NASA confirms first Earth-sized planets around sun-like star

[Guardsman Bass]

A pity they're not habitable, possessing intra-Mercury style orbits:

MOFFET FIELD, Calif. -- NASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface, but they are the smallest exoplanets ever confirmed around a star like our sun.

The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.

Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit, is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit, would melt glass.

“The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”

The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days. All five planets have orbits lying roughly within Mercury's orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.

The system has an unexpected arrangement. In our solar system, small, rocky worlds orbit close to the sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are organized in alternating size: large, small, large, small and large.

"The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system," said Jack Lissauer, planetary scientist and Kepler science team member at NASA's Ames Research Center in Moffett Field, Calif. "The analysis of Kepler data continue to reveal new insights about the diversity of planets and planetary systems within our galaxy."

Scientists are not certain how the system evolved but they do not think the planets formed in their existing locations. They theorize the planets formed farther from their star and then migrated inward, likely through interactions with the disk of material from which they originated. This allowed the worlds to maintain their regular spacing despite alternating sizes.

The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets crossing in front, or transiting, their stars. The Kepler science team requires at least three transits to verify a signal as a planet.

The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the spacecraft finds. The star field Kepler observes in the constellations Cygnus and Lyra can be seen only from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.

To validate Kepler-20e and Kepler-20f, astronomers used a computer program called Blender, which runs simulations to help rule out other astrophysical phenomena masquerading as a planet.

On Dec. 5 the team announced the discovery of Kepler-22b in the habitable zone of its parent star. It is likely to be too large to have a rocky surface. While Kepler-20e and Kepler-20f are Earth-size, they are too close to their parent star to have liquid water on the surface.

"In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time," said Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University. "We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come."

[Guardsman Bass]

Sorry, I had to post the OP in a rush.

This is an interesting step forward. Neither planet is habitable, but it's still interesting to find a planet with a radius so close to that of Earth (Kepler-20f, with a radius 1.03 times that of Earth).

The Kepler-20 system seems a bit weird, with five planets of varying sizes all orbiting in smaller orbits than that of Mercury. We've seen that before, with Kepler-11.

[Guardsman Bass]

This is my final consecutive post, I swear-

Phil Plait the Astronomer has pointed out some of the info on the other planets in the system. They range from a Super-Earth/Gas Dwarf at 8.7 Earth masses, to what is basically a "Hot Neptune" (~20 masses Earth masses vs Neptune's 17). The planets actually alternate "Big"-"Rocky"-"Big"-"Rocky"-"Big".

[Iroscato]

Fantastic, and finding a planet with an incredibly close radius like that is a great stroke of luck. We'll definitely hit the jackpot one day soon at this rate

[DarkArk]

Fantastic news, it's only a matter of time at this point.

What still amazes me is that society doesn't seem to quite understand just how many planets are out there, and that it's likely worth it to put in the resources to try and get there. Oh well. We'll come around eventually.

[edaw1982]

Fantastic news, it's only a matter of time at this point.

What still amazes me is that society doesn't seem to quite understand just how many planets are out there, and that it's likely worth it to put in the resources to try and get there. Oh well. We'll come around eventually.

I think it's a matter of '1 is a tragedy a million is a statistic'.
Basically people can concieve and picture certain numbers to a degree. 'Five golden rings, two turtledoves' but it gets harder to concieve 'two-trillion-one-thousand-six-hundred-and-eighty-two' turtledoves.
The mind boggles, and so whilst one can say there's "a gajillion planets out there" it goes over people's heads because it's not a number one can mentally picture.

[Guardsman Bass]

Thread Necro, but there's been some additional news since then. They've found the three smallest exo-planets so far around a red dwarf, including one that is around the size of Mars:

ScienceDaily (Jan. 11, 2012) — A team of astronomers led by scientists at the California Institute of Technology (Caltech) has discovered the three smallest confirmed planets ever detected outside our solar system. The three planets, which all orbit a single star, are smaller than Earth and appear to be rocky with a solid surface. Until now, astronomers have found at most only four other rocky planets, also called terrestrial planets, around other stars.

The trio of new planets is too close to the central star to be in its habitable zone -- the ring-shaped region around a star where the temperature is mild enough for liquid water, and possibly life, to exist. But the planets are the first rocky ones to be found orbiting a type of dim, small star called a red dwarf, the most common kind in the Milky Way. Their existence suggests that the galaxy could be teeming with similarly rocky planets -- and that there's a good chance that many are in the habitable zone.

The red dwarf, called KOI-961, was first flagged as a potential planetary system by the Kepler mission, a space telescope that looks for planets around sunlike stars by scanning the sky for stars that periodically dip in brightness -- the result of one or more planets passing in front of them. Although Kepler reported 900 potential planetary systems in February, only about 85 of those were red-dwarf systems. The fact that a relatively small sample of red dwarfs produced three terrestrial planets means that either the Caltech-led team was really lucky or, more likely, that these planets are commonly found around red dwarfs.

