Define 'earthlike, Marsophile' Rather a contentious term these days!
If you just mean Earth sized than the one at Centauri B takes the cake, though I'd warrant it's more of a 'super Mercury'.
There's also Gliese 581 C a bit farther off, along with Tau Ceti, Epsilon Eridani, etc, which are probably a little (relatively) less nasty..

At a rough guess, about 14 light years - taking in the twenty-or-so "Sun-like" G and K-class stars within 22.7 light years, and assuming a fifth of them have "Earth-like" planets.

Andy

As I'd mentioned above, there's a vast ambiguity regarding what "earthlike" is.

One way to cut the Gordian Knot on that ambiguity is to speak in terms of the incidence of planetary occurrence in a portion of parameter space centered on Earth itself. If we defined the radius to be 0.8 to 1.2 Earth radii (RE), we would naturally get about twice as many as if we defined the radius to be 0.9 to 1.1 Earth radii. But if we put equilibrium temperature on one coordinate and radius on the other, we could measure area in something like RE times degrees Kelvin, and measure incidence in terms of planets per unit area in that abstract parameter space. That's liable to be hopelessly confusing to laypeople, but unambiguous, at least.

The Exopag conference last month had some nice presentations devoted to this topic, and several of them are online here:

http://exep.jpl.nasa.gov/exopag/exopag8/agenda/

I see that at least one really interesting one was taken down after I copied it to my home computer. Perhaps the author will re-post that information at an appropriate time.

It appears likely that M dwarfs as a group will have far more total planets that fulfill any given definition of size and equilibrium temperature than FGK dwarfs (i.e., sunlike stars). The catch is that with smaller stars, the planet's rotation may be tidally locked, which may be destructive to the climate. But we will likely find more earthlike stars circling nearby M dwarfs than sunlike stars, so the answer will have to be qualified in those terms.

Very roughly speaking, the expected value is likely to be in the low tens of light years (~30 LY?) for sunlike stars and for red dwarfs, much closer (~15 LY). That's subject to a lot of caveats, but as you note, the cube root reduces the effect of those unknowns quite a bit.

Does being larger than Earth (as long as we're still in the rocky-planet size range) make them any less interesting? I mean, you probably couldn't walk there due to gravity, but these worlds are so far away anyway...

Also, I'd read tidally locked was probably OK for a world with oceans and reasonably thick (~Earth) atmosphere since those would distribute heat well enough to keep the ocean from boiling off the dayside or the air from freezing on the nightside. Do other people say otherwise?

Surface gravity depends on a great deal of factors, not just size.
At Saturn you would weigh less than on Earth (assuming you had a place to stand), because of the planet's fantastically low density. Mercury and Mars in our own solar system have nearly identical surface gravities (1/3 Earth gees), despite the latter being far larger, because of the differences in density. Mercury is essentially a core with a bit of crust on the top, a "cannonball world" as some call it.

Mission managers have revealed their proposal for an extended mission - K2:
http://www.nytimes.com/2013/11/19/science/...cience&_r=0
The spacecraft will shift to the ecliptic - primarily so the spacecraft won't have to fight the uneven push from incoming sunlight at other angles (the article also suggests it could even be a stabilizing force when positioned properly - which might explain why it can do science with only 2 reaction wheels still). Observations of regions on the ecliptic will be around 90 days, and mission managers suggest there is 7-10 years of propellant still available for the craft to operate (assuming the other 2 reaction wheels keep working).

This is all pending approval and budget from NASA, of course. But exciting that a resurrected mission is possible!

Oh that is good news. And yes I see the reason that Kepler will have to change target after those 90 days. To maintain that optimal position for the solar wind & light.

But I think I do see some drawbacks, shorter observation time - or repeated but interrupted ones, might get lets say one planet at a given star, and then only the kind that orbit very close in to the star ie: 45 days or less. (Two detections necessary to confirm.)
But we might miss out on many other ones, and so not learning what that -potential- planetary system is really like. In addition the solar system is aligned in such a way that Kepler will look out of the galactic plane most of the time - and so observe fewer stars.
Even so, I do hope they will attempt this, absolutely!

