New method uses molecules to detect exoplanets

Technique can also reveal data about a planet’s temperature.

A new technique developed by an international team of astronomers enables scientists to detect exoplanets by searching for specific molecules in their atmospheres.

Exoplanets can rarely be directly observed due to the overwhelming brightness of the stars they orbit. The SPHERE instrument on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile and a few similar instruments have directly observed a small number of exoplanets in distant orbits around their stars.

Led by Jens Hoeijmakers, a scientist at the University of Geneva (UNIGE), who is also a member of the National Centre of Competence in Research PlanetS (NCCR PlanetS), the research team pioneered a technique that traces molecules present in planets’ atmospheres but not in their parent stars.

“By focusing on molecules present only on the studied exoplanet that are absent from its host star, our technique would effectively ‘erase’ the star, leaving only the exoplanet,” he said.

The instrument used in this technique is sensitive to only a few molecules, such as water, carbon monoxide, methane, and ammonia.

Planets cannot be found with this technique if the same molecules in their atmospheres are also present in their stars.

Hoeijmakers’ team tested the technique on Beta Pictoris, a star known to be orbited by a giant planet, designated Beta Pictoris b. Using images taken by the ESO Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI), they compared the spectra found in individual pixels with those that correspond to specific molecules.

If a specific molecule was found in one of the pixels, that meant it is present in Beta Pictoris b’s atmosphere.

The planet became visible during searches for water and carbon monoxide but could not be seen when the researchers looked for methane and ammonia, meaning the first two are present in the planet’s atmosphere while the latter two are not.

None of these molecules are present in Beta Pictoris itself, as the star is so hot that it destroys them.

Discovering molecules in a planet’s atmosphere also reveals data about the planet’s temperature. Beta Pictoris b is known to have a temperature too high for its atmosphere to have methane and ammonia.

Hoeijmakers described the new technique as “in its infancy” and said he expects it to change the way planets and their atmospheres are characterized.

A paper on their findings has been published in the journal Astronomy and Astrophysics.


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