The widely circulated Kepler results, announced yesterday, tell us that over twenty percent of Sun-like stars in the Milky Way have Earth-sized planets in the habitable zone, where liquid water could exist on the surface. Work out the math and it turns out that the nearest Sun-like star with a planet like ours in the habitable zone is probably on the order of twelve light years away, an energizing thought for those of us who ponder future technology and interstellar probes. Imagine: One in five Sun-like stars with a planet the size of Earth in the zone where liquid water can exist.
Image: Analysis of four years of precision measurements from Kepler shows that 22±8% of Sun-like stars have Earth-sized planets in the habitable zone. If these planets are as prevalent locally as they are in the Kepler field, then the distance to the nearest one is around 12 light-years.zone. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.
But how did we get here? Kepler, which was launched in 2009 to look for planets transiting their stars, examined over 150,000 stars for four years and turned up more than 3000 planet candidates. It’s been a fascinating ride, but finding ‘hot Jupiters’ and Neptune-class worlds and even intriguing super-Earths always reminded us that the primary goal was to learn what fraction of stars have Earth-sized planets at just the right temperatures for life. Ideally, retrieving data for G-class stars like the Sun would have helped us look for close twins of our planet.
Kepler’s malfunctions challenged but did not end that effort. The team involved in the present work includes Geoff Marcy and Erik Petigura (UC-Berkeley) and Andrew Howard (University of Hawaii, Manoa), who have been working with the 10-meter instruments at the Keck Observatory (Mauna Kea) to obtain data from the HIRES spectrograph, focusing on 42,000 stars in the Kepler field that are only slightly smaller and cooler than the Sun. 603 planets turned up, with 10 being between one and two Earths in diameter and orbiting in the habitable zone. Remember that only a small number of systems are oriented so that transits occur as viewed from Earth. The team’s algorithms yielded the estimate of 22 percent of all Sun-like stars with Earth-sized planets in their habitable zones, plus or minus eight percent depending on the habitable zone definition.
On the latter point, Erik Petigura defined the habitable zone for this study as that region where a planet receives between four times the light the Earth receives from the Sun and one-quarter of that amount. Kepler’s stuck reaction wheels have meant that extending the mission to analyze G-class stars like the Sun was not possible. Instead, the potentially habitable planets the team found in its survey all occur around K-class stars (Alpha Centauri B is the nearest example of a K-class star, though not in the Kepler field). The team’s analysis demonstrates that the results for K stars can be extrapolated to G-class stars, and thus we arrive at the 22 percent figure.
Telling us how common potentially habitable planets are around Sun-like stars is prime-time for Kepler, and now we have a reading that’s highly encouraging. As we look forward to missions to characterize exoplanet atmospheres and look for the signatures of life in their spectra, we can now assume that only a few dozen nearby stars will need to be observed before we detect an Earth-sized planet in the habitable zone, a fact that will play into the design of telescopes for such missions. Geoff Marcy, though, is quick to point out that just because a planet is Earth-like in size and in the habitable zone defined here, it isn’t necessarily life-bearing:
“Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms. We don’t know what range of planet types and their environments are suitable for life.”
The caution is understandable, and we are a long way from being able to make the kind of observations that help us pin down the presence of life on an exoplanet. We also need to remember that without information about the mass of these planets, we can’t say anything about their density and thus can’t be sure that they are in fact rocky worlds like our own. The discovery that Kepler-78b has the same density as the Earth, announced just last week, does tell us that at least some of these planets are likely to be rocky.
In any case, the idea that there are tens of billions of potentially habitable worlds in a galaxy of 200 billion stars is exhilarating, as Andrew Howard notes:
“It’s been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then, we have learned that most stars have planets of some size orbiting them, and that Earth-size planets are relatively common in close-in orbits that are too hot for life. With this result, we’ve come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way Galaxy.”
We should also put these findings in a broader context. Red dwarf stars are not included in the study, but they represent 75 percent of the stars in the Milky Way. The question of whether life could exist around such a star, given the problems of tidal lock and stellar flare activity, is an open one, but we do know from previous work that 15 percent of these stars are expected to have Earth-sized planets in their own habitable zones (this is based on work by David Charbonneau and Courtney Dressing at the CfA; Ravi Kopparapu at Penn State obtained an even higher estimate). We don’t know yet whether life exists on any of the worlds around any of the stellar classes, but it does appear that the cosmos is stuffed with planets where the great natural experiments that lead to life can be run again and again.