Looking out of the galactic plane is not sub-optimal. The galactic plane is so dense with stars that it's not optimal for Kepler's methodology because it would increase the number of cases where the pixel containing a star has a background star making results ambiguous. In fact, Kepler's main mission was dedicated to a region near the edge of the galactic plane, with one corner featuring a significantly higher density of stars than the opposite corner (~3x).

One exciting and strong possibility is that galactic latitude (how far above/below the galactic plane) could be a very important variable in the kinds of systems that evolve, because there is more metal (elements with atomic number >2) close to the galactic plane. I would suspect that the main value of this extension would be to probe the inner system of lots of stars of different metallicity and thus survey how the inner systems vary as a function of this. It's not trivial, though, to determine the metallicity of a star, and the existing estimates for the Kepler field are very shaky. But once any particular stars are determined to have/lack planetary systems, we have arbitrary possibilities to determine, later, the stellar properties with good precision.

Naively one might guess that a system with higher metallicity would have more rocky planets because there is more raw material to create them. However, there's some suggestion in the existing data that this is, surprisingly, not the case.

So the value of the extended mission, I expect, will be primarily to pump up the "n" of inner systems surveyed. I wouldn't expect, otherwise, any surprises in the results.

Thank you for that explanation JRehling.

I knew Kepler were aimed more or less toward the galactic plane, but not the reason for having it slightly offset.

And yes I have also read about the surprising find of planets at stars with low metallicity.
And this extended mission, -if approved- will improve the statistics for how many planetary systems there might be total in the galaxy. Some of the stars might be regular stars that just happen to be at apogalacticon, and others might be from mergers.
Yet also that is one opportunity, since it indeed might get us data on planetary formation where the conditions originally were different.

Is there a sense of how planets orbit vis-a-vis the galactic plane? Is there a guess as to how many stars spin like Uranus in our system? Can we detect a wobble from a star with planets orbiting vertically to our perspective? Do I only ask questions?? lol I'd love any thots from our esteemed gang here!

We see both transiting systems and radial-velocity detections all over the sky, showing that planetary systems have no strong preference for having their orbital planes oriented any particular way relative to the galactic plane (ours is inclined by about 60 degrees, for example). Radial-velocity detections are biased toward systems seen edge-on (by a sine factor), so we are insensitive to orbits seen like a bullseye, but not affected by the orientation of the planets' orbital axes to a particular direction on the sky.

(Using a single plane for a system is a simplification, good to about 8 degrees for the major planets in our own system and much worse including dwarf and minor planets).

The issue of planetary system orientation in comparison to the galactic plane is probably similar to that of pairs of binary stars, which is a much easier case to observe, and this has been shown to be essentially random.

http://adsabs.harvard.edu/full/2004SerAJ.169...59P

I've got a bombshell to share with everyone from the 223rd AAS.

The first transiting earth mass planet has been discovered. The mass has been weighed with TTVs, and is not just an estimate. I feel it is the most important find since the discovery of the Kepler 11 system. A lot of research has been dedicated to finding the transition point between rocky planets and mini neptunes. I think this find has given us an answer. There isn't one.

SpaceRef
http://spaceref.com/exoplanets/newfound-pl...-the-first.html

Harvard
http://www.cfa.harvard.edu/news/2014-01

HEK website
http://www.cfa.harvard.edu/HEK/koi314.html

Edit: It seems that KOI 314 is close enough (200 light years) that it can be studied by the James Webb after it launches. What an exciting discovery.

Great find, as it demonstrates the ability to detect Earth sized planets.
They bagged this one while looking for exo-moons even.
Yet they do say it seem to be a mini Neptune that have lost a substantial part of the atmosphere.

Note that its diameter is 60% greater than Earth's. It's an Earth-mass planet, but not an Earth-sized one. Though they've already found Earth-sized planets; the trick is finding a Goldilocks world.

