Kepler's Outrageous Six-planet System

I have covered developments in the field of extrasolar planets since 1995, when the first planet around a Sun-like star was announced around 51 Pegasi. With the pace of discovery advancing at a seeming exponential rate (the current tally is 525 and rising fast), I feel a bit jaded whenever a new exoplanet result is announced. It takes a lot to impress me.

Consider me impressed. In what astrophysicist Jack Lissauer (NASA/Ames Research Center) calls "the most important exoplanet discovery since 51 Peg," scientists on the team for NASA’s Kepler space telescope have announced an entirely new planetary system that seems so outrageous I almost don’t want to believe it.

The system consists of at least six planets, all of which orbit the host star in such perfect alignment to one another and to our line of sight that they periodically cross the face of the star’s disk, an event known as a transit. Not only that, five of the six are packed so closely near the star that they would easily fit inside Mercury’s orbit if they were going around the Sun. Their orbital periods range from 10.3 to 46.7 days.

If that isn't impressive enough, the planets themselves are unlike anything in our solar system. The five innermost planets range in mass from about 2.3 to 13.5 times that of Earth, and vary in diameter from about 2 to 4.5 times that of Earth. Being so close to the host star's heat and wind, one might expect them to be scaled-up versions of the terrestrial planets, like the recently discovered "cannonball planet" Kepler-10b. But the five worlds have surprisingly low densities, ranging from 3.1 grams per cubic centimeter at the high end (slightly less dense than our Moon) to only 0.5 g/cm³ (less dense than Saturn, which consists almost entirely of gas).

As Lissauer points out, the planets are more like mini-Neptunes than super-Earths. They probably have rock-iron cores that are surrounded by thick envelopes of ice and gas. Adds team member Jonathan Fortney (University of California, Santa Cruz), "I think of them as being like a marshmallow with a ball bearing in the center. Most of the mass is in the core, but most of the volume is in the atmosphere."

Kepler scientists led by Lissauer discovered the planets by seeing how much each one slightly dims the host star’s light every time it transits. The amount of dimming reveals the planet’s diameter. Normally, astronomers follow up Kepler planet candidates by looking for subtle changes in the star’s spectrum as it wobbles under the gravitational influence of one or more planets. These radial-velocity measurements can confirm a planet’s existence and reveal its mass.

But in the case of Kepler-11, the star is so faint (magnitude 14) that performing these measurements will be extremely challenging. Instead, Lissauer and his colleagues have measured the mass of the five innermost planets by seeing how they gravitationally jostle one another, perturbations that show up as subtle variations in the transit timings.

Unfortunately, the sixth planet, which orbits considerably farther out (a little beyond where Mercury orbits the Sun, and with a period of 118 days), hasn’t affected the inner planets enough yet for its mass to be measured. The planet is about 3.7 times the diameter of Earth, but all the Kepler team can say about its mass is that it’s less than 300 Earths (a little less than a Jupiter mass) — otherwise they'd have detected its gravitational influence by now.

Nobody was predicting that Kepler would discover such a system, or even that such a system might actually exist in nature. As exoplanet researcher David Charbonneau (Harvard-Smithsonian Center for Astrophysics) notes, "It's quite simply one of the most beautiful data sets I have ever seen! I knew it was possible in principle to find a system of six transiting planets, but when it materialized I could barely believe it."

Computer simulations of the system’s long-term evolution show that the configuration is gravitationally stable for the 8-billion-year estimated lifetime of the host star. The star itself is very similar to the Sun in mass and temperature, but is older and slightly puffed up. It’s about 2,000 light-years from Earth in the constellation Cygnus.

The planets appear to trace nearly circular orbits, and their orbital planes are aligned to within 1° or 2° — prerequisites for maintaining long-term stability. Surprisingly, none of the planets are locked in orbital resonances with their siblings, meaning their orbital periods don’t come in ratios of simple integers.

All of these factors indicate that the planets formed farther out than their current orbits, and slowly and gently migrated inward within a gas-rich disk that possibly contained huge numbers of asteroids. The gravitational influence of gas and small bodies would tend to circularize the orbits and keep their orbital planes well aligned, and would help prevent the planets from locking one another into resonances.

The planets generally become less dense the farther they orbit the parent star, a relationship that hardly comes as a surprise. The host star’s heat and wind is blasting the planets, a violent interaction that is slowly stripping away their atmospheres, with the innermost planets suffering the most. "This is exactly the kind of system you want in order to study this mass-loss process," says Fortney. "Six planets around the same star; it’s ideal for comparative planetary science."

Lissauer also points out that there could be additional undetected planets, either inside the innermost planet, between the fifth and sixth planets, or outside the known planets. If the planet is massive enough, it should eventually reveal itself through its gravitational perturbations of the known worlds.

Exo-Earths Galore

If the new multiplanet system doesn’t yet impress you, take note that Kepler scientists today also announced hundreds of new exoplanet candidates, bringing the mission's total to 1,235. Of the newly-announced candidates, dozens are Earth-diameter or smaller. Team member Geoff Marcy (University of California, Berkeley), who has been involved in more planet discoveries than anyone in history, estimates that "90% to 95% of these candidates are bona fide planets." Adds team member Daniel Fabrycky (University of California, Santa Cruz), "There are a ton of multiple-planet candidates from Kepler. There are 115 doubles, 45 triples, 8 quadruples, 1 quintuple, and 1 sextet (Kepler-11). So Kepler-11 is just the extreme end of a distribution, not a freak."

Of Kepler’s 1,235 planet candidates, 68 are about the size of Earth. Five of those orbit within the habitable zone of the host star — the region around a star where water on a planet's surface can exist in a liquid state. Overall, 54 of the new planet candidates orbit within their star’s habitable zone, 49 of which range from about twice Earth’s diameter to larger than Jupiter.

All of this suggests that Kepler's most exciting results lie in the future. And who knows, after the exoplanet count has risen into the thousands, and astronomers have characterized individual planets and systems to the point where they truly understand the full diversity of what’s out there, maybe it will turn out that our solar system is the freak.

Here's more from NASA on the six-planet system and on Kepler's overall new data release.