杏吧原创

The planet-hunting has found its first extrasolar planets: three alien worlds that had been previously discovered with ground-based telescopes. The finds confirm that Kepler鈥檚 instruments are sensitive enough to detect Earth-like planets around sun-like stars 鈥� but they might also be unexpectedly sensitive to charged particles in space that can zap circuitry.

Kepler launched on 6 March with a simple charge: Stare at a swatch of sky for three and a half years, and look for Earths. The telescope will hunt transiting exoplanets, planets that pass in front of their stars and dim their brightness at regular intervals.

It鈥檚 focused on a 100-square-degree patch of the Milky Way between the constellations Cygnus and Lyra that contains about 4.5 million stars, 100,000 of which are prime candidates for planets.

In the first 10 days of its calibration period, Kepler collected data on 52,496 stars, three of which were known to have transiting planets. 鈥淲e expected to be able to see those instantly from the first data that we took,鈥� says project manager Jim Fanson at NASA鈥檚 Jet Propulsion Laboratory. 鈥淎ny planet that you can detect from the ground will be very obviously visible to Kepler.鈥�

One of these planets, , provided some good news: Kepler is indeed sensitive enough to detect alien Earths.

Brightness dip

HAT-P-7b is so hot 鈥� more than 2300 掳Celsius 鈥� that it radiates its own light. Deputy principal investigator David Koch, of NASA鈥檚 Ames Research Center, says: 鈥淭he planet is glowing red.鈥�

Kepler detects the light emitted by both the sun and the planet. This means it can tell when the planet passes behind the star as well as in front of it. When the planet is in front, it blocks a bit of the star鈥檚 brightness; but when the star is in front, the planet鈥檚 light disappears.

Kepler measured the dip in brightness when HAT-P-7b hid behind its star with extreme precision 鈥� more than enough to detect other Earths, Koch says.

鈥淜epler is operating at the level required to detect Earth-size planets,鈥� Koch and colleagues write in a new study. 鈥淭he signal from a Sun-Earth analogue will be at a comparable level of statistical significance.鈥�

Vulnerable components

But Earth-sized planets aren鈥檛 the only thing Kepler is sensitive to. It may be more susceptible to harmful cosmic rays than scientists had hoped.

Kepler鈥檚 computer has mysteriously entered a standby, or 鈥渟afe鈥�, mode twice since launch, once on 15 June and once on 2 July. This happens when the computer unexpectedly fails to communicate with the telescope鈥檚 electronics, and automatically reboots itself as a precaution.

鈥淚t鈥檚 like if you鈥檙e driving your car and your 鈥榗heck engine鈥� light comes on, you don鈥檛 want to start your cross-country trip. You want to open the hood and have a look at the engine,鈥� Fanson told New 杏吧原创.

Voltage spike

杏吧原创s are investigating several possible causes for the hiccups, both of which probably had the same root source.

The prime suspects are energetic charged particles known as cosmic rays. Earth鈥檚 atmosphere shields us from these particles鈥� potentially dangerous effects, but they bombard spacecraft at a rate of thousands per second.

If a cosmic ray hits a vulnerable spot in Kepler鈥檚 electronics, it could cause a voltage spike that mimics a request from ground controllers to reboot the spacecraft鈥檚 computer. 鈥淚t could be that the computer is just chugging along doing everything fine, and then a cosmic ray comes sailing through,鈥� Fanson says. 鈥淎ll of a sudden it thinks it鈥檚 been asked to reset, so it resets.鈥�

Alternatively, cosmic rays could toggle chips in the computer鈥檚 memory, making it misinterpret instructions. The reboots could also be caused by a bug in the software, or half a dozen other things, Fanson says. 鈥淭here are many, many things you have to look at that could be causing it. These systems are very complex,鈥� he says.

Accelerator test

杏吧原创s will begin testing duplicates of Kepler鈥檚 electronics in September, bombarding them with charged particles from .

If the culprit does turn out to be cosmic rays, mission managers could tell the telescope not to shut down all the way when it sees this sort of signal. That would reduce the time it takes to start back up. 鈥淲e can鈥檛 modify the electronics now, so we鈥檇 just have to live with it, and modify the way the spacecraft responds to mitigate the impact to the timeline,鈥� Fanson says.

But not to worry 鈥� the time spent in safe mode 鈥渂asically has had no impact鈥� on data collection. The mission team allocated 12 days per year as a buffer in case anything happened. Of the 150 days in the mission so far, only three and a half have been lost to safe mode.

鈥淲e expected that we would spend some time in safe mode. Every mission does,鈥� Fanson says. 鈥淓ven if we did nothing [to mitigate the problem], it would not affect the mission.鈥�

Journal reference: (vol 325, p 709)