Kepler: The Search is On

by Edna DeVore - Deputy CEO

Is the Earth rare, unique among worlds, or does our home planet resemble uncounted others that orbit distant stars? Do distant planets provide comfortable places for living beings? A new NASA space mission will help answer these key questions in the search for extraterrestrial life. On December 21, NASA announced two new space missions in the Discovery series: Dawn and Kepler. Dawn will explore two large asteroids in our own solar system. Kepler, on the other hand, will look beyond the neighborhood of the Sun to seek evidence for Earth-sized planets circling distant stars.

Do other solar systems exist like ours -- with inner reaches populated by small rocky planets and outer zones populated by giants? Is our solar system rare or unique? Today, we cannot answer that question as we lack sufficient data. The almost 80 extrasolar planets discovered so far are giants like Jupiter, which is almost 318 times more massive than Earth. Many orbit their parent stars in hours or days, rather than the months or years the planets in our system take to orbit the Sun. So we know of other solar systems, but they are very different than ours. The discovery of giant planets rather than planets of all sizes results as a consequence of the Doppler-shift technique used to find them rather than an indication that our solar system is distinctive.

The Kepler Mission will seek evidence for small planets like Earth, orbiting stars similar to the Sun. Kepler is a photometer -- an astronomical light meter -- that will measure the light output of many stars simultaneously. It will measure the brightness of stars with great precision, but it will not take photographs or spectra. When Kepler launches, it will soar into an Earth-trailing orbit and fix its sights on a piece of the Milky Way in the direction of a starry field in the constellation Cygnus. There, Kepler will view continuously more than 100 square degrees -- about 500 times the area of the full moon -- and monitor the brightness of about 100,000 stars in our galaxy.

The spacecraft will look for planets as they cross in front of their parent stars and block a part of the light from the star. This activity is called a transit.

From Earth, we periodically see Mercury and Venus transit the Sun, and early observations of these transits helped to establish the size of these planets and their orbits. When the Moon crosses between Earth and the Sun, blocking out the Sun's light, we call this special kind of transit an eclipse. We have learned to predict these local events from observation and by employing the laws of planetary orbits discovered by Johann Kepler. By observing the transit of a distant planet, the Kepler science team can determine both the size and the period of the planet's orbit -- its year. The duration and depth -- amount of starlight blocked by the planet -- distinguishes giants like Jupiter from small planets like Earth.

Why stare at the same group of stars for the planned four years of this mission? First, we don't know which stars will have transiting planets, and those must be discovered. Second, when a possible transit is detected, it must be confirmed by observing subsequent transits. For example, think about finding our planet Earth by this method. An alien astronomer on a planet circling a distant star observes the first transit of the Earth across the face of the Sun. One Earth-year later (365+ days) a second transit is observed. Is this the same planet, or another, or perhaps a variation in the star's light output? After waiting for a third Earth-year to pass, the alien astronomer is rewarded by a third transit of the same period, duration, and brightness change. Thus, the Earth is discovered and confirmed. Like the alien astronomer, the Kepler science team will seek this robust confirmation before announcing the discovery of planets orbiting other stars. The team anticipates discovering planets of all sizes, but particularly hopes to discover smaller planets like Earth.

When Kepler sees the transit of an Earth-sized planet, the light from the star will decrease in proportion to the size of the planet and its distance from the star. A Jupiter-sized planet can diminish the star's brightness by as much as 1-2%. To date, one Jupiter-sized planet has been observed to transit its star: HD209458. In 1998, amateur astronomers confirmed this measurement and in November 2001, astronomers announced the discovery of sodium in the atmosphere of this planet from observations made using the Hubble Space Telescope during a transit. A terrestrial-sized planet orbiting at a comfortable distance (as Earth does the Sun) would dim the star's light by 1/10,000th comparable to watching a gnat fly across the beam of a searchlight. The human eye could not detect such a subtle change. But, the highly sensitive charge-coupled devices (CCD's) -- the electronic eyes of the satellite -- onboard Kepler can detect this tiny decrease and provide evidence of distant worlds.

Over the duration of the mission, Kepler will gather sufficient data for us to understand the general characteristics of other solar systems. By observing 100,000 stars, Kepler will conduct a census of sorts. Is our solar system unique, or is it common? Is the Earth a rare type of planet? If Kepler finds that "Earths" are prevalent, the discovery points toward many potential sites for life beyond our own solar system. If Kepler finds few or no small, terrestrial-sized planets, this too is a successful outcome, as we will better understand the place of our Earth and our solar system in the universe.