NASA’s Kepler Dishes Up A Triple Planet Treat

The top graphic shows the orbits of the three known planets orbiting Kepler-18 as compared to Mercury's orbit around the Sun. The bottom graphic shows the relative sizes of the Kepler-18 and its known planets to the Sun and Earth. Credit: Tim Jones/McDonald Obs./UT-Austin

What has a super-Earth and two Neptune-like planets? This tempting dessert belongs to the latest Kepler discovery found orbiting Kepler 18. Bill Cochran and a team of researchers have found the resonance they were looking for… and this very Sun-like star may have even more planets dancing around it.

Kepler 18 is a prime candidate for a solar system. The host star is approximately 97% the Sun’s mass and only about 10% physically larger. For now, the transit method has detected three planetary candidates named b, c and d which orbit within a zone smaller than Mercury’s. The “Super Earth” is about twice our size and its year only last three and a half days. At about six times and seven times our size, gaseous planets c and d have rough orbital periods of seven and half and fifteen days respectively.

While the two larger planets have similar transits, their times “are not staying exactly on that orbital period,” Cochran says. “One is slightly early when the other one is slightly 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. “It means they’re interacting with each other,” Cochran explains. “When they are close to each other … they exchange energy, pull and tug on each other.”

By using the transit method, the Kepler mission is able to watch for periodic brightness changes that signal orbiting bodies. Imagine a bright flashlight moving steadily behind a picket fence in the dark and you’ll get the picture. If each board were a slightly different size, the times the flashlight would be seen would vary. Resonance occurs – very simply put – when there’s a pattern like two wide boards and then a small one. But there’s more that can pass in front of our flashlight than just boards. There could be a line-of-sight star with a binary companion… and it’s just variables like these that makes confirming Kepler’s findings crucial.

In a process called “validation”, Cochran and his team utilized the Palomar 5-meter (200-inch) Hale Telescope and its adaptive optics to take another look at Kepler 18 and its system. “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.” The findings were negative. The planetary trio survived the next stage of identification.

“There’s a small possibility that [planet b] is due to a background object, but we’re very confident that it’s probably a planet,” Cochran says. With a seven hundred times probability factor that the Kepler findings signify a planetary signature, chances are good this trio is going down on the records as a validated system – with perhaps more yet to be discovered.

“We’re trying to prepare the astronomical community and the public for the concept of validation,” he says. “The goal of Kepler is to find an Earth-sized planet in the habitable zone, with a one-year orbit. Proving that such an object really is a planet is very difficult. 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.”

Original Story Source: McDonald Observatory News Release.

Super Earths

An artist’s impression of Gliese 581d, an exoplanet about 20.3 light-years away from Earth, in the constellation Libra. Credit: NASA

The holy grail in the search for extrasolar planets will be the discovery of Earthlike planets orbiting other stars. With better telescopes and techniques, astronomers will eventually be able to even detect the atmospheres of extrasolar planets and determine if there’s life there. Although Earth-sized planets are impossible to detect with current observatories, astronomers are now finding super earths.

A super Earth is a terrestrial planet orbiting a distant star. But instead of having the mass of our own planet, it might have 2, 5, or even 10 times the mass of the Earth. Although that makes them large, very massive planets, they’re not as large or massive as gas giants.

And just because they’re called super Earths doesn’t mean they’re habitable, or even Earthlike in climate at all. Super Earths could be orbiting close to their parent star, or well outside the solar system’s habitable zone.

Scientists haven’t completely settled on a definition for super Earths. Some believe a planet should be considered a super Earth if it’s a terrestrial planet between 1 and 10 Earth masses, while others think it should be between 5 and 10 Earth masses.

The first super Earth ever discovered was found in 1991 orbiting a pulsar. Obviously that wouldn’t really be a very habitable place to live. The first super earth found orbiting a main sequence star was found in 2005, orbiting the star Gliese 876. It’s estimated to have 7.5 times the mass of the Earth, and orbits its parent star every 2 days. With such a short orbital period, you can expect that it’s orbiting very close to its parent star. Temperatures on the surface of the planet reach 650 kelvin.

The first super earth found within its star’ habitable zone was Gliese 581 c. It’s estimated to have 5 Earth masses, and orbits its parent star at a distance of 0.073 astronomical units (1 AU is the average distance from the Earth to the Sun). That’s pretty close to the star, and Gliese 581 c would probably have a runaway greenhouse effect, similar to Venus. But right beside that is Gliese 581 d, with a mass of 7.7 Earths and an orbit of 0.22 AU. This planet could very well have liquid water on its surface.

The smallest super Earth discovered so far is MOA-2007-BLG-192Lb, which has only 3.3 times the mass of the Earth, and was orbiting a brown dwarf star. But this record will probably be beaten by the time you read this, as planet hunters get better. It’s only a matter of time before a true Earthlike planet is discovered.

We have written many articles about super Earths. Here’s an article speculating on the kinds of atmospheres that super Earths might have, and another article about how similar super Earths really are to our own planet.

Here’s an artist’s impression of a super Earth features on NASA’s Astronomy Picture of the Day website, and here’s an article from NASA about super Earths.

We also recorded an episode of Astronomy Cast dealing with the different kinds of extrasolar planets you can find. Listen to it here. Episode 125: A Zoo of Extrasolar Planets.

Source: Wikipedia