"When you combine that with the fact that these are some of the most numerous stars in the galaxy, you realize this type of system could be common," says Philip Muirhead, a postdoctoral scholar at Caltech. "There's no question that it's exciting." Muirhead is the lead author on the paper describing the discovery, which has been accepted by the Astrophysical Journal. The team will also present their results in two talks on January 11 at the meeting of the American Astronomical Society in Austin, Texas.

"Red dwarfs make up eight out of every ten stars in the galaxy," adds John Johnson, assistant professor of astronomy and one of the paper's coauthors. "That boosts the chances of other life being in the universe -- that's the ultimate result here. If these planets are as common as they appear -- and because red dwarfs themselves are so common -- then the whole galaxy must be just swarming with little habitable planets around faint red dwarfs."

This report comes just a few weeks after the Kepler team announced it had detected two rocky planets around a sunlike star -- Kepler-20e and Kepler-20f -- the first Earth-sized planets ever found and the smallest known at the time. In January 2011, the Kepler team reported the discovery of the first unequivocally rocky planet around another star, Kepler-10b. Another planet-Corot-7b, which was found in 2009 -- could also be a rocky planet.

With the exception of Kepler-20e, which is about the size of Venus, the other previously discovered planets are all bigger than Earth. All three of the ones found by the Caltech-led team are smaller-the outermost one is about half the size of Earth (similar to Mars) and the other two are three-fourths the size of Earth (smaller than Venus). In fact, the entire KOI-961 planetary system is remarkably tiny. KOI-961 has a diameter one-sixth that of the sun's, making it just 70 percent bigger than Jupiter. Each of the three planets needs less than two days to zip around their star, and all three are about one hundred times closer to that star than Earth is to the sun. And because they're so close to their star, they're hot-the outermost planet is estimated to be about 200 degrees Celsius (400 degrees Fahrenheit) while the innermost planet is a scorching 500 degrees Celsius (more than 900 degrees Fahrenheit).

"The really amazing thing about this system is that the closest size comparison is to Jupiter and its moons," Johnson says. "This is causing me to have to fully recalibrate my notion of planetary and stellar systems."

Kepler's initial measurements -- which are automated to help it sift through roughly 150,000 stars -- underestimated the size of KOI-961 and any planets it might have had. No one realized this until amateur astronomer and paper coauthor Kevin Apps alerted Muirhead and his team to the idea that KOI-961 bore a remarkable resemblance to another red dwarf called Barnard's Star, a nearby star that's one of the most well-studied. "That's what blew the case wide open," Johnson says.

When the astronomers used telescopes at the Palomar and Keck Observatories to take a closer look at both stars, they found that the two are practically twins. The characteristics of Barnard's Star allowed the team to infer the properties of KOI-961, which is needed to deduce the nature of the planetary system from the star's light curve -- a plot of how the star dims over time due to transiting planets. In particular, the depth of the light curve -- that is, how much the curve dips -- reveals the planets' sizes.

Because the planets are so small, the only way they could have enough gravity to hold themselves together is if they are balls of rock, like Mercury, Venus, Earth, and Mars. "Just three years ago, just talking about a rocky planet would have been pure speculation," Johnson says. "But these are unambiguously rocky."

Still, before they could make any conclusions, the researchers had to confirm that the dips in light detected by Kepler really were due to planets -- and not something else, such as a pair of background stars in orbit around each other. To do so, they turned to old photographs taken by Palomar Observatory's 48-inch Samuel Oschin Telescope in 1951. Because KOI-961 is relatively close -- about 130 light-years away -- it appears to drift across the sky relatively quickly, so that a photo taken in 1951 would show it in a different location than a current image would. By comparing pictures of KOI-961 over the years, astronomers can check whether there are any stars behind it that could produce the light dips they saw. They found none.

Instead of a planetary system, the dips could also be caused by stars in orbit around each other. But the researchers analyzed the statistics of such a scenario and found that it's very unlikely. Combining these statistical results with the observations that show a lack of background stars, the astronomers concluded that the light dips from KOI-961 are indeed produced by three small, terrestrial planets.

The other Caltech authors on the Astrophysical Journal paper, "Characterizing the Cool KOIs III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets," are graduate students Tim Morton, John Pineda, Michael Bottom, and David Levitan; postdocs Justin Crepp and Evan Kirby; and astronomer Lee Armus and postdoc Tanio Diaz-Santos of the Infrared Processing and Analysis Center (IPAC) at Caltech. There are ten other authors from the Harvard-Smithsonian Center for Astrophysics, Cornell University, UC Santa Cruz, the American Museum of Natural History, Vanderbilt University, and UC Berkeley. This research was supported by NASA, the National Science Foundation, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Company.

[Simon_Jester]

One thing that I've been thinking about through all the exoplanet searches of the past ten to fifteen years is detection threshold. It's much easier to spot massive planets, and planets in tight, short orbits- those are the ones that make stars wiggle the most visibly.

