Two Saturn-sized planets travel in synchronised orbits around the star Kepler-9 (Image: NASA/Ames/JPL-Caltech)

NASA’s Kepler space telescope normally cannot “weigh” the extrasolar planets it finds, instead measuring their physical size. But the discovery of its first multiple planet system shows the telescope can weigh planets that are gravitationally linked with their neighbours.

Kepler hunts for periodic dips in a star’s brightness, a signal that a planet may be crossing between the star and Earth in an event called a transit. The technique measures a planet’s width, since large planets block out more of their stars’ light. But it does not reveal how much mass is packed into a planet of a given size. Without that information, it may be hard to tell the difference between a water world and a planet that has been stripped down to its iron core.

Instead, Kepler astronomers must rely on other telescopes to measure how much the planets tug on their host stars, causing them to wobble forwards and backwards.


Now, the telescope has found a planetary system that demonstrates it can weigh planets for itself.

Keeping time

Using data collected over seven months, researchers led by Matthew Holman at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, have uncovered two Saturn-size planets orbiting a star called Kepler-9, which lies more than 2000 light years from Earth. A planet about 1.5 times as wide as Earth may also be part of the system.

The Saturn-sized planets travel in orbits that “keep time” with each other – the outer planet takes 38 days to orbit, twice as long as its sibling. But this “resonance” isn’t exact – sometimes the ratio of the planets’ periods is more than 2-to-1, and sometimes it is less. The discrepancy comes from the planets’ tugs on each other, providing an estimate of their relative masses.

This is a new capability for Kepler, and it could prove useful in measuring the masses of Earth-sized objects, Holman noted in a press conference.

That’s because Earth-sized planets that orbit far enough away from their stars to be able to support life as we know it will tug relatively weakly on their stars. That will make it difficult for other telescopes to weigh the planets by searching for how they make their host stars wobble.

“With Kepler, in all likelihood, you’re going to have these tantalising Earth-sized planets, but you’re going to have a very difficult time measuring their masses,” says Greg Laughlin at the University of California, Santa Cruz.

Subtle effect

But if an Earth-sized planet is in a resonance with larger planets, measuring variations in the times between their transits could give Kepler a way of determining whether it has really found Earth’s twin.

The subtle effect may require many transits – and therefore years – before the variations can be detected, though, Laughlin points out.

Exoplanets also made news earlier this week. On Tuesday, the European Southern Observatory announced the discovery of a sun-like star about 130 light years away that boasts as many as seven planets. Wobbles in the star’s position suggest that one planet candidate may be just 1.4 times as massive as Earth. If confirmed, the planet would tie the record for the smallest exoplanet found so far (a record that doesn’t include one tiny body found orbiting a dead neutron star).

Journal reference: Science, DOI: 10.1126/science.1195778