Oh yes that is the big price, though we're not allowed to say why here.

Anyhow, in related news, astronomers at the Univ of Toronto report about another in-between class planet. This one that might be the missing link between a brown dwarf and a gas giant.

By the way, Kepler was featured on Nova this week.

Nova is a Public Broadcasting System (PBS) show produced in the U.S. - not sure how widely it is showing in other countries.

William J. Borucki (the Kepler PI) is going to give a public lecture in BC on February 1st.



http://www.meetup.com/astronomy-131/events/161140292/

I'll be dropping in, that's for sure!

(Not sure if it's more appropriate here or in the conferences subforum)

Kepler has probably found dozens of planets that are close to the same size and mass as Earth, but:

1) In most cases, these are much, much hotter than Earth.
2) Kepler data itself does not indicate the mass of planets.
(In certain cases, the pull of planets on neighboring planets may provide an indication of mass.)

Determining the mass of an Earth-sized planet by the pull of the planet on its star is only feasible for planets very close to the star, so it is essentially impossible with current technology to ascertain that any planet is earthlike in all three of mass, size, and temperature. It is, at the outside, possible to determine that a planet is earthlike in size and equilibrium temperature, and given enough such planets, you'd certainly expect some of them to also be earthlike in mass.

For planets that are earthlike in size, I don't think anyone expects the norms to be wildly outside of what we see with the solar system's terrestrial planets. Perhaps the high end of the range is higher than the densest planet in the solar system (which happens to be Earth), but it's probably not too far beyond that.

If the optical radius corresponds to a high haze layer and not the lithosphere, we may see some terrestrial planets which appear less dense than Mars and perhaps less dense than the Moon. I would lay odds though that most terrestrial planets around other stars have densities within or close to the Mars-to-Earth range.

Kepler successfully observed a transiting exoplanet, WASP-28b, in a test for the K2 mission in January!

With the K2 mission moving forward on reserve funds through May, we are looking good. The money quote from Tom Barclay:
I’ve also being doing some work on estimating what sizes of planet we are going to be sensitive to. It looks we are going to be able to find Earth-sized planets orbiting relatively bright G and K type stars and well as around fainter M-dwarfs. This is great because these are the stars we are going to be able to get follow-up ground based observations of. I can’t wait to get some more data on the ground.

Just to let everyone know that Kepler is holding a teleconference tomorrow at 1:00pm EST.

http://www.nasa.gov/ames/kepler/nasa-hosts...s/#.Uwyd_V4x_UQ

http://www.jpl.nasa.gov/news/news.php?release=2014-057

A study on super Earths, with a conclusion I question to some degree, pressure is not a showstopper for "interesting" chemistry to happen on such a world.
Since this make references to Kepler I added this piece to this thread.

Conference in a few minutes:
http://www.nasa.gov/news/media/newsaudio/i...ml#.Uw4qZYWldMJ

Edit: 715 candidates validated! http://www.nasa.gov/ames/kepler/nasas-kepl...planet-bonanza/
More slides: http://www.nasa.gov/ames/kepler/digital-pr...a/#.Uw4urYWldMJ

Just listened - 94% of the planets are sub-Neptune sized. This vastly increases our knowledge about smaller planets.
The new verification technique allows them to more quickly verify multiple planet systems, thus the huge increase. They expect more verification as they go through older data.

So all these are parts of new systems, many which resemble our inner solar system!

speaking of Kepler and its follow-on mission, this was recently published in arXiv: The K2 Mission: Characterization and Early results

This strategy do make perfect sense after all, a survey is the right method since this field of research is rather new and in fact have been defined by Kepler itself.

I've web-published the first of two posts I'm going to do on the performance of Kepler and the search for possible earthlike planets in its data. The quick takeaway of this post is that Kepler's data was noisier than expected, and this has helped to bury any real possible earthlike planets in lots of false positives. There's a really pretty illustration (I think) that shows the artifactual nature of the Kepler telescope's detector.

http://sciencepiazza.blogspot.com/2014/03/...ts-lost-in.html

Such a shame that no actual water worlds have been found, covered in water. Who knows if there is one out there...

