Astronomers Test an Exoplanet Instrument on Jupiter

The European Southern Observatory’s Very Large Telescope (VLT) has a high-resolution spectrograph called  ESPRESSO, designed specifically to detecting and characterize exoplanets. Astronomers recently ran a test with the instrument, studying the atmosphere and winds of Jupiter. They used a technique called Doppler velocimetry to measure the reflection of light from the Sun in the planet’s clouds, allowing for instantaneous measurement of the clouds’ wind speeds. The technique has also been used on Venus and will guide the future study of exoplanets.

ESPRESSO is the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations.  The instrument combines the light from all four of the VLT telescopes, capturing the light from each of the 8.2 meter mirrors in the four Unit Telescopes of the VLT. That combination makes ESPRESSO, in effect, the largest optical telescope in the world.

This type of observing power is being used to measure the variation of the fundamental physical constants in the Universe and to analyze the chemical composition of stars in nearby galaxies. And as a planet hunter, it can search for Earth twins in the habitable zone of solar-like stars.

Researchers from the Institute of Astrophysics and Space Sciences (IA) at the Faculty of Sciences of the University of Lisbon (Portugal) (Ciências ULisboa) developed the Doppler velocimetry method originally to study Venus, allowing them to model its atmosphere for several years.

They wanted to test out a new method which might allow them to study and monitor the atmospheres on distant gaseous exoplanets.

The test was conducted in July 2019, and for five hours, the team pointed the VLT at the equatorial zone of Jupiter, where light clouds are located at a higher altitude, and at the planet’s north and south equatorial, which correspond to descending air and which it forms bands of dark, warmer clouds in a deeper layer of the atmosphere.

“One of the difficulties centered on ‘navigation’ over Jupiter’s disk, that is, knowing exactly which point on the planet’s disk we were pointing to, due to the enormous resolution of the VLT telescope,” said researcher Pedro Machado, in a University of Lisbon press release. “The difficulty was related to the fact that we were determining winds with an accuracy of a few meters per second when Jupiter’s rotation is on the order of ten kilometers per second at the equator and, to complicate matters because it is a gaseous planet, and not a rigid body, it rotates at different speeds depending on the latitude of the point we observe.”

With ESPRESSO, the team was able to measure winds on Jupiter from 60 to 428 km/h with an uncertainty of less than 36 km/h.

“Comparing the results between cloud-tracking methods, based on previous reference observations, and our new Doppler velocimetry approach,” the team wrote in their paper, “we found a good agreement between them, demonstrating the effectiveness of this technique.”

“Jupiter’s atmosphere, at the level of the clouds visible from Earth, contains ammonia, ammonium hydrosulfide and water, which form the distinct red and white bands,” Machado said. “The upper clouds, located in the pressure zone of 0.6 to 0.9 bars, are made of ammonia ice. Water clouds form the densest, lowest layer, and have the strongest influence on the dynamics of the atmosphere.”

Their work not only contributed to the temporal and spatial variability study of Jupiter’s troposphere, but it also validated the use of their Doppler technique to study the dynamics of planetary atmospheres. This paves the way for exploration the atmospheres of distant exoplanets. While Jupiter is “only” 43 light minutes away from Earth, the same technique should be feasible for exoplanets that are hundreds to thousands of light years away.