The paper is Petigura et al., “Prevalence of Earth-size planets orbiting Sun-like stars,” Proceedings of the National Academy of Sciences, published online 4 November 2013 (abstract). Dennis Overbye’s report in the New York Times is well worth reading. I love this quote in Overbye’s article from Geoff Marcy: “This is the most important work I’ve ever been involved with. This is it. Are there inhabitable Earths out there? I’m feeling a little tingly.” Me too.
The print edition of the NY Times , on the continuation page (A15) has the Drake equation at the top of the page.
I remember when , some experts, thought that the second term (the number of stars with planetary systems was 1!).
Now the third term is about to be nailed.
I think Copernicus (or his principal) should be approached with the utmost respect!
Exciting news indeed. I also heard they were proposing a plan under which 2 wheel Kepler would still be able to continue finding Earth-like planets. Is this it?
I have to say that this sounds like theyre declaring victory and calling it a day. There is no way that a RE 1.5-2.0 world is earth like. Just do the math. 1.33 x PI x R^3 Even a 1.4 RE world is a world with 2.7 times more
innards compared to Earth. IMO Kepler proved that Terrestrial Planets are common but they tend to be Larger than Earth and mostly close to their parent Star. I am disturbed that a substantial disappointment has been whitewashed. More so because by declaring the mission sucessfull it is unlike we will have follow up space mission on this theme. Critics of such mission will just point to this Kepler paper and declare the Science is Settled.
hold on. Earth sized was defined by kepler team as planets with 0.75-2 earth mass or max 1.3r way back when the mission started. I dont see any candidates in the HZ like that they’re all super earths.
On their webiste they present these results by saying “Ten of these candidates are less than twice the size of Earth and orbit in their sun’s habitable zone” and “one in five stars like the sun is home to a planet up to twice the size of Earth, orbiting in a temperate environment.”
i assume theyre talking about radius so its not “earth size” as they defined it years ago. Are the lack of true earth size planets in the HZ a result of kepler not performing as well as it was designed to?
Some sloppy reporting even on the National Academy of Science site. They say 22% of stars harbor Earth-like planets while the abstract of the paper right below states 11%. I’d go with the paper…
It’s great that this has been studied, but I agree that the assumptions leading to the 22% estimate seem much too optimistic. Not only are planets of 1.5-2 Earth radii in the HZ likely sub-Neptunes like Kepler-11b, but also their “self-made” definition of “habitable zone” is extremely generous: In the Solar System, it would be 0.5-2.0 AU, almost from the aphelion of Mercury to the perihelion of Vesta!
(And unlike this article (and the NYT) claims, the error range “plus or minus eight percent” is not due to different habitable zone definitions, but is the statistical error for this single optimistic HZ definition, if I understand the paper correctly.)
The paper itself gives more “realistic” values when using the habitable zone as defined by Kasting resp. Kopparapu: Then eta_Earth decreases to 5.8% resp. 8.6%. Restricting also the size range to 1-1.4 Earth radii reduces the latter value by about half (according to the paper), so we have maybe 4.3% as a more “conservative” estimate. And of course this is not based on actual data but on (mild) extrapolation, since Kepler hasn’t actually found any such Earth-analog yet.
“We don’t know yet whether life exists on any of the worlds around any of the stellar classes, but it does appear that the cosmos is stuffed with planets where the great natural experiments that lead to life can be run again and again.”
Worth repeating.
And it’s also worth repeating what many posters here on this site have said before: that “life” means “life”, not “exactly like us”.
I think that pessimistic appraisals of planetary possibilities tend to focus on the question of “how could life like ours evolve in a place like that?”
It goes without saying (and yet bears repeating) that it is very possible to have life not-like-ours.
I am prone to agree with the macro implications of a statistical survey.
Likely there ARE eath-like planets out “there” of nearly the right size and in the habitable zone of a G star.
But Kepler didn’t/hasn’t found any of them.
For statistics to predict that an earth twin “should” be within 12 light years and for one to actually be identified are far from the same thing.
