Astronomers discover an 'odd couple' of planets

Astronomers using NASA's Kepler planet-hunting spacecraft have discovered an Oscar and Felix among extrasolar planets – an odd couple with orbits so close to each other that at times the larger planet looms more than twice as large as the full moon in the second planet's night sky.

The duo orbits a star 750 light-years from Earth, one sun among some 150,000 stars that Kepler monitors in the hunt for Earth-like planets orbiting sun-like stars at Earth-like distances.

This pair, however, occupies a system unlike any detected so far.

Individually, each planet falls into a class detected elsewhere. The smaller, inner planet, Kepler-36b, has about 4.5 times Earth's mass and is about 1.5 times as large. It's a rocky orb with an estimated 30 percent of its mass in the form of iron.

Its larger, outer companion Kepler-36c, is more like a mini Neptune. It boasts eight times Earth's mass, but has a density suggesting a gas-giant wannabe with a "substantial" atmosphere of hydrogen and helium, according to the team making the discovery.

 But the planets' two orbits are separated by an average of only about 1.2 million miles – cosmic overcrowding by the standards of Earth's planetary neighborhood. Mars' orbit is about 49 million miles outside Earth's path, while Venus' orbital path lies roughly 27 million miles inside Earth's.

Two planets of such dramatically different compositions and such tightly packed orbits represent "an exciting system," says Eric Agol, an astronomer at the University of Washington in Seattle and one of the two lead authors on the formal report of the finding published Thursday on Sciencexpress, an online adjunct to the journal Science.

The planets reach their closest encounters – less than 1.2 million miles apart – when they line up on the same side of the star once every 97 days on average, in a configuration known as an inferior conjunction.

Life is highly unlikely on these orbs. They are so close to their host star – with orbits of 13.8 days for the inner planet and 16.2 days for the outer planet – that temperatures hover between 1,200 and 1,300 degrees F.

And they are likely to get worse. Kepler-36b is from 2 billion to 3 billion years older than the sun, the team estimates. The star is a so-called sub-giant, meaning it's burned up the hydrogen in its core and is headed down a path that will turn it into a red giant. The star's core is contracting, allowing it to burn hotter and brighter – hot enough to begin fusing the hydrogen outside the core. Like the sun's end-of-life sequence, Kepler-36's fiery atmosphere will expand much farther into space than it does now, engulfing the odd couple.

As planet-hunters uncover such seemingly bizarre systems, "we're constantly being surprised," says Joshua Carter, the other lead author and a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "But we're just surprised because we're basing our preconceptions on the solar system."

Planets in our solar system tend to have organized into two groups – the rocky inner planets and the giant, gaseous outer planets. The rocky planets formed close to the sun, where any gas the planets' gravity couldn't capture and retain was vaporized. Beyond the so-called snow line, between Mars and Jupiter, temperatures were cool enough for gas to remain a gas, as well as to shift from gas, to liquid, and finally ice.

With orbits so close and so close to their star, but with such a stark contrast in density (an eight-fold difference) and composition, the odd couple would seem to require a different explanation, the researchers say. It's possible that the rocky inner planet formed inside the frost line, while the mini-Neptune formed outside and migrated inward over time, Dr. Agol acknowledges. Migration seems to be the leading explanation for the large number of gas giants – so-called hot Jupiters and Neptunes – planet hunters have uncovered during the past two decades.

Migration explanations could account for the system the team detected, Dr. Carter agrees. But such an explanation "would have to simultaneously account for the closeness and the disparity in density."

One possible explanation Agol says he finds appealing is a variation of migration where both planets began as gas-giant wannabes beyond the snow line, then migrated in with time and settled into the orbits the team identified. But the inner planet may not have had enough mass to allow its gravity to retain its atmosphere so close to its stellar furnace, while the larger of the two was just far enough away from the star and just big enough for its gravity to keep a grip on an atmosphere.

"One of the most exciting things about this system is that maybe these planets are straddling a line" that determines whether planets, which may have started out with comparable masses when they first formed, will retain or lose their atmospheres, he says.

The team found the system as it analyzed data released early in Kepler’s mission. The craft launched in March 2009 and is now in an extended phase of its mission.

Kepler detects candidate planets by constantly watching the 150,000 stars in its field of view for the slight dimming a planet would impart as it passes in front of its host star. That dimming revealed the presence of the larger of the two planets orbiting Kepler-36. But irregularities in the timing of the dimming suggested that the bigger planet might have a sibling.

During the past year or so, as Kepler gather more data on the system, Carter and Agol have been perfecting a mathematical tool for analyzing such irregularities to tease out information on both planets from the subtle dimming effects Kepler picks up.

Kepler's exquisite measurements of key properties of the host star allowed the team to use the transits to determine the size of each planet, while the orbital periods allowed the team to estimate the masses of the planets – calculations that become more precise when the gravitation interaction between the two planets is included. Armed with these key traits, the researchers estimated the density for each planet, allowing them to distinguish the difference between the mini Neptune and the rocky super Earth.