U. TEXAS-AUSTIN (US) — A “super-Earth” and two Neptune-sized planets have been discovered orbiting Kepler 18, a star that is 10 percent larger than the Sun with 97 percent of the Sun’s mass.
The discovery, made using NASA’s Kepler space telescope, is reported in a special Kepler issue of The Astrophysical Journal Supplement Series in November. There may be more planets than these three, designated b, c, and d, that orbit much closer to Kepler 18 than Mercury does to the Sun, researchers say.
Orbiting closest to Kepler-18 with a 3.5-day period, planet b, considered a “super-Earth,” weighs in at about 6.9 times the mass of Earth, and twice Earth’s size. Planet c has a mass of about 17 Earths, is about 5.5 times Earth’s size, and orbits Kepler-18 in 7.6 days. Planet d weighs in at 16 Earths, at 7 times Earth’s size, and has a 14.9-day orbit. The masses and sizes of c and d qualify them as low-density “Neptune-class” planets.
Planet c orbits the star twice for every one orbit d makes. But the times that each of these planets transit the face of Kepler 18 “are not staying exactly on that orbital period,” says Bill Cochran, senior research scientist at the University of Texas-Austin. “C and d are out of phase. One is early when the other one is late, then both are on time at the same time, and then vice-versa.”
Scientifically speaking, c and d are orbiting in a 2:1 resonance, meaning “they’re interacting with each other. When they are close to each other … they exchange energy, pull and tug on each other,” Cochran says.
The Kepler telescope uses the “transit method” to look for planets, monitoring a star’s brightness over time and looking for periodic dips that could indicate a planet passing in front of the star. A large part of the team’s work is proving that potential planets they find aren’t something else that mimics the transit signature.
Kepler 18’s planets c and d did astronomers a favor by proving their planet credentials up front via their orbital resonance; they had to be in the same gravitational system with the parent star for the resonance to occur.
Confirming the planetary bona fides of planet b, the super-Earth, was much more complicated, Cochran says. His team used a technique called “validation,” instead of verification. They set out to figure out the probability that it could be something other than a planet.
They used the Palomar 5-meter Telescope at the California Institute of Technology to take an extremely high-resolution look at the space around Kepler 18. They wanted to see if anything close to the star could be positively identified as a background object that would cause the transit signal they had attributed to a super-Earth.
“We successively went through every possible type of object that could be there,” Cochran says. “There are limits on the sort of objects that can be there at different distances from the star.”
Astronomers know how many of different types of objects (various kinds of stars, background galaxies, and more) are seen on average in the sky. They didn’t find anything in the Palomar image.
“We’re very confident that it’s a planet,” Cochran says. His team calculated that the likelihood the object is a planet is 700 times more likely than the likelihood that it’s a background object.
“We’re trying to prepare the astronomical community and the public for the concept of validation,” Cochran says. “The goal of Kepler is to find an Earth-sized planet in the habitable zone (where life could arise), with a one-year orbit.
“Proving that is very difficult (with current technology). When we find what looks to be a habitable Earth, we’ll have to use a validation process, rather than a confirmation process. We’re going to have to make statistical arguments.”
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