However, a cool discovery nonetheless! How quickly we get jaded.

    What I’m interested in are any ‘2 transit only planets’ that the Kepler team might be sitting on because 3 are needed in the normal pipeline. Kepler lasted long enough for quite a few Hz G star systems to throw up only 2 transits. I’d be fascinated to know if any of those might be getting followed up, as difficult as that might be….


  • Navin Weeraratne July 23, 2015, 23:26

    Greater internal heat (assuming a higher mass of radioactives), a higher mass to surface area ratio, and a hotter sun, all suggest a warmer planet.

    Is the gravity of 452b enough to keep it’s hydrogen from escaping over time?


    If not, then 452b could dry out. That, and it’s “starting” water content, would be an upper bound for it’s habitable period.

  • Rob Flores July 24, 2015, 1:39

    I think many would agree that one of the most startling finding in the Kepler data is the relative abundance of super terrestrials/mini Neptunes in the range of RE 1.5-3.5,

    The question if these ST worlds were 1/10 less abundant would the Kepler
    data analysts still claim that there many Earth sized planets that Kepler is unable to detect due to its limitations

  • J. Jason Wentworth July 24, 2015, 7:39

    Rangel’s comment deserves amplification. Our Earth is, viewed objectively, far short of being an ideal place for life, yet here we are, thriving anyway. Also:

    Just as there is a “Goldilocks Zone” (the Habitable Zone) around each star, I would posit that there is also a “Goldilocks Spectral Type” of star–a spectral type which, all things considered, is the best all-around type of star to have habitable planets orbiting them. Like Rangel, I think K-class orange dwarf stars are the best, and here is why:

    [1] K-class dwarfs emit very little ultraviolet light (making an ozone layer much less important–some oxygen-bearing planetary atmospheres might contain other gases that, even in trace amounts, would prevent one from forming);

    [2] They burn steadily, with stable energy output, on the Main Sequence for much longer than G-class and F-class stars;

    [3] Their Habitable Zones, while narrower than those of G-class and F-class stars, are still “generously wide,” so that inner planets have good odds of orbiting within them, and such planets don’t have to orbit their stars so closely as to be rotationally-locked;

    [4] K-class dwarf stars have little or no flare activity (either Tau Ceti or Epsilon Eridani–I forget which–doesn’t even have detectable starspots!);

    [5] Although they occur frequently as binaries, many of these (perhaps even most) have either very close or quite wide separations between the components, making habitable planets around them likely as well, and;

    [6] They are bright enough for photosynthesis in plants to work just fine. BUT:

    Even given these advantages of K-class dwarfs, I think it would be a mistake to ignore M-class red dwarfs, F-class stars, and even brown dwarfs as potential abodes of life, because of the un-ideal but entirely workable example of our world. A planet orbiting in the Habitable Zone of an M-class dwarf, if it had a highly-inclined spin axis (think “sideways” Uranus or “upside-down” Venus), need not be rotationally-locked. We also don’t know for sure if tidally-locked M-dwarfs’ planets are truly hostile to life. Also, not all red dwarfs are necessarily “nasty” flare stars–the very smallest ones and the largest ones might be well-behaved in this respect (due to lower stellar energy and higher stellar gravity, respectively). In addition:

    Brown dwarfs–some of them, at least–might emit enough visible light, along with infrared light, for plant photosynthesis to work (life *in* their atmospheres is also a possibility, at least for the smaller and cooler ones). F-class stars are rather like Irish Wolfhounds as stars go (being shorter-lived), but life does not necessarily have to follow Earth’s timetable (if Earth’s is average, some biologies will develop faster [and some more slowly, of course]). As well:

    The only way to truly judge the life potential of *any* star, regardless of its spectral type, is to observe its planets spectroscopically (which we should be able to do before too long). If a planet has a spectral signature of oxygen coexisting with methane in its atmosphere, someone would almost certainly be at home there (whether they could communicate intelligently with us is another matter). Even A-class stars bear observing (at least in the radio portion of the spectrum), because some of them might be orbited by artificial worlds (space colonies) whose founders settled there from elsewhere. Such stars would provide plentiful amounts of light, heat, ultraviolet light, and X-Rays for all kinds of (shielded) agricultural and (un-shielded) industrial processes.

  • ljk July 24, 2015, 8:31

    Eniac said on July 23, 2015 at 19:36

    “Note that even if the solar system were perfectly positioned for Kepler, not a single of our own solar system’s planets would be noticeable. I sure hope our next planet-finding mission will be luckier in the Earth-like department!”

