Kepler, Planets and Singing Stars

Kepler was launched into space by a predominantly American team in March 2009. For a period of at least 3.5 years, it will make precise measurements of the light from 170,000 stars, simultaneously. The data do not only boost the discovery of planets orbiting other stars, including those resembling Earth, but they also open a window on the inner workings of the stars themselves. Probing the inner structure of stars is a European speciality. It is time to join forces and get to know our cosmic neighbours!

The German astronomer Johannes Kepler first discovered the harmonic patterns in planetary motion as they occur in our solar system. The first objective of the NASA satellite named after him is to probe far deeper into these patterns to detect planets around other suns. It does this through the so-called transit technique. Every time a planet passes in front of its host star, it causes a slight dimming of the star¡¯s light. This dimming may be incredibly subtle, like a mosquito passing in front of a street lamp located at a distance of 1000 km, but Kepler¡¯s instruments are able to detect it!

To date we have spotted more than 540 confirmed exoplanets ¨C planets orbiting distant stars ¨C with Earth-based surveys. Before the Kepler mission there were about 350 known exoplanets. In the Kepler press release of February 2011, it was announced that Kepler has found 1235 planetary candidates circling 997 host stars. Confirmation of these candidates is on its way. A more recent press release, in May 2011, announced that over 400 of them reside in systems containing two or more planets, and that most of those do not resemble our solar system at all.

Stellar orchestra

Johannes Kepler¡®s discoveries strengthened his belief that harmony must be present all through the universe. Almost 4 centuries later, we know that many stars actually ¡°ring¡± like giant bells. Indeed, sound waves lead to oscillations of the whole star. The Universe is filled with islands of sound!

Some stars pulsate very slowly, producing deeper sounds (the tubas of the star world), while others oscillate rapidly and with a higher pitch (the stellar piccolos). The oscillations of a star cause its brightness to change periodically. Astronomers divide stars into different classes, and the waves that can be propagated inside a star are determined by its structure. Red giants, white dwarfs, blue supergiants, and yellow sun-like stars: can you imagine a more colourful orchestra?

The Kepler satellite captures the sound of these stellar instruments by recording their variations in brightness. A team of scientists then carries out "stellar seismology¡± or ¡°asteroseismology¡±, in a similar way that geologists probe the Earth's interior. When earthquakes send waves through the Earth's interior, we analyse them to understand the structures deep beneath our feet. Asteroseismology allows us to do the same thing for the internal structures of incredibly distant stars. For the sun, our own star, ¡°helioseismology¡± has led to major insights, which we would not have been able to obtain in any other way.

Every star is unique. But an experienced ¡°asteroseismologist¡± can make sense of them, just as a violin expert can tell a Stradivarius apart from any other just by listening.

European specialty

In order to maximise the scientific use of the data to come, the Kepler Asteroseismic Science Consortium has been formed. It includes about 500 researchers from all over the world, but most of its members are based in Europe. Asteroseismology is a European-dominated field of research. Thanks to CoRoT (Convection Rotation and planetary Transits), a smaller, French-led European space mission focused on planet search and asteroseismology and operational since 2007, European scientists are optimally prepared to explore the Kepler stellar data. Joining forces is crucial to succeed in such a big endeavour.

The KASC team uses seismic techniques to probe the cores of a large number of stars. This allows us to determine the size of the star and to measure a star's chemical composition as well as its rotation rate. For many of the stars where accompanying planets are detected, we use stellar seismology to determine the parent's age, because their planets have about the same age but this cannot be determined by other means. This will answer some crucial questions. For example, are all stars that host Earth-like planets similar to our sun? Or is there diversity?

Exploring distant stars has taken on a new life thanks to a wealth of data from NASA's Kepler spacecraft. Several star-shaking results have already been presented at press conferences worldwide. They prove that Kepler can make precise measurements of the radius and age for individual stars. The data also allow us to better understand the future of our Sun when it will become a red giant and the mysterious ringing in variable stars used as cosmic lighthouses.

Who would have guessed in Kepler¡¯s days we would one day be listening directly to the hearts of stars with telescopes in space, allowing us to understand their intriguing lives.

Kepler is working well enough to detect planets like our own Earth. Does a twin Earth exists somewhere in our galaxy? Based on Kepler data, an estimate of around 100 million habitable planets in our galaxy may be realistic. And then the next questions driving research will be, ¡°Could that planet host life? Does it have an Earth-like atmosphere?¡± Answering these will not be easy, but scientists are already working on the next steps.