Please see here :
http://spaceref.com/extrasolar-planets/one-in-five-stars-has-an-earth-sized-planet-in-its-habitable-zone.html
Specifically where it says :
“The team focused on the 42,000 stars that are like the sun or slightly cooler and smaller, and found 603 candidate planets orbiting them. Only 10 of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures suitable for life. The team’s definition of habitable is that a planet receives between four times and one-quarter the amount of light that Earth receives from the sun.”
Not only 1.5-2x the size of earth is dubious at best, but 4x the solar flux is completely ridiculous for the HZ. This is the same solar flux at 0.5 AU. Venus is 0.7 AU.
I wanted to verify they said this and yes, its even in the abstract.
At a time where science is questioned so much for climate change, it is important that precision and honesty prevail.
I can only see this as data manipulation. Think of the general public that now knows that there’s an “earth-like planets in 22% of G stars” and there is no basis for this.
That’s rather generous. The incident flux received by Venus is only 1.91 times more intense than that of the Earth, though I suppose Venus does have pressures and temperatures compatible with liquid water — 60 kilometres above the surface, at the level of the sulphuric acid clouds.
It would be interesting to know how the value changes with more realistic habitable zone models.
Maybe, maybe not. Kepler-78b is likely to have suffered more severe atmospheric erosion at such a close distance to the star.
Very exciting news… in the long term I suspect that it will be finding habitable planets around other stars that will drive interest in interstellar travel, so these results are very encouraging!! 10s of billions of potentially habitable rocky exoplanets in a galaxy of 200 billion stars… that is a very exciting estimate.
Of course, we can’t be sure what a planet will be like just from knowing its size… just look at Earth and Venus. Apparently twins, yet so different. We must wait for the next generation of space telescopes to hunt for the true Earth analogues…
two papers I saw presented on the live feed mentioned anything above 1.6 Radia as being a mini neptune with a gas atmosphere
or
anything above 1.6 is not a rocky planet
If I recall correctly most of the other presenters for the most part followed this convention and used something like this figure to describe the “earth’s” in the HZ
also there was a online community that I enjoyed chatting with and I came up with a question
would we be able to use doppler or something like exomoons “timing” to measure mass or size of exoplanet ring systems?
and rotation rates of the ring system?
I know, I know the ring system unlike a exomoon would not have its mass in a single object but distributed “evenly” ( shepherding moons?) but after a while our online community came up with ideas that might make ring systems detactable,
here I got lost…………………
someone mentioned a prograde orbit?
how about an inclined orbit that still does a transit, would a ring produce a blue/ red doppler shift?
certainly they do exaggerate with their announcements. In that sense that, contrary to the declaration, not many (if any) true earth sized planets (0.8-1.2 earth radius) has been discovered in the HZ of K or G type stars. Kepler simply did not get it in the time of operation. They seem to extrapolate the occurrence freq of earth sized planets with shorter orbits. Probably it is OK to do so, but nevertheless the general conclusion that there is 20% of HZ earth-twins around Sun like stars is an obvious exaggeration. If only 0.8-1.2 criterion was applied this number would have been much smaller – about less than 5% probably. Unfortunately no journalist has asked such critical question. Maybe Greg or some other competent Centauri Dreams follower could kindly explain me if I am wrong with this criticism or not
ALL of the Earth size kepler planets of RE .5 to 1.2 have been found in tight orbits around their primary, and well inside HZ, with very hot surfaces
ALL but one terrestrial kepler planets in the HZ is atleast 1.6 RE Most are toward the 2.0 RE value. There is a lone 1.4 example in the HZ.
It sounds to me like RE .90-1.1 in HZ’s in K & G stars are closer to 1% in frequency. So the closest twin earth is still something like 100 LY away.
Really those paltry numbers for RE1 to RE2 in HZ planets should give one pause, just by itself. I would give more credence to the optimists if there a veritable smogarboard of Terestrials of all sizes there.
I can’t see why they had to go with the range of *4 insolation to *0.25. Is it for the press? that 22% leads to 12LY which is “visitable” [ whereas *2 to *0.5 would yield 25LY which is not doable??]
Actually, a good paper should present a range of estimates depending on
various insolation ranges ie *2 to *1/2; *1.5 to *1/1.5 etc. each of these would lead to a percentage and a figure for #-of-light-years-to-nearest.
