NASA’s Kepler mission was launched in March 2009. Its mission, to survey our region of the Milky Way galaxy looking for Earth-size and smaller planets. It does this very simply by pointing a 0.95-meter aperture Schmidt telescope with a 12 degree diameter field-of-view (FOV). It is pointed at a single group of stars for the three and one-half or more year duration of the mission and recording data. In the first 16 months of operation Kepler has detected 2,326 candidate planets, 48 of which lie in or near the habitable zone around other stars, that is the region where a planet of Earth-like size, composition and atmospheric pressure can maintain liquid water on its surface.
On December 5 NASA announced that Kepler had confirmed its first planet in the habitable zone. Kepler-22b, is the smallest planet, found so far, to orbit in the middle of the habitable zone of a star similar to our sun. But a word of caution, despite the media hype of a ‘Twin Earth’ that followed the announcement, there is a lot we don’t know about Kepler-22b. We do know that it is about 600 light years away and 2.4 times the radius of Earth. It orbits its parent star Kepler-22 every 290 days. This puts it well within the habitable zone as, though its orbit is closer than ours, Kepler-22 is of lower mass and cooler than our Sun. What we don’t know about Kepler-22b is its mass, so we have no idea if it is a rocky or a gaseous planet or even if it has an atmosphere. If Kepler-22b is rocky then its gravity would be 2.4 times that of Earth.
Kepler-22b was detected using the transit method which measured the dimming of light from Kepler-22 as the planet passed in front of the star. This gives us the planet’s size. To find the mass we need to measure the radial velocity of the star: as the planet orbits it exerts a small but detectable gravity tug on its parent star, that causes the star to wobble and this can be measured to give the mass of the planet.
Our methods of detecting exoplanets are becoming more and more sophisticated and we are finding them almost everywhere we look. Over 700 have been discovered since 1995 when the first exoplanet, 51 Pegasi was found. Whereas before we were only able to detect ‘Hot Jupiters’, large gas giants orbiting close to a star, we are now able to detect smaller and smaller planets and new technology like ExoplanetSat, that comprises very small and inexpensive satellites, will extend the search. It is perhaps not helpful if every new discovery is met with such overblown hype. When the time comes that we do discover Earth’s twin, and I have no doubt that we will, the public will feel that they have heard it all before.
“the next step, once Kepler has looked at a lot of planets, will be to see what their atmospheres are made of, using infrared spectroscopy.
“If one or two of them have oxygen in the atmosphere, it may be a transient thing – like Venus, undergoing a runaway greenhouse effect – but if we find, say, 20 Earth-like planets, all with the signature of oxygen in their atmosphere, then that would be very unlikely. Life would be the more reasonable explanation.”
I hope we can quiz Lewis further on this subject in the second part of our podcast. Watch this space.