Scientists spot hidden companions of bright stars

Photographing faint objects close to bright stars is incredibly difficult. Yet, by combining data from ESA's Gaia space telescope with ESO's GRAVITY instrument on the ground, scientists managed just that. They took the first pictures of so far unseen dim companions of eight luminous stars. The technique unlocks the tantalizing possibility to capture images of planets orbiting close to their host stars.

Have you ever tried to take a picture of a firefly next to a bright streetlight? Chances are that all you will see in your snapshot is the glare from the lamp post. This is the same problem faced by astronomers chasing faint, small, stars or planets next to a bright star.

To tackle the problem, an international team of astronomers led by Thomas Winterhalder, scientist at the European Southern Observatory (ESO), started by searching through the catalog produced by Gaia, which lists hundreds of thousands of stars that are suspected to have a companion.

Although the companion objects are not bright enough to be seen by Gaia directly, their presence leads to tiny wobbles in the paths of the more luminous host stars, which only Gaia can measure.

In Gaia's catalog of stellar orbits, the team identified eight stars to be targeted by GRAVITY, the advanced near-infrared interferometer at the Very Large Telescope of ESO, at Cerro Paranal in Chile. GRAVITY combines infrared light from different telescopes to pick up tiny details in faint objects, in a technique called interferometry. The research is published in the journal Astronomy & Astrophysics.

Jackpot

Thanks to GRAVITY's uniquely sharp and sensitive eye, the team caught the light signal of all eight predicted companions, seven of which were previously unknown. Three of the companions are very small and faint stars, while the other five are brown dwarfs. These are celestial objects in between planets and stars: more massive than the heaviest of planets but lighter and fainter than the lightest of stars.

One of the brown dwarfs spotted in this study orbits its host star at the same distance as Earth from the sun. This is the first time a brown dwarf so close to its host star could be directly captured¬.

"We have demonstrated that it is possible to capture an image of a faint companion, even when it orbits very close to its bright host," explains Thomas. "This achievement highlights the remarkable synergy between Gaia and GRAVITY. Only Gaia can identify such tight systems hosting a star and a 'hidden' companion, and then GRAVITY can take over to image the smaller and fainter object with unprecedented accuracy."

In an earlier study, astronomers used Gaia data and a different ground-based observatory to capture the image of a giant gas exoplanet. This planet orbits its host star at about 17 times the distance of Earth from the sun, tracing an angle in the sky considerably wider than the typical separation of the companions imaged by GRAVITY in this new result.