Kepler mission scientists would like to delay the release of exoplanet data. Credit: NASA/Kepler mission/W. Stenzel

Kepler, the NASA mission manoeuvring to spot the first Earth-like extrasolar planet, is supposed to publicly release data in June for the 156,000 stars at which the orbiting telescope stares. But on Monday a NASA advisory panel recommended that Kepler be allowed to censor 400 "objects of interest" — presumably good planet candidates — until February 2011, giving the mission team more time to firm up discoveries, rule out false positives and publish. If enacted, the new policy would represent a selective editing of data on the basis of its science content, rather than its quality — unprecedented for such NASA missions.

As Kepler astronomers get ever closer to the prize — an Earth-sized planet orbiting in the habitable zone of a parent star — some astronomers are advocating open sharing of data, with its benefit of bringing additional eyes and ideas to bear on ballooning data sets that swamp the resources of any individual team. Others, however, want to maintain more control over the candidate planets, which can remain in limbo for years while awaiting confirmation. This closed approach ensures not only ultimate bragging rights in the scientific literature, but also enables more cautious media announcements in a field that has suffered embarrassing retractions.

"It's come to a head," says William Borucki, Kepler's principal investigator at NASA Ames Research Center in Moffett Field, California.

Half-baked candidates

Since its launch on 6 March 2009, Kepler, with its 0.95-metre telescope, has been staring at the same field of stars near the northern star of Vega, looking for tiny reductions in starlight caused by a planet passing in front of a star's face. In January, the Kepler team announced the discovery of five new exoplanets. Borucki says that the team, as of last week, has found 328 more candidates — but that as many as 50% of these may be false positives, where objects such binary stars confuse the picture.

I think Kepler is being far too conservative, and far too closed about what's going on. ,

The option to delay Kepler's data release, made by NASA's astrophysics advisory subcommittee, was a compromise between giving the Kepler team its full preference — to censor 500 objects until the mission's end in November 2013 — and sticking with the original data policy, enacted last year, that would have required Kepler to turn over its first 43 days worth of data on 15 June. NASA astrophysics director Jon Morse says he will make a final decision based on the committee's advice in the next week or so.

Many astrophysics programmes allow researchers a proprietary period with the data. For instance, guest observers on the Hubble Space Telescope get exclusive use of their data for a year before public release. But the tradition for NASA Discovery missions — small, principal-investigator-led missions like Kepler — is to make calibrated data available immediately. That policy has already been changed once for Kepler, last year, when the team was given more than a year to pursue confirmations and work out the kinks in its data processing.

But Borucki says more time is needed because a mission launch delay meant that the team missed out on a season of the ground-based follow-up observations that are needed to verify candidate exoplanets. He also worries about releasing "half-baked" candidates that the media will jump on without an understanding of their uncertainty. "My worry is less of being scooped than it is of putting out inaccurate estimates of what exoplanets are really like out there," he says.

Keeping secrets

Exoplanet astronomers outside the mission, however, are critical of what they say is an overly cautious approach. Scott Gaudi, an astronomer at Ohio State University in Columbus, says external astronomers might help the Kepler team, as it will be unable on its own to follow up and confirm all its candidate planets. "I think Kepler is being far too conservative, and far too closed about what's going on," he says, "and I think it's to the detriment of science".

The Kepler team also shouldn't worry too much about scientific competitors taking the team's data and beating it at its own game, says Malcolm Fridlund, the European Space Agency (ESA) project scientist for the COROT spacecraft (Convection, Rotation and Planetary Transits), a French mission that also looks for planets but which has a 0.3-metre telescope — smaller than Kepler's. Fridlund says there is "no way" the COROT team was planning to use the public Kepler data as it is too busy trying to confirm its own backlog of several hundred candidates.

COROT has a similar data policy to Kepler's current plan — a proprietary period of one year. But Fridlund, who is ESA's study scientist for a planned follow-up planet-hunting mission called PLATO (Planetary Transits and Oscillations of Stars), wants to do things differently next time. The policy for the future mission will require the immediate dissemination of data. "You get a larger community and you get a bigger workforce for free," he says. "It's clear that the more people you get involved, the more support you get."