[i think they couldnt use R_e ranges because it’d have to be 0.8-1.2 R_e within say 50% insolation to 200% insolation and Kepler didnt collect enough data for that]
I do hope that this does not turn out to be hype that results is a lot of push back. Also, as we have talked about before, “earth like” in this context does not mean a world that looks like ours and is biotic or pre-biotic. The general public is going to think this means that there are likely living or inhabitable worlds out there, quite close by. Is this a calculation on NASA’s part to push for more funding for more missions to detect those worlds?
some snippets from yesterday and today,
M class stars will have dry planets in the HZ, carbon planets I guessed? no !
its a lack of a large planet to shepherd in ice to a HB planet in the M star HB
In the kepler II chat function I conjectured engineering the ice planetoids into the dry world
in computer modeling some of the M dwarf snow line worlds where in fact larger then Ceres
one speaker discussed as far as I could tell ( sound issues) the idea of ice core worlds, IE no rocky inner core, some of my chat participants questioned the ice core theory and they brought or the speaker mentioned a “steam core”
I brought up the ideal gas law and if it be a very large planet how any gas could be heterogeneous at ice planet pressures?
so in the spirit of our KSC II I objected,
would not H2O under such pressure degenerate into a metallic H and produce a super critical Ozone layer above?
during our “chat” I brought Fomalhaut B on a number of occasions, is this a gas ball, planet with a large ring system? one speaker discussed large and small planet vortices, large planets produce a stable outer system regimes (NICE)
Kepler II is commencing BRB afternoon session PST
https://connect.arc.nasa.gov/kepler?launcher=false
(I regret that I’ve been so quiet here. Real life and the search for a livelihood matching my aspirations has kept me in the weeds.)
One thing these studies suggest is another potential answer to Fermi’s question: If habitable worlds are truly common, then it is conceivable that a species might find so much low-hanging fruit (colonization-wise) so near at hand that it would feel it redundant to move much further afield.
If so, of course, this suggests either that explosive radiating growth is not a foregone conclusion once toeholds are gained, or else that expansion to a mere handful of nearby systems is enough to answer to long-term risks… at least enough so that some kind of transcension hypothesis variant becomes possible.
http://accelerating.org/articles/transcensionhypothesis.html
(I’ve considered this answer to Fermi interesting, but unsatisfying in light of its failure to answer to the “all your eggs are still in one basket” complaint. A handful of local systems would deal with this complaint, barring multi-system gamma ray bursts and similar events.)
I think Kepler has given us a taste of what the frequency of Earthlike mass planets in the HZ is, but until we can get transits down to 1/2 Earth radius we won’t really know.
We now have some idea of the frequency and distribution of planets Earth sized and up, but no idea of that below Earth’s size; the bulk of Earthlike planets may be between 1/2 Earth mass and 1 Earth mass (or .75 to 1 Earth radius). And getting a bit below .75 Earth radius would give us a much better frequency/size curve for terrestrial planets. Is it linear or is it exponential?
False positives for Alien biospheres in the Habitable Zones of M-dwarfs…
Just to add to the confusion, around M-dwarfs things get complicated. Surprisingly the Far-UV to Near-UV ratio of Red-dwarfs is much, much higher than our own Sun. This allows CO2 to be dissociated by FUV light to produce O2 & O3, potential biomarkers, but reduces their loss via photolysis by NUV light. Thus Red Dwarf planets will have detectable oxygen and ozone in their atmospheres even without photosynthetic life to resupply them. Conceivably there could be lifeless planets with ~breathable atmospheres.
For some idea of how alien the observed Kepler planets in the 1-2 Earth radius range, there’s this study’s results:
Densities and Eccentricities of 163 Kepler Planets from Transit Time Variations
Authors: Sam Hadden, Yoram Lithwick
(Submitted on 29 Oct 2013 to ApJ)
We extract densities and eccentricities of 163 sub-Jovian planets by analyzing transit time variations (TTVs) obtained by the {\it Kepler} mission through Quarter 12. We partially circumvent the degeneracies that plague TTV inversion with the help of an analytical formula for the TTV. From the observed TTV phases, we find that most of these planets have eccentricities of order a few percent. More precisely, the r.m.s. eccentricity is 0.018+0.004?0.008, and planets smaller than 3R? are around three times as eccentric as those bigger than 3R?. We also find a best-fit density-radius relationship ??2.2 g/cm3×(R/3R?)?1.8 for the 64 planets that likely have small eccentricity and hence small statistical correction to their masses. Planets larger than 3R? are mostly less dense than water, implying that their radii are largely set by a massive hydrogen atmosphere.
