Hunting for water on exoplanets

Grazie ad una serie di osservazioni mediante il Very Large Telescope (VLT) dell’ESO, un gruppo di astronomi sono stati in grado di rivelare alcune tracce della presenza di molecole di acqua nell’atmosfera di un esopianeta. La scoperta dà credito ad una nuova tecnica che permetterà agli astronomi di cercare l’acqua in maniera efficiente su centinaia di mondi alieni senza far uso di telescopi spaziali.

Since the early 1990s scientists have found almost 1000 planets in orbit around other stars. These so-called exoplanets are mostly much larger than the Earth and many are much closer to their stars than we are to the Sun, leading them to be described as ‘hot Jupiters’. In the new work the team studied the exoplanet HD 189733b, a world that orbits its star every 2.2 days and is heated to a temperature of over 1500 degrees Celsius.

Astronomers usually find exoplanets by measuring the gravitational influence of the planet on the star, which acts to pull the star around in a very small orbit, at velocities of a few kilometres per hour.

This movement causes a small shift in the lines of the stellar spectrum (known as the Doppler shift), which move back and forth with the wobble of the star. The Leiden University-led team have flipped the technique on its head by measuring the gravitational influence of the star on the planet, which is much larger, hurling the planet around its orbit at some 400,000 km per hour. They measured this by tracing the Doppler shift of the water lines in the exoplanet’s spectrum as it orbited the star. Despite the much larger velocity of the planet, it is nearly a thousand times fainter than the star, which makes detecting it very difficult. The team were able to detect the spectral line of water in the exoplanet atmosphere by using the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) instrument mounted on the VLT. Using the same technique, scientists were recently able to find the simple molecule carbon monoxide (CO) in the atmosphere of the same planet, but this is the first time it has been used to identify a more complex molecule like water (H2O). The detection means that the door is now open for a much more detailed census of the chemical make-up of many other exoplanet atmospheres, including molecules such as methane (CH4) and carbon dioxide (CO2), which are key ingredients for unravelling a planet’s formation history. It also paves the way for future observations with the coming generation of large telescopes like the European Extremely Large Telescope (E-ELT) that will begin operations from its site in Chile in 2020.

These instruments will be able to use the technique to hunt for potential signs of life, such as oxygen, in the atmospheres of planets similar to the Earth.

Jayne Birkby, who led the team, said, “We knew our technique worked for simple molecules at shorter wavelengths, but in order to hunt for water, we had to go to longer wavelengths where the Earth’s atmosphere really starts to obstruct the signals we are looking for, so we weren’t sure we would find anything. Of course we were delighted when we saw the signal jump out at us. It means we can do much more with this technique. In the next decade our work will help astronomers refine their search for Earth-like planets, and even life, in orbit around other stars. It’s incredibly exciting to think that in my lifetime we will reach a day when we can point up to a star and say with confidence that it has a world just like our own”.

RAS: Novel technique boosts hunt for water on planets around other stars

arXiv: Detection of water absorption in the dayside atmosphere of HD 189733 b using ground-based high-resolution spectroscopy at 3.2 microns

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Kepler, SOPHIE & HARPS-N detect two new exoplanets

An international team of astronomers, including Alexandre Santerne of the EXOEarths team at CAUP, has identified and characterized two new exoplanets thanks to combined observations from the Kepler space telescope plus the SOPHIE and HARPS-N spectrographs. These planets, named KOI-200b and KOI-889b, are among the first detected with the new high-accuracy spectrograph HARPS-N, the northern hemisphere counterpart of the most prolific exoplanet hunter, HARPS (ESO). CAUP researcher Alexandre Santerne commented: “The SOPHIE spectrograph was already playing an important role in the characterization of Kepler planets by unveiling the true nature of the candidates and measuring the mass of giant planets. With the new HARPS-N spectrograph, with an even better accuracy, we expect to characterize much smaller exoplanets, hopefully down to the size of the Earth“.

The new planets have about the size of Jupiter but eccentric orbits with periods of less than 10 days. These new results help to further understand the evolution of orbits of these planets located very close to their star, known as “hot Jupiters.” There are currently more than 850 known exoplanets, but as seen from the Earth, only some of them are oriented in a way that they are passing in front of their star every orbital period. These periodic transits of the planet in front of its star produce a small dip in its brightness. These micro eclipses allow astronomers to know the diameter of the planet and some details about its atmosphere. The Kepler space mission (NASA) has identified more than 2,000 stars that have a great chance of hosting transiting planets. However, most of them need complementary ground-based observations to establish their nature and to complete their characterization. The team participated in these ground-based observations since 2010, using the SOPHIE instrument, which has already participated in the detection and characterization of more than fifteen Kepler planets, through the radial velocity method. Their observing program is now completed by new observations with the more accurate HARPS-N spectrograph. KOI-200b is slightly bigger than Jupiter and slightly less massive. With a low density, this gaseous planet is orbiting around its star in less than one week. The planet KOI-889b is of the size of Jupiter but is ten times more massive. This very-massive planet is orbiting around its star in slightly less than 9 days. These two planets have eccentric orbits: during their orbit, their distance to their star is varying. This produces large variation in their equilibrium temperature of several hundred of degrees in a few days. KOI-889b, which is among the most massive planets discovered so far, is also among the most eccentric transiting planets. It could have been formed by a different mechanism than less massive planets. Santerne added: “Even if there are just hot and giant planets as we already know hundreds of them, these two planets are orbiting on a highly eccentric orbit, which is relatively rare for such short-period planets. I prefer to see these two new planets as two other bricks in the wall of our knowledge about planetary systems: bigger is the wall, better we understand planetary formation and evolution“.

arXiv: KOI-200b and KOI-889b: two transiting exoplanets detected and characterized with Kepler, SOPHIE and HARPS-N