Exoplanet under the looking glass

New method allows precise determination of exoplanet's spectra and position

For the first time, astronomers have succeeded in investigating an exoplanet using optical interferometry. The new method allowed astronomers to measure the position of the exoplanet HR 8799e with unprecedented accuracy. Also, the planet's spectrum was recorded as precisely as never before, paving the way for future searches for life on other planets. The measurements, which were obtained with the participation of astronomers from the Max Planck Institutes for Astronomy and for Extraterrestrial Physics, were performed with the GRAVITY instrument at ESO's Paranal Observatory.

Investigating exoplanets in detail and without confounding noise is difficult. In general, with increasing distance, it becomes more and more difficult to image fine details of an astronomical object. Furthermore, exoplanets are typically buried in the glare of their much brighter host stars. Now, a group of researchers led by Sylvestre Lacour of the Observatoire de Paris and the Max Planck Institute for Extraterrestrial Physics, also including MPIA researchers, has been able to demonstrate a new method of investigation that mitigates these problems and thereby provides a new perspective on exoplanets.

Key to the new technique is the GRAVITY instrument, which has been in operation at the European Southern Observatory's Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile since 2016. Using a technique known as interferometry, which exploits the wave nature of light, GRAVITY is able to combine the light of several telescopes to form a common image. Combined, the four 8-metre-telescopes of the Very Large Telescope (VLT) can make images so detailed that a single telescope would need to have a mirror diameter of approximately 100 meters to provide the same level of detail.

The study of the exoplanet HR 8799e that has now been published is the first to demonstrate the potential of interferometric observations for the investigation of exoplanets in practice. The planet is one of only a few (about 120 out of 4000) for which direct images exist; so far, most exoplanets have only been detected indirectly. HR 8977e is part of a young five-body-system, a mere 130 light-years away from us, which consists of the star HR 8799 and four planet (as far as we know, at least). All of the planets are gas giants with between 5 and 10 times the mass of Jupiter.

Among the four, HR 8799e is the one closest to the host star. That is why it is particularly difficult to clearly distinguish light from the star and light from the planet in observations. The star's radiation is about 20,000 times greater than that of the exoplanet ­– under normal circumstances, the star drowns out the planet’s light. Since HR 8799e is so close to its host star, the effect is particularly large.

GRAVITY was able to deliver much more detailed images of the exoplanet than its predecessor instruments. With the help of these high-resolution images, the astronomers were able to calculate the distance between the star and the planet ten times more accurately than before. This allows a more precise determination of the planet's orbit, which, according to the new measurements, appears to be slightly inclined relative to the orbital plane of the other planets of the HR 8799 system.

Interferometry is a particularly powerful way to distinguish between the light of the planet and the light of the star – the separation of the two is much cleaner than with conventional method of blocking out the star’s light using a mask ("coronagraphy"). With this clean separation, the astronomers were able to measure the spectrum of HR 8799e much more accurately than before. The spectrum shows that the atmosphere of the relatively young gas planet, which is 30 million years old, has a temperature of respectable 880 degree Ceslius. But the spectrum also had a surprise in store. Silvia Scheithauer from the MPIA, who contributed to the GRAVITY project, says: “Going by the planets of our own solar system, we would expect large amounts of methane in the atmosphere of a gas planet this hot. But surprisingly, the atmosphere of HR 8799e hardly contains any methane it all. Instead, we found major amounts of carbon monoxide!“ This shows how much astronomers still have to learn about planet formation ­– but it also underlines the key role of atmospheric spectroscopy for learning about exoplanets.

Currently, the astronomers are planning long-term follow-up observations with GRAVITY. With this additional data, the astronomer should be able to reconstruct the orbit of HR 8799e with such great accuracy that there would be another first: the first time where the motion within a spatially resolved exoplanet system would reveal not only the gravitational influence of the central star, but also the mutual attraction of the gas planets. Such observations should allow for an accurate estimate of the masses of the four gas planets. To the best of our current knowledge, HR 8799e needs between 40 and 50 years for one complete orbit.

The new observations are also of interest for future searches for traces of life in the universe. The main current search strategy aims to detecting tell-tale signs of life in the spectrum of an exoplanet's atmosphere. The successful GRAVITY observation opens up a way for taking spectra of this kind with greater accuracy.

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