Expanding on the empirical analysis in the previous paper linked to, is this theoretical modelling work:
Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
Punchline: We suggest 1.75 Earth radii as a physically motivated dividing line between these two populations of planets [Earth-like vs Sub-Neptunes].
Interestingly this almost fits the empirical density-radius result – the fitting function matches the density of an Earth-like planet at about ~1.53 Earth radii. At that size an Earth composition solid planet has a density of ~7.4.
Question: is it not the case that Kepler broke down before the existence of truly Earth-sized planets in the HZ could be confirmed? That being the case, surely no real conclusion can be reached? (And, you could also say that the major objective of the mission was failed, and that is why there are papers like this?)
The perfect colony world? No planetary protection concerns, and no need to terraform for a breathable atmosphere.
Four year Plus 2 months, should have been enough time to find
RE .9 to RE 1.1, Planets in the HZ of stars in the Kepler field.
Especially since most yellow suns and heavyer Orange suns tend to
give fewer false positives than Red Dwarfs.
Note that for stars even slightly smaller/cooler than our sun say G4-G8 the luminosity decrease brings into play shorter orbital periods for the terrestrials. This means that Kepler SHOULD have spotted their transits from 5 to 7 times, by now. But nobody is officially pointing this out.
I still think the odds do not favor a closeby Twin Earth (w/o our O2 levels obcourse.) . But as they say that’s why we roll the dice, lets see if we get lucky and find a close one.
@Rob Flores are you saying that RE 0.9-1.1 in the HZ would have been detected if present (and therefore they are not present). Or, if present, are they still below the detection limit in most cases?
Regarding the number of planets 1.25 Earth radius or smaller the latest Kepler results show the largest increase of detections in that category. That trend will likely continue with a more complete analysis of all 16 quarters of Kepler observations through the next year or two.
If there really are significant numbers of these Earth size worlds orbiting G and K stars in 200 to 400 hundred day orbits is the question. The small numbers of Earth size worlds identified in HZ orbits so far is, I think, more indicative of the limits of Kepler’s capabilities then an actual absence of such planets. These are the most difficult targets right at Kepler’s limits.
Hopefully, further analysis of the Kepler database will provide us with a better understanding of these elusive planets. We haven’t yet heard the final word from Kepler.
If they were there in significant numbers (Esp at the G7-K7 star classification ) a few, maybe 2-3 should have been detected. They
are not supposed to be beyond detection range.
I do not mean to say that no twin Earth’s exist in the whole of the Kepler study star field. But from this article : in the DAILY GALAXY. Some hard
calculations using statistics.
“Out of the 156,000 stars being monitored by Kepler, we are effectively searching only 27 for perfect Earth analogues: If they are less abundant than 3%, we may very likely find none, unless the Kepler’s mission is extended to allow several years more of data collection.” From Dr Rehling. Well the mission did extend to something like a year more after this article.
Let us assume that number is 9 perfect Earths analougues if we just look at G and K type stars. Not only did we not find one, we did not even find a close but no cigar Twin of Earth at .75 RE – 1.25RE in a habitable zone around these type of stars.
RE: Alex Tolley’s remark. I would have to freshen up my rusty chemistry, but I suspect that abiotic oxygen formation would result in a build up of Nitrogen Oxides in the atmosphere through lightening strikes, and, over time, most of the Nitrogen in the atmosphere finishing up as Nitrates. (There been no life to use it as an oxidant of organic compounds.)
Still, the detection of Nitrogen Oxides in the atmosphere may make a good marker as to whether the oxygen detected is of biological origin.
If the occurrence of earth-sized (.8-1.25 Earth Radius) planets in the habitable zone of solar-type stars really is very rare, then why are we so amazingly unique? After all, most work to date seems to very much confirm the core accretion theory of planet formation and both Kepler and HARPS seem to confirm that at least 70% of stars have planets?
