The movement of a star on its way through the galaxy has led to the discovery of a new extraterrestrial world.
Using data from the Gaia spacecraft, which is mapping the Milky Way, astronomers spotted a kink in the motion of a star called HIP-99770, suggesting the presence of a nearby planet.
Follow-up observations then yielded another treasure: a rare, direct image of the so-called exoplanet, later dubbed HIP-99770b. It’s the first time astrometry – the tracking of a star’s movement – has been combined with direct imaging to detect an orbiting planet.
This effort represents a new and effective way to search for worlds outside the solar system to better understand both Diversity of planets and planetary systems out there, chasing another world where life may have flourished.
“Performing both direct imaging and astrometry allows us, for the first time, to gain a complete understanding of an exoplanet: measuring its atmosphere, weighing it and tracking its orbit all at once,” says astrophysicist Thayne Currie the Subaru Telescope and the University of Texas at San Antonio.
“This new approach to planet-finding shows how we will one day identify and characterize an Earth twin around a nearby star.”
Finding exoplanets is not an easy task. They are very small and very faint compared to the stars that orbit them and very distant. That means we don’t usually see them directly. Whatever light they emit or reflect is drowned out by the brilliance of their star. Instead most more than 5,300 confirmed exoplanets previously identified have only been indirectly proven.
There are two main ways to do this, and both are better at finding exoplanets orbiting close to their stars. The first is to look for transits, the faint and regular dips in starlight that reveal the presence of an orbiting exoplanet passing in front of the star.
The second is to look for changes in the wavelength of a star’s light, known as Doppler shifts, as it moves around the common center of the orbit that it shares with the exoplanet.
This is because the exoplanet’s gravity also affects the star; around this center the two bodies move in a kind of dance, like an Olympic hammer thrower. This movement in the star can also be detected in other ways.
Stars are not stationary in the Milky Way; They orbit the galactic center like the planets of the solar system orbit the sun. So instead of using changes in wavelengths, the astrometric technique looks for deviations from a straight line of motion as the star moves through the galaxy. This method can be used to detect exoplanets that Doppler shifts are not displayedlike exoplanets orbiting their stars in larger orbits.
And exoplanets that are farther from their stars are better candidates for direct imaging because they are far enough away to be able to distinguish them from the stellar glow.
“Indirect planet detection methods are responsible for most exoplanet discoveries to date. Using one of those methods, precision atrometry, told us where to look to try to image planets.” Currie says. “And as we’ve found, we can now see planets much more easily.”
Researchers looked for direct imaging candidates in data collected by the Gaia and Hipparcos spacecraft: European Space Agency projects tasked with mapping the Milky Way, including the positions and movements of the stars. This gave them a 25-year record of star positions.
Several stars showed evidence of a serpentine wobble that could indicate the presence of a giant exoplanet. The Subaru Telescope and the Keck Observatory in Hawaii were used to search directly for these worlds. And they caught a big one on one of the first stars they looked at.
HIP-99770b is an exoplanet 14 to 16 times the mass and 1.05 times the radius of Jupiter, orbits a star twice the mass of the Sun at a distance of 17 astronomical units. That’s more than three times Jupiter’s distance of 5 astronomical units from the Sun and just a little closer than the distance of 19.8 astronomical units from Uranus.
However, it receives about the same amount of radiation that Jupiter receives because the star HIP-99770 is so much brighter than our Sun. Because the team was able to image the exoplanet directly, they were able to take measurements of its properties. It’s warmer and less cloudy than previously directly imaged exoplanets orbiting the star HR 8799and its atmosphere has both water and carbon monoxide.
The direct images also revealed the presence of an icy debris disk orbiting the star at a distance of about 150 astronomical units, similar to the Solar System’s Kuiper Belt. Further research will no doubt continue to probe the exoplanet to see what else we can glean from direct imaging.
In the meantime, the researchers continue to observe and analyze the approximately 50 candidate stars they identified in the Gaia Hipparcos data. HIP-99770b represents a proof of concept for their techniques, but they’re far from finished.
“This is the first of many discoveries from our Keck and Subaru imaging program that uses astrometry to select targets. We already have additional discoveries to be announced later this year and next.” Currie says.
He adds“We are now in a new era for mapping other worlds.”
The research was published in Science.
