NASA hosted a telephone press conference Thursday to announce that the Kepler space probe had discovered two Saturn-size planets and a possible “Superearth” orbiting a star 2,300 light-years away. But the meaning of the session came when a reporter asked whether Kepler had confirmed any Earth-size planets in a habitable zone.
“Come back in a couple of years and we’ll give you an answer,” said Bill Borucki, Kepler’s principal investigator for science.
[An artist imagines the two Saturn-size planets of the ‘Kepler-9’ system. Image credit: NASA/Ames/JPL-Caltech]
Kepler, presently looping around the sun about 18 million miles from Earth, is taking continuous readings of the light from more than 150,000 stars, searching for the slight dimming that could signal a planet passing in front of its star. Kepler’s goal is to discover Earth-size planets in the habitable zone, the region far enough from the star where liquid water could exist, but not so far that the water would be ice.
The new finding is disappointing, since the space agency had announced Monday that it was holding the teleconference “about an intriguing planetary system.” What was intriguing to NASA is simply that for the first time, two Saturn-like planets had been confirmed eclipsing the same star from Kepler’s vantage point; in addition, the system, called Kepler-9, might have a closer planet only 1.5 times the size of Earth.
“What’s going on now is to try to confirm that object as a planet … we’re trying to rule out what we call an astrophysical false positive” on the Superearth, said Matt Holman, associate director of the Theoretical Astrophysics Division, NASA Ames Research Center, Moffett Field, Calif. Superearths are planets larger than ours but smaller than, say, Neptune. A false positive could be a distant eclipsing binary star causing a regular dimming of starlight.
Kepler-9, in the constellation Lyra, is far from the only other star than Sol known to shepherd a flock of planets — the European Southern Observatory says at least 15 such systems are known so far. Kepler-9 is merely the only one where more than one planet was detected and confirmed through the eclipsing method.
Kepler identified 700 potential planets in its first 43 days of observations, and scientists are working to confirm planets among them. Five potential multiple-star systems were noted in the 700, and now the team has confirmed that a sixth suspect really does have more than one planet.
But on Tuesday, the ESO announced that its High Accuracy Radial velocity Planet Searcher had “discovered a planetary system containing at least five planets, orbiting the Sun-like star HD 10180.” The discovery was far more exciting than NASA’s latest news.
HARPS uses a nearly 12-foot diameter telescope at La Silla, Chile, to detect planets by studying their gravitational tugging on stars; calculations based on a star’s wobbles give evidence of planets of varying distances and masses.
ESO added that within the same star system, its researchers also found “tantalizing evidence that two other planets may be present, one of which would have the lowest mass ever found.” That would be a so-called Superearth only 1.4 times the size of our own planet. If confirmed at seven planets, the ESO discovery would make the system similar to our own solar system, which has eight (sorry Pluto).
“Furthermore,” ESO added, “the team also found evidence that the distances of the planets from their star follow a regular pattern, as also seen in our Solar System.”
The latest information from ESO makes the NASA announcement seem pretty drab.
Unfortunately for the search for habitable locations beyond our solar system, the two confirmed planets NASA announced Thursday are closer to their star than Mercury is to our sun, making them roasting hot. The planet that is nearer to the star is estimated to be heated to more than 1,000 degrees Fahrenheit while its sister is more than 800 degrees. The third possibility, if it does prove to be a planet, is in a tighter orbit yet, whipping around the star in 1.6 days; it would positively sizzle.
Each of the two confirmed new planets is nearly ten times Earth’s diameter. The larger is so close to its star that it orbits in 19.2 Earth days, while the other goes around in 38.91 days. NASA believes it’s no accident that the orbital period of Kepler 9-C is almost exactly twice that of Kepler 9-B.
The theory is that they formed in the outer reaches of the planet’s debris field, as Jupiter and Saturn did. Then, unlike our hugest gas giants, they migrated toward the star and fell into a harmonic two-to-one orbit.
“These planets probably had to form further from the star and then migrate inwards,” said Alycia Weinberger, an astronomer with the Carnegie Institution of Washington, D.C.
Kepler was launched in 2009. So far, it has confirmed the presence of seven planets. Confirmation could take three years or longer for a planet of Earth’s orbital distance from its star, because it would pass in front only once a year and several transits are needed for confirmation.
Since 1995, several detection methods have discovered more than 470 extrasolar planets.
A NASA fact sheet adds, “Number of habitable exoplanets discovered to date: zero.”