Could the reason (or reasons) be:
1. In many instances, close binaries disrupt planet formation.
2. Jupiter type planets migrate in and destroy, absorb, or scatter the building blocks of earth-type planets. But this only affects 10%-12% of stars within 3 AU.
3. Neptune and Super-Earth type planets migrate in and destroy or scatter the building blocks of earth-type planets.
4. Kepler is simply not sensitive enough to detect smaller planets beyond 100 day orbits. This has been suggested because of the discrepancy between Kepler and HARPS results within .25 AUs.
5. God has simply made us unique. I hope this is not the case. Otherwise, why did he make all that empty real estate out there?
As an aside, does the newest Kepler estimate of 5.8% for 1-2 Earth Radii planets in the habitable zone of solar-type stars include or exclude the effect of having a binary partner?
@Rob Flores
“This means that Kepler SHOULD have spotted their transits from 5 to 7 times, by now. But nobody is officially pointing this out.”
Good point.I tried to here a while ago, accompanied by calculations that showed that, for lower luminosity stars, Kepler had already observed more than 3 transits.
I don’t remember any replies.
See also here for details :
http://oklo.org/2012/03/11/lights-in-the-sky/#comments
That’s when Kepler had observed for 18 months.
Did Kepler really observed for 4 years and 2 months ? I though that there was an initial commissioning period.
I think that the bar was raised to 5 transit because G stars were more noisy than expected. However, knowing exactly how long Kepler observed, it should be possible, as I have shown, to calculate for what star mass it has observed 5 period of the habitable zone.
Heath Rezabek:
Put yourself in the shoes of the inhabitants of a world at the boundary of this local neighborhood you are describing. They live in their home system because their ancestors colonized it, millenia ago. They know they can do this again and just 12 light years away another uninhabited system beckons. Now you are telling me they will find it redundant to go there just because there are inhabited worlds in the opposite direction? I do not find this plausible at all.
Kepler was launched March 7, 2009 I believe it took a month to put it into
operation. The 2nd reaction wheel stopped on May 11, 2013. Yes maybe the figure is closer to 4 year plus 1 month.
As to Theories as to why Earth maybe “special” I favor
a Supernova burst nearby early in our solar system formation.
clearing out the debris field a bit and lowering the masses of
potential terrestrial planets.
I am confident that NASA has the integrity to call it as it sees it. If the data really points to a strong confidence to declare Earth-like planets are probably very rare and it was de-emphasized because It might seem to promote religion then it would be a crime against science.
Spot on Dave, denitrifying, NOT nitrifying, is the dominant action of life on Earth. If life disappeared, all oxygen would combine into nitrates in a few million years – that’s two tons of it for every square metre’s surface! But will that be replicated in other biospheres?! Biology texts seem to imply ‘don’t worry be happy’, just assuming sufficient nitrates will always cycle through anaerobic regions carrying heaps reduced feedstock. Given how special conditions actually need to be I have my doubts.
Rob Flores, Mike, Enzo -surely no-one has said there should be a sudden dip in the frequency distribution of planet sizes around 1 RE. That seems unlikely to me. Looking at the replies on here, to me it looks like true Earth analogues (which are in fact there) have simply not been detected reliably enough in the data to be announced as discoveries. I think Enzo has a point, perhaps the amateurs could accept a lower degree of confidence in detection, and work the statistics on that.
Four year and one month means 5 periods of 300 days. That’s a lot of stars, including a lot of G stars.
Maybe there is a problem with Kepler Also, there are reasons to believe that the HZ extends outwards a lot more than inwards where larger earths, with thicker atmosphere could be warmer than otherwise expected. These zones are less explored by Kepler because they require longer observation times.
By the way, personally, I HATE the fact that earths look like they are rarer than expected. But I hate changing parameters to fit my liking even more.
I remember looking at the output of ACRETE and thinking how it was likely that there would be so many solar systems like ours. I certainly preferred that scenario to the current one.
And after nearly 20 years of radial velocity, where are the cold jupiters ? Our solar system looks weirder by the day.
I would love to see some follow-up on Intermittent Near Earth
Twins signals that are too low confidence to publish. Surely the
Kelpler team would hand these off to ground based observers.
I mean It would take the fortitude of Thor to know that one
signal is some what > close< to confirmation confidence (but the grand poobah forbids any announcement) And resist the urge to leak such information.
