Another Tatooine: Kepler discovers multiple planets orbiting two stars

Diagram showing the comparison of orbits of the two Kepler-47 planets with the inner planets in our own solar system (planetary sizes and orbits to scale). Credit: NASA / JPL-Caltech / T. Pyle (SSC / Caltech)

Not too long ago, the Kepler space telescope discovered the first exoplanet which orbited two binary stars (a circumbinary star system). Such planets had been thought possible, but this was the first confirmed detection of a world reminiscent of the planet Tatooine in the Star Wars films, a planet with a double sunrise and double sunset.

Now another such planetary system has been found by Kepler, but this one consists of not just one, but two known planets. The planets orbit the binary star system Kepler-47, which is about 4,900 light-years from Earth.

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If all of Kepler’s exoplanets orbited one star, this is what it would look like

Artist’s conception of Kepler-22b, an exoplanet which is less than two and half times the size of Earth and orbits in the habitable zone of its star. Credit: NASA / JPL-Caltech

As the number of exoplanets being found by the Kepler space telescope continues to grow into the thousands, it can start to feel overwhelming (in a good way though). So many worlds found so far, and many more, probably millions, waiting to be discovered.

The Kepler planetary candidates are of a wide variety, orbiting different kinds of stars. Some are large gas giants like Jupiter or Saturn, and others are small and rocky, like Earth. The most current observations now suggest that they are common in the universe, maybe even outnumbering the stars themselves.

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Kepler finds alien solar system similar to our own

Diagram of Kepler-30 solar system, showing alignment of the three known planets.
Credit: Cristina Sanchis Ojeda

The Kepler space telescope has been finding a wide variety of alien worlds, as well as solar systems that they reside in. In our own solar system, the planets are in nice, neat orbits which are aligned with the Sun’s equator, consistent with the idea that they formed from a relatively flat, spinning disk of gas and dust around the Sun billions of years ago.

So far, other solar systems found haven’t tended to follow that pattern. Did that mean our solar system was a fluke? But now, Kepler has found one, about 10,000 light-years away. Scientists at MIT, the University of California at Santa Cruz and other institutions made the discovery from analysis of Kepler data.

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Is Kepler getting close to finding another Earth?

Size comparisons of Kepler planetary candidates. Credit: NASA Ames/Wendy Stenzel

The longer the Kepler mission keeps searching for exoplanets, the more amazing discoveries it makes. The Kepler mission team just released their third catalogue of new planetary candidates – 1,091 to be exact. This bring the total number of planetary candidates found by Kepler so far to 2,321, which orbit 1,790 stars. Confirmed planets so far now number 760, from Kepler and other telescopes.

These are the same candidates which had been previously discussed at the Kepler Science Conference last December, but now the official catalogue is available to the public.

A primary goal of Kepler is to find out how many planets among the stars being studied are Earth-sized and how many orbit within a star’s “habitable zone” where it would be possible for liquid water to exist. Ideally, planets which fit both criteria would be the “holy grail” of the mission.

Over 200 Earth-sized planets have now been added to the catalogue, and over 900 planets that are less than twice the size of Earth. There are also 46 planets in the habitable zone, and of these, ten are Earth-sized.

With the latest results, the number of planets found that are less than twice the size of Earth increased by 197%. Planets larger than twice the size of Earth, however, increased by 52%. This confirms the earlier trends seen that smaller rocky planets outnumber larger gas or ice giants.

There are also more multi-planet systems being discovered; 20% of the stars studied compared to 17% last year.

One of the Earth-sized planetary candidates found would seem to be of particular interest – KOI-494.01. It is still awaiting final confirmation, but the data so far support it being a real planet. It is estimated to have a mass 15% greater than Earth and a radius only 5% greater. The mean surface temperature is now estimated to be only about 1 ºC (33.8 ºF) cooler than on Earth. The estimated mean surface temperature (Ts) of KOI-494.01 is listed at 287 K (14 ºC / 56.93 ºF). Not bad!

According to the Earth Similarity Index (ESI), an estimated measure of how similar a planet is to Earth, KOI-494.01 now has an ESI rating of 99%. In the previous catalogue, it was listed at 97%. The confirmed planet with the highest rating so far is Gliese 667Cc, at 85%.

There is also a growing list of other Earth-sized candidates with ESI ratings close to that of KOI-494.01. See the summary lists here, updated March 3, 2012.

According to Natalie Batalha, Kepler deputy science team leader at San Jose State University in California, “With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant.”

The updated findings continue the trend of exoplanets which are smaller like Earth, some of which orbit in their stars’ habitable zones. While these are likely to be of a wide variety, depending on other factors like composition, star type, etc., the discovery of planets like KOI-494.01 is an encouraging sign that there may indeed be other worlds out there similar to our own.

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Kepler finds three smallest exoplanets so far

Artist's conception of the KOI-961 system. Credit: NASA/JPL-Caltech

The Kepler space telescope has been finding exoplanets by the thousands, and as the mission progresses, it has been able to detect smaller and smaller planets over time. Indeed, it was just recently announced, among other significant discoveries, that the smallest exoplanets have (again) been found.

These three planets all orbit one star, KOI-961, a red dwarf star only about one-sixth the diametre of our Sun. They all orbit very close to the star, taking less than two days to complete an orbit, and are only about 0.78, 0.73 and 0.57 times Earth’s radius. The smallest is actually close to the size of Mars!

According to Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington, “Astronomers are just beginning to confirm thousands of planet candidates uncovered by Kepler so far. Finding one as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us.”

Size comparison of smallest known exoplanets to Earth and Mars. Credit: NASA/JPL-Caltech
Scale comparison of KOI-961 planets to Jupiter and its four largest moons. Credit: NASA/JPL-Caltech

The discovery was made by a team led by astronomers from the California Institute of Technology in Pasadena.

The easiest exoplanets to detect are larger gas giants, like Jupiter or Saturn, and planets with very tight orbits around their stars (and smaller, dimmer stars). As Kepler accumulates more data, it is able to detect both smaller planets like Earth or even smaller as in this case, and planets that orbit farther out from their stars.

“This is the tiniest solar system found so far,” said John Johnson, the principal investigator of the research from NASA’s Exoplanet Science Institute at the California Institute of Technology in Pasadena. “It’s actually more similar to Jupiter and its moons in scale than any other planetary system. The discovery is further proof of the diversity of planetary systems in our galaxy.”

Interestingly, the new planets were found by comparing KOI-961 to another very similar star, Barnard’s Star. This aided in determining the planets’ sizes.

Red dwarfs are the most common type of star in the galaxy, so discoveries such as this help to reinforce earlier conclusions that smaller rocky planets are common. If this one red dwarf star has at least three planets, than how many other red dwarfs do also?

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