Team discovers unusual multi-planet solar system with Kepler spacecraft

Oct 04, 2011
Team discovers unusual multi-planet solar system with Kepler spacecraft
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

(PhysOrg.com) -- A team of researchers led by Bill Cochran of The University of Texas at Austin has used NASA’s Kepler spacecraft to discover an unusual multiple-planet system containing a super-Earth and two Neptune-sized planets orbiting in resonance with each other. They will announce the find today in Nantes, France at a joint meeting of the American Astronomical Society’s Division of Planetary Science and the European Planetary Science Conference. The research will be published in a special Kepler issue of The Astrophysical Journal Supplement Series in November.

Cochran’s team is announcing three orbiting Kepler-18, a star similar to the Sun. Kepler 18 is just 10 percent larger than the Sun and contains 97 percent of the Sun’s mass. It may host more planets than the three announced today.

The planets are designated b, c, and d. All three planets orbit much closer to Kepler-18 than Mercury does to the Sun. Orbiting closest to Kepler-18 with a 3.5-day period, planet b weighs in at about 6.9 times the mass of Earth, and twice Earth’s size. Planet b is considered a “super-Earth”. 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,” 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.”

Kepler uses the “transit method” to look for planets. It monitors a star’s brightness over time, looking for periodic dips that could indicate a planet passing in front of the star. A large part of the Kepler science team’s work is proving that potential planets they find aren’t something else that mimics the transit signature (such as a perfectly aligned background star, specifically either an eclipsing binary star or a single star orbited by a giant planet).

That follow-up work to Kepler is done by scores of scientists using ground-based telescopes the world over (including several at The University of Texas at Austin’s McDonald Observatory) as well as Spitzer Space Telescope.

Kepler-18's planets c and d did astronomers a favour by proving their planet credentials up front via their orbital resonance; they had to be in the same planetary system as each other 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.

First, they used the Palomar 5-meter (200-inch) Hale Telescope with adaptive optics 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.

“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. 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.

The process is called “planet validation,” rather than the usual “planet verification”. Cochran says it’s important to understand the difference – not just for this system, but for future discoveries from Kepler and other missions.

“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 [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.”

Explore further: Image: NGC 6872 in the constellation of Pavo

Provided by McDonald Observatory

4.7 /5 (3 votes)

Related Stories

Two more kepler planets confirmed

Aug 08, 2011

Hot on the heels of confirming one Kepler planet, the Hobby-Eberly Telescope announces the confirmation of another planet. Another observatory, the Nordic Optical Telescope, confirms its first Kepler planet ...

Exomoons could be excellent incubators

Jun 20, 2011

With the arrival of the Cassini–Huygens mission in 2004 to Saturn’s satellite Titan, we terrestrials became acutely aware that similar moons could be orbiting similarly large planets in other solar ...

Amazing image: Kepler’s transiting exoplanets

Mar 30, 2011

Wow. This remarkable visualization shows every Kepler planetary candidate host star with its transiting companion in silhouette. Jason Rowe from the Kepler science team created the image, and the sizes of ...

Recommended for you

Image: NGC 6872 in the constellation of Pavo

14 hours ago

This picture, taken by the NASA/ESA Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2), shows a galaxy known as NGC 6872 in the constellation of Pavo (The Peacock). Its unusual shape is caused ...

Measuring the proper motion of a galaxy

14 hours ago

The motion of a star relative to us can be determined by measuring two quantities, radial motion and proper motion. Radial motion is the motion of a star along our line of sight. That is, motion directly ...

Gravitational waves according to Planck

Sep 22, 2014

Scientists of the Planck collaboration, and in particular the Trieste team, have conducted a series of in-depth checks on the discovery recently publicized by the Antarctic Observatory, which announced last ...

Infant solar system shows signs of windy weather

Sep 22, 2014

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have observed what may be the first-ever signs of windy weather around a T Tauri star, an infant analog of our own Sun. This may help ...

Finding hints of gravitational waves in the stars

Sep 22, 2014

Scientists have shown how gravitational waves—invisible ripples in the fabric of space and time that propagate through the universe—might be "seen" by looking at the stars. The new model proposes that ...

How gamma ray telescopes work

Sep 22, 2014

Yesterday I talked about the detection of gamma ray bursts, intense blasts of gamma rays that occasionally appear in distant galaxies. Gamma ray bursts were only detected when gamma ray satellites were put ...

User comments : 0