Astronomers Use a Novel Technique to Spot a Huge Exoplanet
By teaming direct imaging with an indirect technique, astronomers struck gold.
University of Texas at San Antonio Professor of Physics and Astronomy and exoplanet hunter Thayne Currie isn’t shy about his ultimate scientific goal.
“I want to find the next Earth,” he tells Inverse, “so I am interested in trying to study planets around other stars as a step to be able to do that.”
Specifically, Currie studies exoplanets by directly imaging them, which is no easy feat: The small size of distant planets, the intense glare of their star, and the problem of even knowing when and where to look make direct images of planets outside our Solar System difficult and relatively rare. Most exoplanets are discovered indirectly, by watching the star for signs a planet’s gravity is tugging on it, or a dimming in the star’s light indicating a planet is passing in front of it.
“We've discovered maybe 20, maybe a few more planets by imaging,” Currie says. Currie adds that about 5300 planets have been detected by the other indirect means.
That imbalance in exoplanet discoveries may soon change. Currie is the lead author of a new study published Wednesday in the journal Science that not only describes the discovery and direct imaging of a new exoplanet, but showcases the first time researchers have discovered an exoplanet by combining the powers of imaging and indirect detection methods.
“I think this opens up a new era in trying to study planets by imaging,” Currie says.
Another new world
The newly discovered exoplanet — HIP 99770 b — orbits a massive star about 13 times as bright as our Sun, located about 125 light years from Earth.
The planet itself is about 14 to 16 times as massive as Jupiter and may contain water and carbon monoxide in its atmosphere, according to Currie. (That could place it not as a planet, but as an object intermediate in mass between a planet and a star called a brown dwarf.)
Interestingly, it gets about the same amount of light from its star as Jupiter does from our Sun. In fact, he says, if you scale up our Solar System to account for the sun becoming as large as HIP 99770 b’s star, the two systems look remarkably similar, making it a planetary system worth studying as a contrast to better understand our own.
As scientifically interesting as HIP 99770 b is in its own right, it’s how it was discovered that might be the most revolutionary.
At the core of the imaging portion of the study is the Subaru Coronagraphic Extreme Adaptive Optics instrument on the Subaru telescope on Mauna Kea, Hawaii. Operated by the National Astronomical Observatory of Japan, the Subaru telescope sports a nearly 30-foot-wide mirror, as well as powerful adaptive optics that help sharpen images of space taken through the distorting shroud of Earth’s atmosphere.
“It uses a sensor to be able to figure out how starlight is being blurred by the atmosphere,” Currie says,” and then it uses a mirror that changes shape 2,000 times a second to be able to correct for that atmosphere.”
The telescope also uses a coronagraph, which blocks out the light of the distant star to allow the light from the nearby planet to shine through.
Overall, Currie says the Subaru telescope can better distinguish an exoplanet around a distant star than even the James Webb Space Telescope.
But just having a telescope capable of imaging an exoplanet doesn’t make it easy to find one in the first place. Planets could be orbiting at all kinds of distances from their star or could be swinging behind their star at the time you choose to look.
“Doing this is very hard, and in many large programs to image planets have come up empty, or they have a few discoveries out of hundreds of stars observed,” Currie says. “This is very inefficient.”
So what Currie and his international team of research colleagues did was first use indirect methods of exoplanet detection — known as precision astrometry — to locate stars that could potentially host exoplanets. Only after first detecting the possibility of an exoplanet around HIP 99770 b’s host star using the European Space Agency’s Gaia and Hipparcos spacecraft did they use the Subaru telescope to confirm the discovery, and take the direct image of the exoplanet. That’s a powerful new technique for exoplanet hunters like Currie.
“If we just look at the stars blindly, just pick stars at random nearby, the chances that we're going to find an imageable Earth-like planet is negligibly small,” he says. “But if we know exactly where to look, and get some information about when to look, now we have a chance of succeeding.”
But the approach is not just more efficient in terms of searching for exoplanets, Currie says, but it provides more information about the planet once it's discovered.
“We’re able to simultaneously get information about its atmosphere from direct imaging,” he says, and then through astrometry measure its orbit and directly measure its mass, which is not something you can do with direct imaging on its own.”
Going forward, the discovery of HIP 99770 b could mark an inflection point where the detection of exoplanets and their direct imaging begins to accelerate. Currie’s team has already imaged a second exoplanet that will be described in a future publication, he says.
And the types of exoplanets the technique can discover will help other missions discover even more exoplanets. NASA’s forthcoming Nancy Grace Roman Space Telescope will be able to use exoplanets like HIP 99770 b to test its capabilities once launched in 2027, Curries says.
Meanwhile, every exoplanet discovered and directly imaged helps scientists build a catalog of planets outside our solar system with different masses and ages, “so we can actually understand how planets evolve, how their atmospheres evolve.”
But as for finding and imaging “the next Earth,” that’s something even the Subaru telescope cannot resolve.
“That's gonna have to wait until the next generation,” Currie says, a task for larger telescopes like the Extremely Large Telescope, which, if completed in the deserts of Chile, will sport a primary mirror some 130 feet in diameter.