KBZ maybe you are right. Maybe the Kepler mission in retrospect wasn't
sensitive enough to give a definitve answer on Earth Twins. But this fact
only became apparent after operations were well under way. Which would answer alot questions about how the data from Kepler is being released and
analyzed.
Enzo, sizes of things in nature follow smooth frequency distributions. I can’t see there is any physical reason why there should be sudden dip in the frequency distribution around Earth mass. Couple that with the fact that it is near the detection limit, and I can only conclude it is an artifact of the data. Those planets are there, they just have not been detected yet.
Not many systems have cold Jupiters, but a lot have close-orbiting super-earths. I wonder, maybe in the typical stellar system, it is possible that the metals that would have gone into a Jupiter are instead used to make a large number of terrestrial planets? And that these Venuses and Marses (is that a word?) are orbiting too far out to be detected in these crowded systems?
Rob Flores:
“Kepler was launched March 7, 2009 I believe it took a month to put it into
operation. The 2nd reaction wheel stopped on May 11, 2013. Yes maybe the figure is closer to 4 year plus 1 month.”
Kepler only collected data from May 12,2009 onwards, and lost at least 15 days due to “safe mode” shutdowns inbetween. Substracting a few more days for data downloads, there’ll only be a total of ~ 3 years, 11 months of scientific data. To get the necessary ~6 transits (according to https://centauri-dreams.org/?p=19781), small planets need to have orbital periods <240 days on average. So Kepler could not discover any habitable-zone planet around a Sun analogue.
Does someone know up to which spectral class an Earth analog would have period <240 days?
kzb:
"I can’t see there is any physical reason why there should be sudden dip in the frequency distribution around Earth mass. "
It's not a sudden dip, but a diagram given in the paper (Fig. 3) shows that at least for periods <100 days, nearly Earth-sized planets are somewhat rarer (12.0% of all stars) than larger super-Earths. The most common (logarithmic) size range of planets is 2-2.8 Earth radii, at 18.6%.
Also, the most common (logarithmic) orbital period for their "Earth-sized" (1-2 Earth radii) planets is 25-50 days – "Earth-sized" planets with 100-200 days orbital period (the highest range for which there are good data) are rarer by a factor of 2.5, only occurring at 3.2% of all stars.
(See fig.2; the authors euphemistically call this
"constant (within a factor of 2 level) over the entire range of orbital period"
and then extrapolate this "constant" function to higher periods…)
All these figures are already corrected for observational bias. If the decreases of occurrence rates continue for higher orbital periods and slightly smaller sizes, the number of planets with 0.8-1.2 Earth radii around G-stars in the more conservative habitable zone (orbital period 300-600 days) will be below 1%.
@kzb,
Maybe Kepler is not sensitive enough or broken somehow. An investigation should be directed to understand exactly how because a lot of missions are planned based on photometry, transits etc. if there’s something essentially wrong, we better find out.
Have a look at these interesting graphs on the excellent but infrequent Sytemic blog. It seems to show a sharp drop rather than a continuous tapering :
http://oklo.org/2012/11/10/the-mmen/
http://oklo.org/2012/02/19/regular-systems-of-satellites/
Earth and Venus really stand out.
Kepler had no problem detecting earth size planets in compact systems. This suggests their data should be looked at and the question asked : where they clearly detectable after 3-5 periods or did they emerge after many more periods ? There’s quite a few of these systems and, being short period ones, multiple observations (tens of transits) should be in the data.
If the compact systems’ planets clearly emerged after 3-5 periods only, then this doesn’t point to a Kepler’s problem for longer period planets.
Enzo:
This is simply and only because they are located in a region of the plot where Kepler has no sensitivity (Imagine a diagonal line below and to the right of which there are no points).
We are still in the unfortunate situation where NONE of our own planets would have been detected by Kepler, were they orbiting around stars in its field of view. We came close, but didn’t quite make it. Because of unforeseen extra noise, as I understand. The gyro malfunction didn’t help either, I suppose.
Enzo: “If the compact systems’ planets clearly emerged after 3-5 periods only […]”
They didn’t, as you can easily check from Kepler’s discovery dates. The first Earth-sized planets, Kepler-20e and f, were only discovered in Dec. 2011 after 2.5 years (https://centauri-dreams.org/?p=21091), and even if you discount half a year for data processing and confirmation, this means it took over 100 (!) transits of 20e and about 35 transits of 20f to find them. The recently announced Kepler-78b even had 4,000 transits under its belt (probably the announcement was held back somewhat until its mass could be determined).
