The hunt is on

Nasa’s Kepler mission has discovered its first Earth-sized planet candidates, and the first candidates in a habitable zone, or so New Scientist and Nature magazines reported last week.

The findings are based on the results of observations by Nasa’s Kepler space telescope, conducted between 12 May and 17 September 2009. Kepler’s field of view, which covers approximately 1/400 of the sky, observes more than 156,000 stars virtually continuously. Based on Kepler’s data, Nasa scientists identified 1,235 planet candidates.

Of these planets, 54 are situated in habitable zones in relation to their stars, where liquid water – and thus perhaps life – could potentially exist.
“They selected a direction rich in stars that are comparable in size with our sun, or smaller,” explains Professor Imke de Pater (Aerospace Engineering). The reasons for this were pragmatic: planets rotating around larger stars are harder to detect, and small stars tend to form stable planetary systems.
The Kepler space telescope detects planets around stars by using a method that measures small dips in the star’s brightness. This observational method was first proposed by Nasa’s Dr Bill Borucki, who had to convince many people that his method would actually work.

“The difference in brightness is extremely small,” explains Prof. De Pater, as she scrolls through the Nature article on her laptop. A planet passing in front of a star may reduce its brightness by as little as 0.005 percent. “I wouldn’t have liked to work out the observations,” she says, “but I do enjoy the results.”
A selection of the newly found planets has subsequently been observed by larger, Earth-based telescopes as well. Here the observational method used is to measure the rotation speed of the planets (by measuring the Doppler shift in stellar absorption lines). Combined with the angular velocity, which follows from the observed periodicity of the planet around its star, the velocity allows scientists to calculate the mass. Based on the amount of dimming in the starlight, scientists can then determine the planet’s diameter and thus calculate its density.

As of yet there is no information available about the planets’ atmospheres, although Prof. De Pater believes that some day it should be possible to use spectroscopy for detecting the presence of certain elements in a planet’s atmosphere. Absorption lines should show up in the star’s spectrum when the planet passes in front of the star.

Spectroscopic analysis of the atmospheres of Earth-like planets could well become the next major space mission. “These Kepler findings will undoubtedly give rise to new mission proposals,” Prof. De Pater believes.
One of Kepler’s findings is six confirmed planets orbiting a sun-like star, named Kepler-11, located approximately 2,000 light years from Earth. This is the largest group of planets orbiting a single star yet discovered outside our solar system. All six of the confirmed planets have orbits smaller than Venus’, and five of the six have orbits smaller than Mercury’s. The Kepler-11 findings were published in the February 3rd issue of the journal Nature.

“This is a very special discovery,” says exo-planets expert Daphne Stam, who works for the Netherlands Institute for Space Research and is one of the teachers of the MSc planetary science programme at the faculty of Aerospace Engineering.
“These planets are orbiting so closely to one another that they influence each other’s orbit,” she explains. “It’s therefore also possible to calculate their mass.”

Researchers can deduce the sizes of the planets from their speed of rotation and the decreases in the star’s brightness when the planets pass in front of them. By combining this information with the mass, they can calculate the density of the planets and subsequently deduce what the planets are made of. “In this star system all information comes together,” Stam concludes.

Link to article in Delta