by Ruth Titz-Weider (DLR), Juli 2024
Prehistory
One of the most successful methods for finding extrasolar planets is the transit method. When a planet passes in front of its star, it shadows it a little during this time. For an observer, the star is dimmed by a fraction. This phenomenon is repeated regularly, depending on the planet’s orbital period. Transits of Mercury and Venus can be observed from Earth – however, the sun can be recognised as a disc. Distant stars only appear as individual illuminated pixels. The dip in intensity is often difficult to recognise because of instrumental effects and star changes.
To discover planets with the transit method, you need to be patient. The light from as many stars as possible should be observed simultaneously without interruption in order to achieve a hit. But telescopes located on Earth have one limitation: you can only observe at night. But what if the transit of a planet falls during daylight hours? Yes, that’s bad luck. We don’t know how many transits we have missed because they took place during the day.
Idea for the space mission
With the discovery of the first planet around a sun-like star – previously planets had been found around pulsars – in 1995, the search for exoplanets really took off. The idea of a space mission developed from the desire to observe planets not only from Earth and with the inevitable restrictions – day and night, full moon. The first mission to systematically search for exoplanets in space was CoRoT, a mission organised by the French space agency CNES with substantial contributions from Germany, but also from Austria, Belgium, Spain, Brazil and the ESA.
The acronym CoRoT stands for Convection, Rotation and Planetary Transits and this abbreviation shows that this satellite was not only intended to observe planetary transits, but also the variability of stars. The changes in the star’s brightness can be caused by convection and rotation of the star and the knowledge of the intensity and periodicity of these changes allows conclusions to be drawn about the age of the star and therefore also of the planetary system. Astroseismology and transit searches are both based on the highly sensitive measurement of changes in starlight, photometry. Once a planetary candidate has been discovered, the astroseismological investigations of its star can be used to draw conclusions about the age of the entire system and thus gain insights into the formation of the planet.
Instrument
CoRoT was equipped with a wide-angle telescope with a primary mirror 27 cm in diameter and a focal length of 110 cm. There were four CCDs in the focal plane, two for the transit search and two for astroseismology. CoRoT detected the light of the stars in the visible wavelength range. The field of view was 2.7° x 3.05° – the full moon at half a degree would have fitted into this field 30 times.
Illustration of the CoRoT satellite. Credit: ©ESA
Satellite
At launch, the entire satellite weighed around 630kg and fitted into a box measuring 4.1m x 2m x 2m. Once it had reached its polar orbit, 696 km above the Earth, the two solar sails were unfolded and the protective cap over the telescope was opened like a tin can lid. The telescope received its first light in mid-January 2007, three weeks after the launch.
Course of the mission
CoRoT was launched with a Russian Soyuz-Fregat carrier rocket on 27 December 2006 from Baikonur into its polar orbit at an altitude of around 900 km. After commissioning the electronics and fine-tuning, the first observation phase – initial run – began, which lasted from 7 February to 2 April 2007. After that, measurements were taken in phases of different lengths, 150 or 20 days. CoRoT could not be aligned with a field for longer than 150 days or six months because otherwise the instrument would have been dazzled by the sun or shadowed by the earth.
The mission was initially designed to last three years and was extended twice, in 2009 and then again in 2012. From 2 November 2012, CoRoT no longer sent any data to the ground station1. Presumably, the bombardment of high-energy radiation during the almost six years of operation damaged the instrument to such an extent that it could no longer be put back into operation. The satellite was officially decommissioned on 17 June 2014.
Successes
The first planet was not long in coming: in May 2007, during the first long observation phase, the first candidate was discovered. CoRoT-1b1, a planet with a radius of 1.5 Jupiter radii and an orbital period of just 1 ½ days. Like many of the planets found up to that point – albeit only with ground-based telescopes – it belonged to the hot Jupiter family.
Transit light curve of CoRoT-1b, the first exoplanet discovered from space. Credit: COROT exo-team via ESA
CoRoT-7b was a real sensation. It was the first planet for which the radius and mass had been precisely determined. The mass and radius can be used to determine the density and this value clearly indicated that CoRoT-7b was a rocky planet. For orientation: the rocky planets of our solar system, Mercury, Venus, Earth and Mars, have densities between 4 and 5 g/cm³, the giant planets around 1 g/cm³. For many planets that have been discovered, we do not have a complete profile. We only know the radius and mass of a quarter of all planets and can therefore determine their density. You can check this yourself at any time. In the Encyclopaedia of Extrasolar Planets (https://exoplanet.eu/home/) or the NASA Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu/), all planets can be found with their properties and the corresponding sources.
Another planet that had remarkable properties was CoRoT-9b1: it has an orbital period of 95 days. This is a size that we know from our solar system: Mercury, the fastest planet in our solar system, takes 88 days to orbit the Earth. This discovery was proof that CoRoT was fundamentally capable of measuring such long periods. This relatively long orbital period was the longest determined using the transit method in 2010. 95 days correspond to one third of the distance between the Earth and the Sun (1 AU). In contrast to the hot Jupiters with much smaller distances, this planet receives significantly less radiation and the temperature of its outer shell is estimated to be between – 23° Celsius and +160° Celsius. This is moderate – even if not on an earthly scale – and makes it an interesting object for further observations.
40 planets have been discovered with or through CoRoT. They are spread across 36 stars. Three of the stars, CoRoT-7, CoRoT-20 and CoRoT-24, harbour more than one planet. But these planets were not all discovered using the transit method. CoRoT-7c, CoRoT-7d and CoRoT-20c are based on subsequent measurements with telescopes from the ground using the radial velocity method. The radial velocity measurements are obligatory for a planet candidate, as they are the only way to determine the mass. The measurements show curves that can only be explained by the presence of a second or third planet that does not cause a transit. Several planets do not necessarily have to run in the same plane. The planetary orbits can be tilted in such a way that the innermost planet causes a transit, but the others no longer do.
Accompanying measurements with ground-based telescopes
Even before the satellite was launched, the targeted CoRoT fields were observed with ground-based telescopes, for example to identify double stars that could simulate a transit with their radiation pattern. In addition, possible candidates were signalled by the alarm mode during the ongoing mission and then investigated by subsequent measurements. Photometric observations of the CoRoT fields with the BEST1 and BEST II telescopes were helpful in identifying eclipsing binaries. In order to further determine a planet candidate, the stars were observed spectroscopically. Regularly changing spectral lines can be caused by a planet (radial velocity method). The Thuringian State Observatory in Tautenburg has made decisive contributions here. Other telescopes that have investigated CoRoT candidates are located in southern France and Chile.
To learn more about the mission, visit the CoRoT mission page at ESA or the CoRoT mission page at the CNES.
Other missions: ARIEL – CHEOPS – Gaia – Hubble – James Webb – Kepler – PLATO – TESS