    No even the Jovian worlds? Then how has Kepler been able to detect smaller exoworlds than them?

    That is unfortunate to learn that Kepler is not quite as sensitive as it was supposed to be. Let us hope better telescopes will be sent up soon.

    I also cannot help but wonder if looking for the equivalent for our home if the goal is to find alien life is a mistake? And if the real goal is to find the equivalent of another Earth to one day colonize, that is also a potential mistake for multiple reasons. One of them being an Earthlike world might already have residents.

  • Mike Jude July 24, 2015, 9:18

    OK….we’ve found a possible Earth. It seems to me a simple SETI experiment would be to observe the star’s spectra for anomalies. The easiest way to announce your presence would be to aim a laser at the sky on your equator tuned to a line in the star’s spectral signature. Anyone observing would a see a periodic spectral change that corresponded to your planet’s rotation. It probably wouldn’t even take that much power.

  • Herr Weh July 24, 2015, 10:03

    @ljk: Eniac’s right, Kepler wouldn’t have noticed our own solar system planets. But that’s not because they’re too small but because it wouldn’t have had enough time to record the three transits necessary for confirming a detection before it broke down, unfortunately.

  • ljk July 24, 2015, 13:04

    Things That Could Go Wrong With Habitabilty Of Kepler 452B – & Could We Detect Intelligent Life There?

    By Robert Walker | July 24th 2015 07:35 AM

    There is so much over enthusiastic hype about this planet today, I thought could do with a bit of more sober reporting of the results, interesting though they are. Much of that speculation derives from just one phrase in the press release I think, where they say: “Today, and thousands of discoveries later, astronomers are on the cusp of finding something people have dreamed about for thousands of years — another Earth.”

    The idea of what that means by “another Earth” for astronomers who know the capabilities of Kepler, is rather different from what most of the general public would think of when you say “another Earth”.

    Kepler is great for getting an idea of what proportion of stars have planets, and what types of stars typically have planets, because it focuses on a small patch of the sky, using transit method, looking at lots of stars likely to be far away. However it can’t tell much about these planets, except the diameter of the planet and its orbital period, and spectral type of its parent star. Anything else such as its mass is informed guesswork.

    So, anyone with that background knew in advance that this announcement could only tell us things like that, and understood the press release accordingly. Just from knowing that it was a Kepler press release. Unless just through luck it happened to find some nearby star, that is, but typically you expect them to be distant stars, at their closest, hundreds of light years away, so far away any light signal due to the planet’s atmosphere or surface would be very faint indeed and hard to analyse.

    But now that it’s found that some G type stars have planets like Earth – that means that perhaps it gets a bit more likely that we will find similar planets around some of the nearby G type stars, maybe even Alpha Centauri, or Tau Ceti or some such.

    Full article here:


  • Garry Van Amburg July 24, 2015, 13:37

    Have any of these possibly Earth-like planets been found at such a distance that we could reach them using our current technology in a few hundred years? I know the hope is we’ll have the ability to send probes at higher speeds but, even then, if something is hundreds of light years away it may be interesting to philosophize about how those planets might be but visiting them isn’t feasible. Also, as with SETI, we assume advanced life forms would be capable of sending and receiving radio signals. But, at least here, that’s only been going on for about a century. Anyone aliens trying to send radio signals to Earth when the Egyptians were building pyramids and the Romans their roads, aqueducts and coliseum would have thought no one was home. Yes, I know there are lots of people who believe we have been visited by ETs but I don’t and the great distances involved may be the reason. Did humans always report visits by aliens? No. That’s a fairly recent phenomenon. In the Middle Ages when demons were on everyone’s mind people reported being possessed by and violated by demons. So, it seems our view of our world has a lot to do with our delusions? I know, much of this is off topic.

  • ljk July 24, 2015, 13:54

    Kepler’s mission scientist Natalie Batalha says there are an estimated one billion planets similar to Earth in the Milky Way galaxy:


  • Brett July 24, 2015, 14:21

    You probably couldn’t find the outer solar system worlds in the solar system without either a very lucky gravitational lensing, or direct imaging the entire solar system perpendicularly on using a telescope with a coronagraph/occulter (like this). The orbital periods and distances are just too far to get transits on any reasonable period of time.

