10 January 2011

Astronomers discover Earth-sized exoplanet

A solid planet approximately 1½ times the size of the Earth has been observed outside our solar system. It is the smallest planet found so far in the hunt for planets in our galaxy, the Milky Way. The planet was observed by NASA’s Kepler Mission, and Danish researchers from the Niels Bohr Institute at the University of Copenhagen and Aarhus University took part in the research. The results are published in The Astrophysical Journal.

The newly discovered planet has been named Kepler-10b. Planets outside our solar system are called exoplanets and the exoplanet Kepler-10b orbits a star that is approximately 564 light years away. The Milky Way is a large spiral galaxy that is 100,000 light years in diameter and 1,000 light years thick, so the planet is in our galactic neighbourhood.

Ligt curve of star when planet passes it
The figure shows the dimming in the light
of the star caused by the exoplanet
Kepler-10b which is 1.4 times the radius
of the Earth and has a mass 4.6 times the
Earth’s. Due to the small size of the planet
there is a very small dip in the brightness
of the star (152 parts per million – PPM).
The eclipse lasts just under two hours.
The data is from NASA’s Kepler satellite.  

Measuring diminished light

Observations of exoplanets with the Kepler satellite are done by measuring the light curve of a star. When 'something' moves in front of the star, the light of the star is dimmed a bit. From the observations you can directly see a decrease in brightness, but this could be due to several things.

"We analyse the light curve to find out what is causing the drop in the light curve. It could be a planet, but it could also be a system of several stars orbiting each other. It is a very common phenomenon in the universe to find two or three stars rotating around each other. But in addition to studying the brightness of the star, we also study the star’s light spectrum, that is to say its colour scale and using that we can determine whether it is a planet", explains astrophysicist Lars Buchhave at the Niels Bohr Institute at the University of Copenhagen.

But how large is the planet and how massive is it? It is interesting to understand whether it is 'just' a large gas planet or whether it is a solid planet like the Earth, where there might be potential for life as we know it.

Planet moves in front of star

When a planet moves in front of a star it, the light dims a bit.
In observations you can directly observe a decrease in
brightness. The newfound exoplanet has an Earth-like size.
ESO: Artistic rendering.

To learn more about the planet, we study the star that the planet is orbiting around. Researchers from Aarhus University have studied the star using astero-seismology, where you measure the stellar oscillations. A star is a giant, glowing ball of gas with a nuclear power plant inside. The star vibrates and the vibrations or oscillations paint a picture of its size and density. It is a 10 billion year old Sun-like star and is slightly smaller than the Sun.


Due to mutual gravitational pull the star 'wobbles' a bit as the planet orbits around it. And when you know the size and mass of the star you can calculate how large and how massive it is and with that, how dense the planet is.

"It is a super-Earth planet as we call the category of planets that are 1-10 times the mass of the Earth", explains Lars Buchhave. The newfound planet is 1.4 times the size of the Earth and it is 4.6 times more massive than the Earth, so the conclusion is that its average density is the same as iron. Its orbital period is 0.84 days, so its “year” is less than one day. It may very well have a rocky surface like the Earth, but it lies very close to the star so it is very hot on the surface – around 1560 degrees C, so there is no possibility for life as we know it on Earth.

"Out of the 100 dimming exoplanets that have been found up until now, there are only 2 other small Earth-sized planets, so it is an important discovery, as we are now coming closer and closer to finding a planet that resembles our own planet Earth", explains Lars Buchhave, who is expecting a great deal from the coming years’ hunt for exoplanets that are habitable for life.