artist rendering of yellowish planet

Artist’s impression of HR 8799e, the exoplanet observed by the GRAVITY instrument on ESO’s VLT. [Image: ESO/L. Calçada]

Ever since astronomers first found extrasolar planets by indirect methods in the early 1990s, researchers have been striving to “see” them and study their composition, temperature and other properties. Imaging dim bodies orbiting bright stars and recording exoplanetary spectra without contaminating the signal with stellar photons continue to pose significant challenges.

Using the European Southern Observatory’s Very Large Telescope (VLT) array high on a mountain in northern Chile, a multinational research team obtained infrared spectra of a planet orbiting a star some 39.4 parsecs or 129 light-years from Earth. The feat, accomplished with a state-of-the-art, adaptive-optics-assisted interferometer, is the first direct observation of an exoplanet with optical interferometry and yielded new insight into the body’s mass and atmosphere (Astron. & Astrophys., doi: 10.1051/0004-6361/201935253).

Assisted by GRAVITY

One crucial team member was GRAVITY, the optical interferometer designed to work with the four main telescopes, each with a primary mirror 8.2 m in diameter, that form the VLT array. (The VLT has four movable 1.8-m auxiliary telescopes, but they are not always needed.) GRAVITY picks up wavelengths between 2 μm and 2.4 μm, which astronomers call the K-band, from two separate objects in each telescope’s field of view—in this case, the planet and its central star for fringe tracking and phase referencing.

schematic drawing

Schematic layout of the VLT Interferometer. The light from a distant celestial object enters two of the VLT telescopes and is reflected by the various mirrors into the interferometric tunnel. Two delay lines with movable carriages continuously adjust the length of the paths so that the two beams interfere constructively and produce fringes at the interferometric focus in the laboratory. [Image: ESO] [Enlarge image]

The signals from these two objects pass into separate single-mode fibers. Fiber “delay lines” running underground between the telescopes continuously adjust the path lengths for constructive interference, producing fringes at the interferometric focus of the instrument.

To test GRAVITY’s exoplanetary mettle, the astronomers pointed the array at HR 8799, a relatively bright star already known to have four massive planets and a dusty debris disk. The team focused on HR 8799e, the farthest planet from the central star, with an apparent separation between the two (from Earth’s perspective) of roughly 390 milliarcseconds. Measuring the spectra of both the planet and star allowed the astronomers to remove the latter from the former.

Characterizing exoplanets

Astronomers generally infer the properties of large exoplanets by comparing them with the different spectral classes of brown dwarfs—objects larger than Jupiter, but not massive enough to sustain their own nuclear fusion. The data for HR 8799e indicated a temperature of roughly 1150 K, and the derived surface gravity measurement implied a radius slightly larger than Jupiter’s and a mass about 10 times that planet’s. The planet contains far more carbon monoxide than methane—important clues to its evolutionary stage.

The research team suggests that infrared interferometry could characterize most of the known exoplanets that have been imaged already, as long as their separation from their host star is greater than 100 milliarcseconds and their apparent infrared brightness is above a certain threshold. Resolving the surfaces of exoplanets, in the way astronomers take photographs of our solar-system neighbors, would require optical interferometry with baselines not just 100 m like the VLT’s, but on the order of 10 km—perhaps to be achieved later this century.

The GRAVITY interferometric package was built by researchers from the Max Planck Institute for Extraterrestrial Physics, Germany; LESIA of Paris Observatory-PSL, CNRS, Sorbonne Université, Université de Paris Diderot and IPAG of Université Grenoble Alpes, France; the Max Planck Institute for Astronomy and the University of Cologne, Germany; the CENTRA-Centro de Astrofisica e Gravitação, Portugal; and ESO. Authors of the HR 8799e study are from France, Germany, Portugal, Switzerland, the Netherlands, the United States, Ireland, Belgium, Mexico and the United Kingdom.