Exoplanet observations reveal unexpected atmosphere


Direct imaging of a distant planet refines size, temperature and composition, illustrating exciting new approach. Andrew Masterson reports.


An artist's impression of the gas giant exoplanet HR 8799e.

ESO/Luis Calçada

A new approach to imaging an exoplanet has delivered results of unprecedented clarity, including at least one major surprise.

Using a method called optical interferometry – courtesy of the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile – researchers have been able to directly observe a young gas giant dubbed HR 8799e, one of four planets orbiting a star, HR 8799, some 130 light-years from Earth.

Optical interferometry is an important advance in exoplanet study because it allows astronomers to screen out the bright light and loud noise of a host star and, for the first time, directly measure the spectra of a target planet.

Researchers led by Silvia Scheithauer from the Max Planck Institute of Astronomy in Germany directed their attention to HR 8799e, a planet with a mass between five and 10 times that of Jupiter, orbiting close to its star.

They were able to determine the planet’s distance from its host 10 times more accurately than previous estimates. They also confirmed that its orbit, which takes about 40 Earth-years to complete, is slightly out of alignment with those of the other three planets in the system.

The planet has a temperature, Scheithauer and colleagues estimated, of about 880 degrees Celsius, very much in line with expected measurements for a gas giant thought to be only about 30 million years old.

Looking more closely at their readings, however, the scientists came across a very unexpected finding.

“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,” says Scheithauer. “But surprisingly, the atmosphere of HR 8799e hardly contains any methane it all. Instead, we found major amounts of carbon monoxide!”

In a paper published in the journal Astronomy & Astrophysics, the researchers say the result illustrates starkly how much still needs to be learned about factors governing planet formation.

Continued observations using optical interferometry, they state, will allow them to accurately calculate not just the gravitational interaction between HR 8799e and its host, but also between it and the other three planets.

Applied to other exoplanets, they suggest, the approach could be used to detect gas signatures that could indicate the presence of life.

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  1. https://arxiv.org/pdf/astro-ph/0307036.pdf
  2. https://dx.doi.org/10.1051/0004-6361/201935253
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