03 February 2011
The universe is teeming with planets
More than a thousand potential planets have been found in the universe using NASA’s Kepler Satellite since the mission began in March 2009. The Kepler mission is now publishing the first 4 months of observations. Found in this data are over 1200 possible planets with very different characteristics. One of these is Kepler-11 which is a planetary system with no fewer than 6 planets. Danish researchers from the Niels Bohr Institute at the University of Copenhagen are taking part in the research by analysing data about the planets. The results have been published in the journals Nature and The Astrophysical Journal.
Planets outside our solar system are called exoplanets. Observations of exoplanets using the Kepler satellite are made by measuring a star’s light curve. When a planet moves in front of the star there is a small decrease in its brightness. If this slight dip in brightness occurs regularly there may be a planet orbiting the star and dimming its light. But the detection requires further observations.
"The next thing we do is to observe the star from Earth where you can study its light spectrum, i.e. its colour scale. This way we can find out what type of star it is and you can determine the velocity of the planet’s orbit around the star, if it is possible", explains astrophysicist Lars Buchhave, Niels Bohr Institute. He has studied the light spectra from a number of the stars from the Nordic Optical Telescope in the Canary Islands in Spain.
After the satellite observations and further observations from telescopes on Earth, computer models are made using the principles of physics and then you can calculate a lot more information. The calculations can show the size of the planet and its mass density, i.e. whether they are large gas giants like Saturn and Jupiter or whether they are solid planets the Earth and Mars.
Due to the huge number of potential new planets that have been found, it has not yet been possible to confirm them with 100 percent certainty. There are also 'planet candidates', some of which might not turn out to be planets. Since the first exoplanets were discovered 15 years ago, around 500 planets have been found. With Kepler’s publication the number of possible known planets has more than tripled.
Planets in the habitable zone
"We can now begin to see what the incidence of minor planets in the universe is and what type of planets are orbiting other stars. It is not the full picture, but it is the first shovelful", says Lars Buchhave, who further explains that of the new candidates for exoplanets, nearly 60 lie in the so-called habitable zone. That is to say that the planets are at distance from their star where the temperature could allow for liquid water. 5 of the planets in the habitable zone are less than double the size of the Earth.
"All of the new data is from Kepler’s first 4 months of observations. This means that we are talking about planets that have an orbital period of less than 4 months, that is to say an annual cycle of just 4 months or less. In future, we will be able to study potential planets with a longer orbital period, which are farther away from their host stars", explains Lars Buchhave.
Solar system with 6 planets
One of the stars that has been observed by the Kepler satellite has no less than 6 planets in orbit and all six are moving in front of the star and dimming it. The 'solar system' has been named Kepler-11. The 6 planets lie in a complicated orbit where their gravitational pulls make them pull at each other when their orbits are close together. You can calculate models for this and from the models you can tell that they are planets. You can also calculate their size and mass. They are all small planets between 2 and 4.5 time the size of the Earth. The results for Kepler-11 have been published in the prestigious scientific journal, Nature.