Exoplanets – Niels Bohr Institute - University of Copenhagen


Exoplanets - planets orbiting other stars than our own Sun

We live in an exciting time in astronomy: We are poised to answer fundamental questions that have been pondered by man for centuries. Is the Earth unique or just one of many habitable worlds? Are Earth-like planets, where water could be liquid and life as we know it exist, common in our galaxy? Can we uncover planets elsewhere in the Universe with signatures suggesting the existence of life?

An artist’s impression of the first Earth-like exoplanet, OGLE-Bulge-2005-390Lb, discovered from the Danish 1.54m telescope at the ESO La Silla observatory in 2005 as part of a large international team.

Fifteen years ago, no planets outside our Solar system were known. The discovery of the first exoplanet orbiting a Sun-like star opened a new field in astronomy: exoplanetary science. Since then, several thousand exoplanets have been discovered, the vast majority of which are quite unlike the planets in our own Solar system and unlike any that had been predicted by theory. During the first many years, instruments and methods allowed mainly for the detection of these "unusual" planets, but today, we are on the brink of identifying planetary systems similar to our own, and it seems that there are billions of exoplanets which are not very different from our own Earth, while planets like Jupiter and Saturn are more rare.

Apart from a very small number of special cases, one does not see the planets directly. The majority of the first exoplanets were discovered because they pull the star they orbit back and forth, which is seen as small shifts in the wavelength of the light from the star. The more mass the planet has, the more it pulls the star. This method therefore determines the mass of the planet. It is called the Doppler method or sometimes the radial velocity method. Another method to detect a planet is if it transits (i.e. gets in front of) its host star. Then it dims the light of the star slightly. The larger the planet, the more it dims the light of the star. This method is called the transit method, and it lets us determine the size of the planet. If we know both the size and the mass of the planet, we can calculate its density, which gives us more deep understanding of the nature of the planet.

Today most new planets are discovered by the transit method, mainly due to measurements from the very successful Kepler satellite.

Kepler-11 is a sun-like star around which six planets orbit. At times, two or more planets pass in front of the star at once, as shown in this artist's conception of a simultaneous transit of three planets observed by NASA's Kepler spacecraft on Aug. 26, 2010. Image credit: NASA/Tim Pyle

A third method takes advantage of the way light gets amplified when it passes a star or a planet. This method is called the microlensing method, because the stars and planets act like lenses. The Doppler method has its main sensitivity to planets with large mass, like Jupiter and Saturn in our solar system, and best if they are in smaller orbits. The transit method is most sensitive to planets in small orbits, like for example Mercury in our solar system, but larger in size. Both methods are pushing toward the ability to see more Earth-like planets. The microlensing method is most sensitive to planets that resemble the planets in our own solar system. The three methods therefore complement one another well, and our group is involved in research with all three methods. Together they can teach us what a "normal" solar system in the Milky Way looks like, and whether our own Solar system is normal or unusual.

Follow-up observations of the discovered planets allow a wealth of further studies to be made, for example the study of their atmospheres, which might ultimately lead to the discovery of signs of life on another planet. Besides the three discovery methods, we are also involved in the study of how the stars they orbit affect the conditions for possible life on the planets. Can life for example exist on planets around very active stars? The near future promises a wealth of exciting discoveries in the exoplanet field, and it will bring us a bit closer to the answer to the question about the possible existence of life outside our own Earth. Do we live at a completely unique place in the Universe, or are there billions of places where technical civilizations we can eventually communicate with have already developed?