So inevitably all our early sightings were of "hot Jupiters:" the most massive planets in the immediate local region of space, orbiting as close as possible to their parent stars.

Only later on did we start detecting lighter planets- and finding planets that orbit far out is more difficult still. If interpreting Newton's law in a rather crude way gives me the right answer, doubling orbital radius of a planet will make it about 1/4 as visible; halving its mass only makes it about half as visible.

So the real question is "when can we look at a solar system and confidently reject the hypothesis that there is a 'twin of Earth' there, i.e. a planet of approximately the mass of Earth in the habitable belt of the star? So far, all we've seen are the biggest and hottest planets, so our sample is biased.

[Zac Naloen]

The other thing is that they've found a few gas giants in the habitable zone, you can't rule out that they may have earth sized moons that are habitable if our solar system is anything to go by.

[Solauren]

Unfortunately, without some form of FTL, the odds are, we'll never confirm if planets outside our planet are inhabited.

[Surlethe]

Unfortunately, without some form of FTL, the odds are, we'll never confirm if planets outside our planet are inhabited.

That may not be strictly true. To find evidence of life, for example, we can look at the composition of a planet's atmosphere. An oxygen-rich atmosphere like ours would suggest that some mechanism is producing molecular oxygen, and IIRC the only known mechanism which does this is photosynthesis. (I am not certain about this.)

[starslayer]

Pretty much. There are other reactions which will liberate oxygen (most of the ones I know of involve stuff like chlorine or fluorine), but none that are persistent over millions of years. Free methane on a warm planet is another marker you can look for.

[darthdavid]

Unfortunately, without some form of FTL, the odds are, we'll never confirm if planets outside our planet are inhabited.

Well not any time soon at least. STL interstellar missions are definitely possible you're just stuck waiting a long time for results and you really need a bunch of orbital infrastructure and/or cheap lift capacity that we just flat out don't have yet to make launching one even remotely viable. Also, even small unmanned probes would be expensive enough to send that we may never actually do it. But if we had the infrastructure and the will it could definitely be done.

Unfortunately, without some form of FTL, the odds are, we'll never confirm if planets outside our planet are inhabited.

That may not be strictly true. To find evidence of life, for example, we can look at the composition of a planet's atmosphere. An oxygen-rich atmosphere like ours would suggest that some mechanism is producing molecular oxygen, and IIRC the only known mechanism which does this is photosynthesis. (I am not certain about this.)

Pretty much. There are other reactions which will liberate oxygen (most of the ones I know of involve stuff like chlorine or fluorine), but none that are persistent over millions of years. Free methane on a warm planet is another marker you can look for.

Of course there's no reason life would necessarily have to use the same chemical 'tool-kit' it does on Earth, so trying to find Oxygen and other indicator chemicals we'd expect to see based on life here could easily miss something that operated on a radically different biology (though where you'd even start looking for, say, Silicon based life I couldn't even begin to guess...)

[GrandMasterTerwynn]

Unfortunately, without some form of FTL, the odds are, we'll never confirm if planets outside our planet are inhabited.

As others have pointed out, there are certain chemical markers you can look for. The signature of water vapor in the atmosphere of a rocky planet within the habitable zone is a promising sign of surface water. Free oxygen or free methane in the atmosphere of a rocky planet is potentially indicative of biological activity. Seeing the distinctive spetroscopic signature of chlorophyll, or another photosynthetic molecule, would also be indicative of biological activity. All of this you can see without leaving cislunar space, if you were willing to build a big enough telescope or a big enough array of telescopes.

If you were patient and you had enough money, you wouldn't need FTL to probe extrasolar planets.

Of course there's no reason life would necessarily have to use the same chemical 'tool-kit' it does on Earth, so trying to find Oxygen and other indicator chemicals we'd expect to see based on life here could easily miss something that operated on a radically different biology (though where you'd even start looking for, say, Silicon based life I couldn't even begin to guess...)

Within the sorts of chemistries that can exist within the range of temperatures where water can remain liquid, there are only so many of these that are suitable for the complex chain of chemical reactions that make up biology. The other possible bases for life all have myriad problems with relative abundance and sorts of permissible chemical reactions that, in environments warm enough to support liquid water, carbon-based life will vastly outcompete them. Esoteric life would be confined to esoteric environments, which present their own challenges. At Earth-like surface pressures, ammonia only works as a solvent at temperatures colder than -33 degrees Centigrade; which imposes the problem that such temperatures are not conducive to high rates of reaction. And in order to get ammonia to be liquid across Earth-like temperatures, you need 60 atmospheres of pressure . . . which imposes the problem that, outside of a gas giant, it's hard to get an atmosphere with that much pressure at such cool temperatures.

[Surlethe]

Remember, the original point was oxygen --> life, not life --> oxygen. Other forms of life might not produce molecular oxygen, but if we observe molecular oxygen, then we are probably observing life.

[Solauren]

Like I said, we'll probably never confirm it. But at least we'll know 'odds are...'