  • Andrew LePage July 24, 2015, 15:23

    For followers of my posts here on Centauri Dream as well as on my own web site about planetary habitability, here is link to my recent independent assessment of the potential habitability of Kepler 452b:


    For a synopsis of earlier finds by Kepler and other teams, I suggest this post in Centauri Dreams from earlier this year:


  • J. Jason Wentworth July 25, 2015, 1:51

    A crazy thought: Since Kepler is in its “latter years” now, what might it be able to detect if it was instead pointed at our nearest stellar neighbors (Alpha Centauri, Epsilon Eridani, Tau Ceti, Epsilon Indi, Delta Pavonis, Barnard’s Star, etc.)? Among other things, maybe it could confirm the existence of Alpha Centauri Bb, if its orbit crosses in front of its star from our solar system’s point of view. Also:

    Even a dedicated mission (Kepler 2) to examine our nearer neighbors in this way seems well worth doing, but I have a feeling that the same attitude that leads SETI researchers to think “We couldn’t be *that* lucky, to find another civilization so close by” would also lead the “exoplanet hunters’ club” to not be enthusiastic about such a mission. But if we never look for transits of closer stars, we’ll never find any planets that may be there (I’m not saying they *are* there, just that they could be there, and are no more [or less] likely to exist than transiting planets orbiting far more distant stars).

  • Abelard Lindsey July 25, 2015, 12:30

    I also think this planet is a mini-Neptune. I think any planet with a radius more than, say, 1.2 that of Earth is like to be a mini-Neptune.

    What has struck me is how shockingly few “Earth-like” planets have been found by Kepler, with most of them likely mini-Neptunes.

  • Andrew LePage July 25, 2015, 13:43

    @J. Jason Wentworth July 25, 2015 at 1:51

    It would be great if Kepler could be turned towards some of the more interesting nearby stars. There was a recently published paper on Hubble photometric observations of Alpha Cen B looking for transits of its unconfirmed planet that suggests there MIGHT be a second planet in a tight orbit around this star:


    Follow up observations are needed and Kepler would be perfect for them. Unfortunately because of the operational limits of Kepler during its K2 extended mission, the spacecraft can not be pointed stably at targets outside of the ecliptic plane and all the stars you cite can not be viewed by Kepler. NASA’s TESS and ESA’s CHEOPS missions scheduled for launch in 2017 should be able to observe them, however.

    @Abelard Lindsey July 25, 2015 at 12:30

    Recent work on the mass-radius relation for exoplanetes smaller than Neptune strong suggests that planets transition from being predominantly rocky to predominantly mini-Neptunes at a radius no greater than 1.6 RE. For a full discussion, see the Centauri Dream article “The Transition from Rocky to Non-Rocky Planets”:


    More recent work does suggest that the transition point from 100% rocky to less than 100% (i.e. the size where mini-Neptunes begin to show up in the population) might occur around 1.2 RE (i.e. planets with masses greater twice that of the Earth) with high metallicity stars being more likely to harbor mini-Neptunes than stars of more Sun-like metallicity or less.

  • J. Jason Wentworth July 25, 2015, 17:20

    Andrew LePage, thank you for that information and for the article link! If the second possible Alpha Centauri B planet exists, it sounds like an analog of the hypothesized intra-Mercury planet named Vulcan. I’m surprised that nothing has turned up around Alpha Centauri A, although radial velocity observations are hampered by its “noisy” surface (which is active enough, rising and falling so much) to effectively mask effects caused by planets. That raises an interesting question:

    Are there any stars that have *nothing*–not even asteroids or an Oort Cloud of comets–orbiting them? O and B type stars might not show any paucity of angular momentum due to even fairly massive orbiting objects because they’re so large and massive (plus they might have long since vaporized or “sublimed away” such objects with their intense heat and X-Ray emissions), but it would seem that all A through M stars (plus brown dwarfs) should have *something* orbiting them, even if only meteoroids and/or dust.

  • Abelard Lindsey July 25, 2015, 18:07

    Yes, but even planets with 1.2-1.3 Earth radii are likely to have thick “Venus-like” atmospheres even if terrestrial.

    On the related subject of habitability, what do we know about the past existence of plate tectonics on Mars?

  • Abelard Lindsey July 25, 2015, 18:15

    Is it possible that the Earth itself would have had a thick, Venus-like atmosphere had it not been for the giant, Moon-forming impact that we had stripping off much of that atmosphere?

  • Brett July 25, 2015, 18:46

    @J. Jason Wentworth

    I’d be surprised if there are many habitable worlds around M-class stars. Even discounting the flaring and sunspots (which are common but not universal among red dwarf stars), there’s the higher luminosity in the extended pre-Main Sequence phase. A billion years of higher luminosity would turn nearly all of the planets in such a star’s habitable zone into super-Venuses unless they migrate in afterwards.

