Astronomers using Maunakea telescopes have developed a technique that they say could revolutionize how to capture images of distant planets.
The study of exoplanets — or planets outside of our solar system — is a critical way for astronomers to understand more about how the celestial bodies were formed.
But because of the vast distances between Earth and other worlds, and because planets don’t radiate light and heat the way stars do, detecting exoplanets is difficult, and directly imaging them is even harder.
“We’ve found something like 5,000 exoplanets,” said National Astronomical Observatory of Japan astrophysicist Thayne Currie. “But for most of those, we only have indirect observations. We have maybe only 20 directly imaged planets.”
Direct imaging of exoplanets has historically been a matter of trial and error. NAOJ senior scientist Julien Lozi explained that previous methods of viewing exoplanets drew from surveys of potential candidate stars: ground-based telescopes would train their sights on distant stars in the hopes of imaging potential planets orbiting them.
“It’s hard to to figure out which stars have planets around them,” Lozi said. “Astronomers would just do blind surveys and hope they would catch a planet. We’d pick younger stars, because newer planets would give off more heat.”
Lozi said astronomers grew to rely on more data points to narrow down their blind surveys. Most significantly, irregularities in a star’s movements could indicate the presence of an orbiting planet exerting its own gravitational pull on its parent star — although Lozi added that they also could indicate a binary star system, or a star orbited by a red or brown dwarf star, which he said was “not as interesting.”
But through the precision astrometry — the science of measuring the positions and movements of stars — made possible by space telescopes, Lozi said astronomers now can quickly identify likely candidates for planet-hosting stars and use ground-based facilities to confirm.
As proof of this point, Currie, an astrophysicist for the National Astronomical Observatory of Japan, led a team to successfully image a distant exoplanet orbiting the star HIP 99770 — located in the constellation Cygnus — using astrometric data from the European Space Agency’s Gaia and Hipparcos satellites and powerful adaptive optics and spectrographic instruments at the Subaru Telescope on Maunakea, and further confirmed using Keck Observatory.
Currie said the planet, classified as HIP 99770 b, is the first exoplanet to be successfully confirmed through joint direct imaging and astrometry.
“From direct imaging, we can get information about the planet’s atmosphere, its composition, things like that,” Currie said. “If we combine that with astrometry, we can determine its orbit and mass.”
Based on their data, Currie’s team determined that the planet is about 14 to 16 times the mass of Jupiter — which itself is roughly 318 times the mass of Earth — with an orbit three times larger than Jupiter’s around a star nearly twice as massive as our sun. The planet also is about 10 times hotter than Jupiter, with an atmosphere containing water and carbon monoxide.
Currie said future use of the technique will be invaluable to discovering exoplanets that are more Earth-like. Because a potentially habitable rocky planet will have to be much closer to its star than any exoplanet imaged to date, it will spend much of its time being obscured by that star, but by analyzing astrometric data, astronomers can predict when it will be visible.
Lozi and Currie said the discovery was only possible through the powerful instruments at Subaru and Keck.
“It’s an example of the kind of science we can only do from Maunakea,” Currie said.
Lozi said the ability of Maunakea telescopes to discover new planets is an opportunity to give more celestial objects Hawaiian names, such as the black hole Powehi and the interstellar object ‘Oumuamua.
“It gives us a way to connect our science with the culture and community here in Hawaii,” Lozi said.
Email Michael Brestovansky at mbrestovansky@hawaiitribune-herald.com.