As I said above, it’s been known since 2011 that Kepler was not sensitive enough to find Earth-sized planets after “3-5” orbits. 5 orbits is only enough for super-Earths, which cause larger and thus more statistically significant dips in the lightcurves of their stars.
Eniac,
Your hypothesis can be partially tested looking at Kepler’s data, as I mention above. It clearly had no problem to find planets the size of earth (down to moon size in fact) for periods of ~10 days. So, the question to ask is, if you looked at Kepler’s data for these planets, can you easily detect them after 1-5 periods or do you need many more? if so, how many ?
If you can detected them from the first, say, 30-50 days of data, then the situation is no different from 3-5 periods of 300 days or less, at least from the stellar noise point of view.
@Holger,
“Does someone know up to which spectral class an Earth analog would have period 30 days. More up to date data will show more dots, but no big change.
@Holger,
Not sure what happened to my post. Anyway, G stars start at 0.8 Ms and 0.6 Ls.
If you plug 0.8 Ms and 240 days in the equation below and solve by d :
P = 2 *pi *d/sqrt(G*M/d)
You get 103MKm. This is similar to the HZ of 0.6 Ls 150*sqrt(0.6) = 116MKm
So, 240 days is on the edge of the HZ of the dimmest G stars.
The graphs on Systemic though, although a bit old, show nothing for p>~30 days. More up to date data will have more dots but no big change.
@Enzo:
Thanks for the answer. So Kepler could observe 6 transits for many (most?) of the K-star Earth analogs, but not for any G-star Earth analog.
Thus, either Kepler needs even more than 6 transits to detect Earth analogs (or very long confirmation time), or K-star Earth analogs are so rare that Kepler’s FOV didn’t contain any transiting ones.
The first possibility seems plausible from the time it took for Kepler-20e and f to be confirmed.
OTOH, according to Rob Flores’ “DAILY GALAXY” quote above, the latter possibility would be the case if Earth analogs are rarer than 3%, which unfortunately seems likely by my “back-of-the-envelope” estimate above.
Enzo:
I think my “hypothesis” is already clearly evident from that plot that you mentioned. You can draw that diagonal line between detections and no detections quite clearly, and nobody would claim that there is n actual drop-off right beyond that line of detection, it just does not make sense.
The researchers whose paper we are talking about here did a proper extrapolation beyond the line, and concluded that Earth or Venus do not stand out at all, but are merely examples of a very common type of planet that is unfortunately just beyond the ability of Kepler to detect. We have to wait for a new generation of instruments to positively confirm this, but at this point there cannot be any serious doubt, in my opinion.
If anything, the fact that our own planets are outside the limit of detection is evidence for most exoplanets remaining undetectable, which bodes well for the next generation of instruments.
Eniac,
I wish I could be as sure as you. Just keep in mind that those graphs include planets from both Kepler and radial velocity. Both methods would have to be broken and broken in a way that shows the same empty areas. In the areas where they overlap, they tend to agree.
Keep in mind that HARPS has been working for 10 years and mini neptunes have been well within its range.
Enzo,
I wouldn’t call a lack of sensitivity “broken”. Radial velocity does lack the sensitivity to find Earth analogs: it can currently only detect planets with RV >=0.5m/s (like Alpha Cen Bb, which was a cutting-edge discovery and is still disputed). Earth has a RV of only 0.1 m/s. According to the table in https://en.wikipedia.org/wiki/Radial_velocity_method#For_MK-type_stars_with_planets_in_the_habitable_zone, radial velocity could only detect Earth analogs around late M-dwarfs.
Eniac:
I agree that there “cannot be any serious doubt” if you take the paper’s results at face value. But as I said above, I find their extrapolation to higher periods rather dubious (though I’d be happy to be proven wrong in this).
It’s also strange that Kepler should have been able to find Earth-sized (<1.2 R_E) planets up to a period of 240 days, but did not find any candidates above 50 days (acc. to the paper); among the confirmed (sub-)Earths none even has a period above Kepler-20f's 20 days.