  • ljk July 25, 2015, 19:47

    SETI pioneer Frank Drake’s take on Earth’s new “cousin”:


    To quote:

    I wondered: what did the pre-eminent searcher for alien life make of the latest planetary news? “It just reinforces what we’ve believed all these years — that what happened in the history of our solar system was in no way unusual and required no freakish events,” Drake told me. “What happened here is going to happen in many places.”

    I cannot believe the reporter said the Drake Equation looked “impenetrable”. It’s a linear equation for crying out loud that has been around since 1961 and is probably one of the most famous math examples right up there with E=mc2. Ye gods.

  • Rob Flores July 25, 2015, 20:15

    Actually, the Window of habitability is what makes these RE 1.25-1.6 worlds
    tenable. What is the estimate time for Any liquid water on Venus to disassociate and recombine to create CO2 and H2SO4 I believe it’s less than 1 billion years.
    In a previous post, I mentioned that tide locking was a hazzard to
    to high life forms development. A distance from the the primary to give a solar intensity similar to earths is what were looking for which is determined by the Age and Mass of a Star. these factors are the fulcrum of benign habitabilty,

    JJ Wentworth, mentioned K type stars as more habitable. I am not sure
    but for an example of a good candidate K star of about 80% the mass than our sun, is CD27 14659A in the constellation Capricorn.
    At .65 AU you are in the sweet spot for habitability. But this is between
    Venus & Mercury Orbit. Does not tide locking, limit the habitability of this world. Albeit it probably would take a few billion years to tide lock it completely. But even a planet the size of earth a rotation
    period lasting 60 or more hours introduces tremendous climatic chaos.

    I would not be surprised to learn that G type and F type are more likely
    to remove tide locking from the habitability equation for planets in their HZ.

  • Andrew LePage July 26, 2015, 10:13

    @J. Jason Wentworth July 25, 2015 at 17:20

    It is logically impossible to prove a negative (e.g. nothing orbits a particular class of star). But given what we know about star and planet formation and how quickly dust apparently gathers to form planetesimals then still larger bodies, even short-lived high-mass stars would likely have something orbiting them even if it is only a large collection of asteroid-like bodies.

    @Abelard Lindsey July 25, 2015 at 18:07

    “Yes, but even planets with 1.2-1.3 Earth radii are likely to have thick “Venus-like” atmospheres even if terrestrial.”

    I do not believe this to be a true statement. Nothing I have seen in the scientific literature on the subject indicates that rocky planets in the 1.2 to 1.3 RE size range must have thick Venus-like atmospheres. So long as there is some yet to be found mechanism that impedes the carbonate-silicate cycle for planets this large (e.g. suggestions that plate tectonics might not operate on planets much larger than the Earth), such planets cloud be habitable, which brings me to…

    @Abelard Lindsey July 25, 2015 at 18:15

    The fact that the Earth does not have a thick CO2 atmosphere like Venus has absolutely nothing to do with the giant impact that is believed to have formed the Moon. Earth does not have a thick CO2 atmosphere because of the carbonate-silicate cycle which has scrubbed most of the CO2 out of Earth’s atmosphere to form climatically inert carbonate deposits. Since Venus lost its water (probably during its first billion years), the carbonate-silicate cycle can not work allowing CO2 to build up in its atmosphere.

  • Michael July 26, 2015, 13:26

    @Andrew LePage July 26, 2015 at 10:13

    With a 30% increase in radius the mass increases to around double the mass of the Earth, so there is more likely to be more low reactivity gases available. The higher gravity would also increase the pressure at the bottom of their atmospheres aiding absorption band broadening. I would think that as planets become more massive their atmospheres also proportionally increase on average.

  • Andrew LePage July 26, 2015, 17:46

    @Michael July 26, 2015 at 13:26

    What you state is not necessarily so. According to “Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass” by Kopparapu et al.(2014), Astrophysical Journal Letters, 787, L29, increasing the mass of the planet (and therefor its gravity) decreases the column depth faster than the effects of pressure broadening, at least in the mass range of interest to us. The result is the inner edge of the conservatively defined HZ is actually CLOSER to the star for planets more massive than the Earth (0.95 AU for an Earth-mass planet orbiting the Sun versus 0.92 AU for a 5 ME planet). The caveat, according to Kopparapu, is “we assume that for these planets, the background nitrogen gas pressure scales according to the planetary gravity. We should caution that volatile delivery to a planet is stochastic in nature, and may be a weak function of planetary mass. Still, this is the best assumption we can make in the absence of a rigorous theory of how planetary volatile content varies with planet mass.”