I know personnel space flight isn't allowed to be discussed but as more of a thought without any discussion, the thought of a craft landing on a distant water world and then the challenge of taking off is just insane!!

MOD NOTE: If you know this, then you should also know that skirting the rules by using some variation of 'I know, but…' before making a statement referring to a subject that is not permissible ain't gonna fly at all. Please don't do this anymore.

Not insane at all.

http://en.wikipedia.org/wiki/Sea_Launch

Launching from the high seas for 15 years now. The equatorial location is great too...

What makes you think none have been found? Some planets found by Kepler are consistent with a water-world composition.

By and large, we know the composition of almost none of the discovered extrasolar planets. In some cases, particularly the very large and very small planets (density is also a great clue), we can make a good guess and probably be right, but that is based on general principles, not direct observation.

It's particularly interesting to contemplate the nature of Super Earths, since our solar system contains none, although the universe contains many. It's also worth noting that "Super Earth" means potentially something quite different if we define that in terms of radius or in terms of mass.

It's also worth noting that the uncertainty in gross measurements of exoplanets usually creates enormous uncertainty regarding density, much less composition, geology, climate, etc. There are many Kepler planets where the uncertainty in radius is inclusive of possibilities ranging from Venus to Neptune... we're in no position to know what that planet is made of. What we know about an exoplanet is usually the orbital period, quite exactly, and the radius OR the mass, with considerable uncertainty. In a few lucky cases, we've measured radius and mass.

All of the cases where composition has been measured spectroscopically have been large planets where we're finding a trace constituent in an atmosphere which is probably dominated by hydrogen and helium.

It's easy for discussions of these topics to (a) violate the rules of this particular board, and (b ) be entirely speculative in any case, since data is lacking.

What planets found by the probe/telescope could actually be water covered?

I guess not until the Web telescope is in orbit, only then can a better understanding of a number of these planets be confirmed. Sadly only a deep space probe can really do that for certain. Oh well.

Most Kepler planets are 500-5000 light years away, and are therefore a poor prospect for detailed follow-up science in almost all cases. Kepler provides a survey of what is typical, and cranks out big numbers. The future of exoplanet science is probably going to use Kepler data to set expectations for searches closer to home (10-200 LY), and those planets will provide vastly superior options for follow-up science.

What we really hope for is the ability to do visual/IR spectroscopy, which requires getting enough light from the exoplanet without light from its star completely masking the signal. That's hard in the best of cases, and is much harder if the planet is small and/or the system is far away. The distance from the Earth to the Sun would appear as one pixel to HST and JWST for a system 90 light years away. So beyond that, the hope of getting a planet isolated from its star is bleak even with a telescope designed for the task (which HST and JWST are not).

The path towards getting compositional data for smaller planets requires us next to find some closer to Earth. Then build the telescopes that can make those observations. It's seemingly not at all close at hand.

Yes, sure I wouldn't expect distant worlds.

Its a long shot how ever you explain it. Detailed world analysis still a few decades off, for a very crystal clear picture of some of the further away worlds, but even so the closer planets too.

Most of what is known will most likely benefit more the next two generations than for us alive now, such as when the time arises with better technology to help make or break ideas on the current properties of the planets found.

Here's my second installment in an analysis of false positives in Kepler's data pipeline:

http://sciencepiazza.blogspot.com/2014/03/...ating-true.html

Others have taken their own direction on this. I hope my take is useful for researchers and other interested parties in understanding the noise in Kepler's instrument.

In a third post yet to come, I'll take a look at the possible earthlike planets that seem to have the best chance of being real rather than artifacts of noise.

Here's my analysis of possible earthlike planets in Kepler results:

http://sciencepiazza.blogspot.com/2014/04/...her-earths.html

The main finding is that we can't point to any particular Kepler discovery and deem it certain to be earthlike, but there are six top candidates which stand a good chance of including one or two that will prove to be pretty earthlike.

