EXOPLANETS

Starship Kepler—seeking out new worlds



By Amir Alexander
Science Writer, The Planetary Society


A flea on a searchlight

Kepler, the space observatory searching for Earth-like planets orbiting distant stars. During 3 years in space Kepler has already detected thousands of planetary candidates, most of which will almost certainly be confirmed as planets. Before Kepler, decades of searching had yielded only a few hundred exoplanets.

As it happened, a method was soon at hand. Known as transit photometry, it looks for the minuscule dimming of a star's light when a planet passes between it and Earth.

If such dimming occurs at precisely regular intervals, then it is very likely that the star is orbited by a planet.

The dimming caused by a transiting planet is so small that it has been compared to a flea crawling across a searchlight, viewed from 1000 miles away.

Not surprisingly, the photometers used initially by planet hunters were not sensitive enough to register such a tiny effect—but, about a decade after the discovery of the first RV planet, new worlds were being discovered by transit photometry as well.

Transit photometry has a number of important advantages. It provides the size—not mass—of a planet, making it a perfect complement to RV searches. It is less sensitive to distance than RV searches, and is in principle capable of finding planets the size of Earth or smaller.

But it has serious drawbacks too. First of all, only a tiny fraction of planets actually transit—that is, pass between their star and the observer on Earth. That means that most planets are simply invisible to this method.

Then there is the problem that telescopes on Earth can only observe a star during the night and will miss a transit taking place during the day or in cloudy conditions. This is a particularly serious problem when trying to establish if a dimming takes place at regular intervals.

And, finally, the transit method tends to produce a lot of false positives—slight dimmings that look like transits but really aren't. These are usually caused by a hidden binary star behind the observed star, whose light increases and decreases, mimicking the effect of a transit.

These are serious drawbacks, so it is not altogether surprising that where­as planet hunters using the RV method produced a steady stream of discoveries, the yield from the transit searches was, by comparison, only a trickle.

Starship Kepler

Within a few years, however, the trickle turned into a flood. In Mar 2009 NASA launched a permanent planet-hunting observatory into space, charged with finding Earth­like planets. Its name was Kepler.

Its method of detection was transit photometry. In the 3 years since its launch, the spacecraft has revolutionized the search for planets around faraway stars and our basic understanding of what is out there in the universe.

The ideas behind Kepler are simplicity itself:

  1. Put a telescope in space with a sensitive photometer, and point it at one star-rich portion in the sky.
  1. Let it sit there, year after year, staring constantly, unblinkingly, at the same batch of stars.
  1. Wait around as the results start pouring in.

It is this extraordinary simplicity that makes Kepler so effective. Because it is in space, it does not rotate with Earth, and can observe its targets without interruption. It is also free of weather and atmospheric effects that limit the sensitivity of Earthbound observatories.

And because it is looking at many thousands of stars at the same time, it hardly matters that only a small fraction of the planets orbiting them will actually transit. Some will, inevitably—and they will be detected. As for the rest, scientists will be able to extrapolate from the planets that they have found to those they haven't.

Kepler 20e, with a diameter of 6900 miles, is the first planet discovered outside our solar system that is smaller than Earth.

The end result will be a clear picture of how many and what kind of planets and planetary systems are out there in the galaxy. Most intriguing of all, Kepler will tell us how many small rocky planets like our own are out there, and how many of them are neither too hot nor too cold to sustain life.

Kepler is not as large, and certainly not as mobile, as Capt Kirk's Enterprise, designed to stay in place while following the Earth around the Sun. Still, by the standards of the early 21st century it is a substantial spacecraft.

It weighs 2300 lbs, nearly all of which consists of a telescope with a diameter of 37 inches and a 12° field of view. Unlike most spacecraft, it has only 1 instrument—a photometer made up of 42 charge coupled devices (CCDs), each with 2200 x 1024 pixels.

The entire apparatus is pointed at a patch of sky near the constellations Cygnus, Lyra and Draco, which contains about 145,000 stars. It is the planets orbiting these stars that Kepler was designed to find.

And find them it has. As of Dec 2011 Kepler had detected no fewer than 2326 different transiting planets. Most of these have not yet been confirmed—a process that involves follow-up RV observations or statistical calculations to eliminate false positives.

But, according to Kepler team member Darin Ragozzine of the Harvard-Smithsonian Center for Astrophysics, these is no doubt that the overwhelming majority of them will ultimately turn out to be planets. This will be nearly 4 times the number of planets that have been discovered by all other methods since the first one was detected 17 years ago!

Kepler has proved particularly good at detecting not just planets but entire planetary systems. Finding such systems has been a challenge for RV searches, but for Kepler the detection of 1 transit very often heralds the detection of others around the same star. Overall, Kepler has already found no fewer than 350 planetary systems.

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