Editorial: “Kepler shows us a galaxy fit for life”
WHAT are the billions of stars and planets beyond our solar system up to? Do they behave like ours or are we a freak of nature?
While definitive answers are far from nailed down, a recent deluge of observations released by NASA’s has tripled the number of candidate alien planets, taking the total to 1235, and has already yielded many tantalising revelations.
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“We’re starting to see what’s out there; the groups of planets that exist,” says at the University of California, Santa Cruz, who is not part of the Kepler team but is studying their data closely. “It’s very exciting.”
When NASA announced the release of the new data on 2 February, the initial headlines focused on possible habitable planets and an intriguing six-planet star system. More recently, excitement at the annual meeting of the American Association for the Advancement of Science (AAAS) in Washington DC centred on a planet candidate that might be an Earth “twin”. Yet the new release from Kepler offers many more insights that have received relatively little attention so far.
Kepler surveys about 150,000 stars, most of which are between 600 and 3000 light years away, and detects planets when they pass in front of their parent stars. Kepler is highly sensitive to these “transits”, which cause the stars to dim periodically, allowing the telescope to see small, Earth-sized planets missed by other hunts.
Because phenomena such as starspots can mimic this dimming, further observations are needed to confirm a planet’s existence. Nevertheless, these planet “candidates” can give a view of the environment beyond our solar system, including a new estimate for the proportion of stars that host planets.
So far, the telescope has seen planet candidates around just 997 of the 150,000 stars that it watches. But there must be vastly more planets out there. As Kepler only detects planets that it catches transiting, it only sees those with orbits that happen to be edge-on from its viewpoint – probably just 1 per cent of all planets. What’s more, the data released so far only covers the first four months since Kepler’s launch in March 2009, so only planets with relatively short orbital periods, which will have transited in that period, will definitely be detected.
Now the Kepler team has corrected for these factors, and estimates that 34 per cent of the stars that Kepler watches host a planet with a period of less than 125 days (). A previous estimate, which put the figure at 12 per cent, was restricted to planets much more massive than Earth, with periods of less than 50 days.
Another unknown was how common multi-planet solar systems such as our own are. Previously, a few dozen were known, but this was thanks to a variety of surveys with various sensitivities and biases so their relative abundance was difficult to estimate. But Kepler has delivered an unprecedented number of candidates, including smaller planets, and can now reveal that of the solar systems it detects, 17 per cent contain more than one planet candidate.
What does all this tell us? Since Kepler only sees planets that transit in line-of-sight with their star, the solar systems it flags as multi-planet systems are solar systems similar to our own, inasmuch as several planets orbit their star in the same plane (see diagram).
However, the figure of 17 per cent is more complicated than it seems. Attempts to model the relative abundances of various multi-planet solar systems predict fewer single-planet systems than the telescope sees. One explanation is that some alien solar systems that look to Kepler like they only have one planet, are in fact multi-planet systems: Kepler is simply blind to these other orbits.
This interpretation would suggest a violent past for many solar systems. Earlier surveys using different measurements have turned up signs that many gas giant planets orbiting close to their stars are very misaligned with the equator of their stars. As planets are thought to form in a disc of gas and dust that circles the star’s equator, these errant planets are thought to have been flung into misaligned orbits by violent encounters with other planets.
“Some of the planets merge, some are ejected, some fall into the star and you’re left with fewer planets for those reasons, and the remaining planets, if there is more than one, will have high inclinations,” says Kepler member of the University of California, Santa Cruz.
“As well as exotic solar-system architectures, the Kepler space telescope has turned up other oddities”
Many of the apparently single-planet systems seen by Kepler involve planets at the lower end of the mass scale, which may be even more vulnerable to violent changes to their orbits than gas giants. So it is not a stretch to imagine that similar violent encounters might explain some of the “single-planet” Kepler systems, says of the Massachusetts Institute of Technology, who is not a member of the Kepler team. “Smaller planets are easier to throw around,” he says.
As well as exotic solar-system architectures, Kepler has also turned up other oddities, which in turn are feeding back clues to how our moon formed (see “Two planets found sharing one orbit”).
Laughlin stresses that these views of activity beyond our solar system, while fuller than ever, are just a start. Kepler will also have the chance to spot planets farther from their stars that transit less frequently, including more candidates that may be capable of hosting life – the count currently stands at 54. “It would be very hard to imagine we’re not going to have all sorts of candidate habitable planets – I think there will be plenty.”
