26 August 2010

Star with planet system and possible super-Earth found

The first Sun-like star with a planet system consisting of two large planets the size of Saturn, both of which transit the star, and potentially another unconfirmed planet the size of a super-Earth, has been observed by NASA’s Kepler Mission, which includes researchers from the Niels Bohr Institute. The results have just been published in the prestigious journal, Science.

The star and planets are located in our galaxy, the Milky Way. The discovery of the Sun-like planet system is based on seven months of observations by NASA’s Kepler satellite, which is designed to measure the frequency of Earth-size planets in the habitable zone around Sun-like stars. Planets outside our solar system are called exoplanets.

The Kepler satellite The image shows the Kepler satellite observing an exoplanet in orbit
around a star. (NASA: Artist impression)

The observations of exoplanets are made by measuring the host star’s light curve and when a planet that is in orbit around the star passes in front of the star, the light dims slightly. Using this method, two large planets the size of Saturn with orbits of 20 and 40 days have been observed.

In addition to the dimming signal from the two large planets, another signal was identified which could be a smaller planet. It is not yet confirmed whether the object is actually a planet, but the signal corresponds to an object with an orbital period of 1.6 days.

"The two large planets are gas planets, whereas the third planet could be a super-Earth with a radius 1.5 times the size of the Earth. With a close proximity to its host star, the planet would be too hot to walk around on – the surface is approximately 2,000 degrees", explains Lars Astrup Buchhave, an astrophysicist at the Niels Bohr Institute at the University of Copenhagen.

Unique knowledge about exoplanets

Lars Buchhave is involved with the Kepler mission and has worked with measurements of the planet system from the Keck telescope on Mauna Kea in Hawaii. By analysing the light spectra from the ground-based telescopes researchers can rule out certain astrophysical phenomena, such as binary stars, and prove that the signals actually originate from planets.

"The observations offer a wealth of fascinating information", says Lars Buchhave, "by analysing the spectra we can determine the star’s physical properties, such as its effective temperature, gravity, metal content and speed of rotation".

By observing how the star’s light dims as a planet passes in front of it, the researchers have a unique opportunity to determine the planet’s mass, its radius and with that its density, temperature and even whether the planet has an atmosphere.

With the Kepler mission researchers have observed over 700 objects that could potentially be exoplanets, but the findings have not been confirmed yet. Eight stars with planet system have been found, but the new discovery, called Kepler-9, is the first planet system where more than one planet transits the star at the same time. The planets influence each other because of their gravity and 'pull' a little on each other so that their orbits are little irregular and changing.

"They are exciting new observations and there is tremendous follow up observation work in progress", explains Lars Buchhave, who is also busy putting the finishing touches on his PhD dissertation about, of course – exoplanets.

Article in Science >>