Three planets recently discovered through Kepler data provide an interesting take on how we look at smaller planets. Not that the planets around the star designated Kepler-18 are all that small — two of them are Neptune-class and one is a super-Earth. But what is becoming clear is that given the state of our current technology, we’ll have to get used to a process different from planet verification as we move to ever smaller worlds. The technique is being referred to as planet validation — it helps us determine the probability that the detected object could be something other than a planet.
Image: The orbits of the three known planets orbiting Kepler-18 as compared to Mercury’s orbit around the Sun. Credit: Tim Jones/McDonald Obs./UT-Austin.
The new system shows how this works. Kepler-18 is a star similar to ours, about 10 percent larger than the Sun and with 97 percent of the Sun’s mass. Around it we have Kepler-18 c and d, which turn up through transits. Planet c has a mass of about 17 Earths and is thought to be some 5.5 times the size of Earth. Its orbit takes it around Kepler-18 in 7.6 days. Kepler-18 d is 16 times as massive as the Earth, 7 times Earth’s size, and orbits its primary in 14.9 days. These two Neptune-class worlds are, interestingly enough, in a 2:1 resonance: Planet c orbits the star twice for every single orbit of planet d. The demonstrable resonance is ample proof that these are planets in the same system and not something else mimicking a planetary signature.
But the super-Earth, Kepler-18 b, is something else again. A team led by Bill Cochran (University of Texas at Austin) went to work with the 5-meter Hale Telescope at Palomar, aided by adaptive optics, to examine Kepler-18 to see whether the transit signal they thought to be a super-Earth was genuine. Finding no background objects that could have influenced the finding, they were able to calculate the odds that Kepler-18 b is not a planet at 700 to 1. Cochran thinks this process of planet validation is going to become much more significant as Kepler brings in new data:
“We’re trying to prepare the astronomical community and the public for the concept of validation. 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 such an object really is a planet 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.”
So we can with a high degree of probability rule out any of the objects — stars, background galaxies — that might in any way compromise the transit data. The planetary signature of the super-Earth seems real enough, though established in a different way than Kepler-18 c and d, whose gravitational interactions can be readily demonstrated. The planet is thought to be 6.9 times Earth mass and twice Earth’s size. All three worlds orbit much closer to their parent star than Mercury does to the Sun, the super-Earth Kepler-18 b being the closest, with a 3.5 day period.
We can also deduce an interesting possibility about Kepler-18 b, as noted in the paper:
The inner, 3.5-day period planet Kepler-18b, is a super-Earth that requires a dominant mixture of water ice and rock, and no hydrogen/helium envelope. While the latter cannot be excluded simply on the basis of the planet’s mass and radius, the evaporation timescale for a primordial H/He envelope for a hot planet such as Kepler-18b is much shorter than the old age derived for the Kepler-18 system, and such a H/He envelope should not be present. Thus, despite its lower equilibrium temperature, Kepler-18b resembles 55 Cnc e and CoRoT-7b… Kepler-18b, together with 55 Cnc e… are likely our best known cases yet of water planets with substantial steam atmospheres (given their high surface temperatures).
The discovery was announced at a joint meeting of the American Astronomical Society’s Division of Planetary Science and the European Planetary Science Conference in Nantes, France. More on the Kepler-18 results in this news release from the University of Texas at Austin. Look for these results in an upcoming issue of the Astrophysical Journal Supplement Series devoted to Kepler, which will appear in November. The paper is Cochran et al., “Kepler 18-b, c, and d: A System Of Three Planets Confirmed by Transit Timing Variations, Lightcurve Validation, Spitzer Photometry and Radial Velocity Measurements” (preprint).