Telecon tomorrow on a new Kepler discovery (embargoed until 11:00 PST):

http://www.jpl.nasa.gov/news/news.php?release=2014-113

This is fairly sudden. Normally they do not announce teleconferences this early in advance.

They might be ready to unveil an interesting planetary system like Kepler 62.

The people speaking and their research areas seem to draw a bullseye around the topic of terrestrial planets with earthlike temperatures, which happened to be the topic of my last blog post:

http://sciencepiazza.blogspot.com/2014/04/...her-earths.html

I guess they're either going to be talking about a new candidate for most-earthlike-exoplanet-yet, or a significant advance in the quality of information about a Kepler discovery that was already on the books.

EDIT: You may want to pay close attention to the seventh Kepler candidate in the table of my post, K00571.05.

Dutch news sites are breaking the embargo:

http://nos.nl/artikel/637138-planeet-zoals...e-gevonden.html

Kepler-186f
Rocky and 10% > Earth
In the "Goldilocks zone"
Orbits a red dwarf once every 130 days
Distance of 500 light years

Yep:

http://www.nasa.gov/ames/kepler/nasas-kepl...f-another-star/

No surprise that its a red dwarf...

The press conference has just begun. Since the embargo is over, I'll link to the post I wrote in preparation, giving my take on the findings:

http://sciencepiazza.blogspot.com/2014/04/...ther-earth.html

The planet in question was seventh on my list of possible "other Earths" in the previous post:

http://sciencepiazza.blogspot.com/2014/04/...her-earths.html

The advantage this one has over many of the other candidates is that it's in a multiple-discovery system, which makes it easier to validate the existence of the candidates.

Note that while the star is a red dwarf, the planet's orbital period is relatively long (130 days), longer than Mercury's, so concerns about tidal lock may not apply in this case.

I suppose that more habitable candidates will be confirmed in the future. If we judge that the smaller stars will be confirmed first, then we have an order of discovery of M, K, then G stars. My question is as the team confirms more candidates, will the Kepler team be releasing more rapid announcements like this one or withhold them for releasing later in a Kepler science conference? I can see the upsides and downsides of both and I don't know which is more likely.

"May not apply" is a good reservation there. =)
The orbital period is longer since the planet move in the weaker gravitational field of a red dwarf.
So it is relatively close to the star, even though it appear to be one of the brighter ones of the class.
Lets wait and see, a lot of instruments will quite likely be aimed at this system in years to come so more facts will trickle down.
The interest for a planet of this kind will be enormous.
None the least because it fits with a hypothesis that Earth type planets could indeed exist orbiting red dwarfs.

And thank you for your blog, I will link it to some people who are interested in more information. =)

[[If we judge that the smaller stars will be confirmed first, then we have an order of discovery of M, K, then G stars.]]

My estimate was that there should be about one of these around an M and about one around a G in the Kepler data. Intrinsically, there should be far more earthlike planets around Ms than Ks or Gs, but Kepler was observing far more Gs (~90,000) than Ms (~4,000).

[["May not apply" is a good reservation there.]]

The specifics of tidal interactions depend upon internal structural factors that we don't even know in the case of Mars, much less an exoplanet.

This one seems very promising not to be tidally locked, but it could go either way. For many smaller red dwarfs, the orbital period of any earthlike planets would be much shorter and almost certainly result in tidally locked rotation.

[[And thank you for your blog, I will link it to some people who are interested in more information.]]

That would be great! I hope it's useful to people.

Formation, tidal evolution and habitability of the Kepler-186 system

Paper is now up on arXiv:

An Earth-sized Planet in the Habitable Zone of a Cool Star

MOD NOTE: Exoplanet designation posts moved here.

K2 is approved, and the first 75 day observation campaign begins May 30th.
I hope those remaining two reaction wheels can last for many campaigns to come...

This is great news.

If K2 can get a single year of observations, I'd consider it a success many times over.