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Tiny planet triplets orbit dwarf star

CALTECH (US) — Astronomers have discovered the three smallest confirmed planets ever detected outside our solar system.

The trio of new planets that are smaller than Earth and appear rocky, orbit a single star too closely 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.


This chart compares the smallest known exoplanets, or planets orbiting outside the solar system, to our own planets Mars and Earth. (Credit: NASA/JPL-Caltech)

This artist’s conception compares the KOI-961 planetary system to Jupiter and the largest four of its many moons. (Credit: Caltech)

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 researchers were 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 California Institute of Technology (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.

“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.”

String of discoveries

The new findings come just a few weeks after the 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.

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.

Practically twins

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.

Researchers from Cornell University, Vanderbilt University, University of California, Berkeley, and the University of California, Santa Cruz contributed to the study that was supported by NASA, the National Science Foundation, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Company.

More news from Caltech: http://media.caltech.edu/

chat6 Comments

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6 Comments

  1. Walt Peterson

    If there are a lot of these little guys around, they could name them after smurfs. It’s more poetic than just giving them alpha-numerical designations.

  2. phil

    Seems like the end is near now that we are confronting our importance:
    Yes, there is a God and he created the universe with a big bang. And he said:
    Let there be a trillion stars with 5 trillion planets and 50 billion
    civilizations with 10 billion people each. And I shall glee at all the
    diverse results. And when all energy in the universe ceases and
    everything is dark and cold, I shall start over and do something
    different. And I couldn’t care less about the welfare of any of those
    civilizations so not to bias the natural path toward destruction of each
    one.
    Knowing our insignificance will be thge catalyst to our agressive self destruction.

  3. Torbjörn Larsson, OM

    So they are even sort-of confirmed. Awesome!

    @ phil:

    Most people are excited about the possibility of life out there. We have known about or mediocrity for a long time (Copernicus), and our insignificance since about a century (Hubble). Yet the world has moved towards more democracy, less poverty and less war over that period.

    Maybe in part precisely because religion lost its significance compared to secular science and moral, allowing us to make things better.

    Nitpick: We know many ways of how universes can come about, and have started to narrow down the possibilities. Magic of gods is not one of those, because magic never works.

    And even if it did, who magicked the magician and so on? It is an insane proposition.

  4. Dialashop

    Its amazing they have now detected such small planets. I don’t think humans will visit these for a least a few centuries, our present space technology is primitive.

  5. Dentist in Pueblo

    The article was really very interesting to read and I have to thank you for sharing this information with everyone. It is nice to know about the new discoveries in the universe and I have read the news about this particular topic some time ago. The topic really made me curious to know more about this new discovery but then the whole thing stopped all of a sudden. So, if you have any future report of this topic I would be glad if you can share it with us.

  6. Andrew Hall

    The conclusions about the commonness of planets around red dwarfs is interesting. These are three planets around a single star, it seems like a jump to infer that rocky planets around red dwarfs are common based of a single result. If there were three star systems each with a terrestrial planet that might be statistically significant, but this one planet could easily be an anomaly.

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