    As I have said, nothing I have seen in the scientific literature on the subject indicates that rocky planets in the 1.2 to 1.3 RE size range must have thick Venus-like atmospheres. If you could cite some more recent work to the contrary, I would be most interested in reading it.

  • ijv July 26, 2015, 20:23

    If wishes were fishes…

    Is there any chance of Kepler 425b being a binary with individual masses closer to Earth? :-)

  • Andrew LePage July 26, 2015, 23:35

    @ijv July 26, 2015 at 20:23

    “Is there any chance of Kepler 425b being a binary with individual masses closer to Earth?”

    The transit of a binary planet would have a photometric signature distinctly different from that of just a single world (e.g. a two-step dip in brightness with variations in the timing and duration of transits as a result of the two planets orbiting each other). I’m afraid that Kepler 452b is a single large planet.

  • Michael July 27, 2015, 15:23

    @Andrew LePage July 26, 2015 at 17:46

    ‘As I have said, nothing I have seen in the scientific literature on the subject indicates that rocky planets in the 1.2 to 1.3 RE size range must have thick Venus-like atmospheres. If you could cite some more recent work to the contrary, I would be most interested in reading it.’

    It is very unlikely a planet would hold say 100 atmospheres of nitrogen and also, depending on the temperature, of carbon dioxide. Carbon dioxide reacts readily in the presence of water to form carbonates reducing atmospheric amounts. Although there is little in the literature about the volatile quantities per planet mass in atmospheres surely increasing massive planets would have increasingly more massive atmospheric envelops as they would have simply accreted more.

    We will have to wait a while longer until we get a better view of these worlds and dimers may provide the tools to workout the pressures on these worlds.


  • The Other David July 27, 2015, 18:06

    One thing to keep in mind is that the various M-R
    relationships were mostly based on planets around
    M-dwarfs. As mentioned above, the UV flux from an
    M-dwarf, especially in its early history is both much higher
    and lasts for a much longer period of time
    than that for a G-type star. Therefore at any given
    distance, the original H-He atmosphere
    for an exoplanet around the former is much more likely to
    have been lost than for an exoplanet around the latter. In other words,
    the M-R relation the authors use may not be the appropriate one so
    I too think that Kepler 452b is more likely to be a mini-Neptune.

  • ijv July 27, 2015, 18:55

    @Andrew LePage

    How distinct would the signature from a close binary be? Especially if the orbital plane of the binary wasn’t aligned with the line of sight to Earth?
    I’m still holding out for a terrestrial double planet ☺

  • sedjak July 29, 2015, 3:42

    The article and comments discuss the potential habitability of Kepler-452b; yet we cannot really know if it is inhabited. However implausible, the idea goes, a super-advanced civilization could have terraformed this world and mitigated a potentially thick unfavorable atmosphere to make it more habitable for itself. We can’t thus really dismiss ETIs or biology out of hand in any star system with known planets. Likewise, this same ETI may have sculpted Kepler-452b into a nursery world for one of its pet projects. The argument goes further in this particular case since Kepler-452a is a K-class star with perhaps more stable conditions for forming life (as we know it). Guess we will have to wait for much further characterization and advanced spectrometry to know any better in this case.

  • Eniac July 30, 2015, 1:08

    Andrew LePage:

    Earth does not have a thick CO2 atmosphere because of the carbonate-silicate cycle which has scrubbed most of the CO2 out of Earth’s atmosphere to form climatically inert carbonate deposits.

    I believe most carbonate minerals today are of biological origin. Limestone made from seashells, mostly. The carbonate-silicate cycle you mention is inorganic, it appears. How can this be reconciled? How can we know there was/would be sufficient CO2 fixation without the life that apparently has been involved in an overwhelming fraction of it while it existed?

  • Andrew LePage July 30, 2015, 11:28

    @Eniac July 30, 2015 at 1:08

    While various life forms certainly aid in the formation of carbonate deposits in Earth’s relatively recent history, they can and do also form abiotically (as is demonstrated almost every day in freshman-level chemistry labs sessions). In fact, that would have been the primary means of forming carbonate deposits on the Earth before the first organisms evolved to use carbonate skeletons around a billion years ago (give or take, depending how one wishes to interpret the genetic and fossil evidence). A more detailed discussion of abiotic carbonate deposit formation can be found in James Kasting’s excellent book on planetary habitability, “How to Find a Habitable Planet” (2010, Princeton University Press) especially in Chapter 3, “Long Term Climate Stability”.

  • ljk August 6, 2015, 13:17

    The SETI Institute is on the case:


    No radio signals so far. Shocking.