The ESA’s Gaia mission is our most accurate star-measuring spacecraft. It’s busy mapping the positions and radial velocities of one billion stars in the Milky Way. The mission’s goal is to create a representative map of the galaxy’s stellar population with unprecedented accuracy. The mission has released 3 sets of data since its inception, leading to many discoveries.
Now a team of astronomers has found an exoplanet with help from Gaia, an unintended result of the ambitious mission.
Most exoplanets are found using the transit method, where an exoplanet passes in front of its star and causes a dip in the light. But that method has its limitations, as every method does. The transit method is an indirect observation of an exoplanet. All observers see is the dip in starlight, not the planet itself, and while the dip provides important information, that information is limited.
Direct observations provide more information but are much more difficult. We’re only now getting telescopes powerful enough to observe exoplanets directly. The powerful James Webb Space Telescope directly imaged the exoplanet HIP 65426 b in 2022. Thanks to the JWST and the powerful ground-based telescopes that are nearing completion, astronomers are getting to a point where they can use both direct and indirect observations of exoplanets to learn more about them, at least in some instances.
This image shows the exoplanet HIP 65426 b in different bands of infrared light, as seen from the James Webb Space Telescope. This was the first exoplanet imaged by JWST. Credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI).
In this new research, Gaia data played a central role, helped by data from the ESA’s now-defunct Hipparcos mission, Gaia’s predecessor. That data told astronomers where to point the Subaru Telescope on Mauna Kea, which provided direct observations and confirmation of the distant exoplanet.
Illustrations of the Gaia spacecraft (l), the Hipparcos spacecraft (m), and a photo of the Subaru Telescope (r). All three facilities contributed to the exoplanet discovery. Image Credits: ESA, ESA, NAOJ.
The team of astronomers that found the planet presented their results in a research article in the journal Science. The article is “Direct imaging and astrometric detection of a gas giant planet orbiting an accelerating star.” The lead author is Thayne Currie from the National Astronomical Observatory of Japan and the NASA Ames Research Center.
“This is sort of a test run for the kind of strategy we need to be able to image an Earth.”
Thayne Currie, NASA, NAOJ
Astronomers have only been able to observe about 20 exoplanets directly, and that’s out of over 5000 confirmed exoplanets. And the 20 all have two things in common: they orbit at a great distance from their stars, and they’re much more massive than Jupiter. With our current level of technology, those are the only exoplanets we can really see directly.
Scientists would like to find and study more of these planets because they’re rare. There’s nothing like them in our Solar System. But they need to know where to look, and that’s where this new method comes in. The Gaia and Hipparcos data revealed the tell-tale wobble of a star as a massive planet tugged on it gravitationally. This brings us to an important distinction between indirect and direct observations of exoplanets.
Indirect observations like the well-known transit method cast a wide net. They survey a large number of stars in one segment of the sky simultaneously, looking for repeated dips in light from stars as planets transit in front of them. This only works when our viewpoint is right. We have to be looking at the system through the orbital plane of the planets; otherwise, the planet doesn’t pass in front of its star from our viewpoint, and there’s no detectable dip in starlight. Other indirect methods target a specific type of star, low-mass red dwarfs, for instance, and hope to detect some transits.
This image shows how NASA’s Transiting Exoplanet Survey Satellite (TESS) surveys large swathes of the sky at once, hoping to detect transits. It’s effective at finding exoplanets, but not the specific types the researchers are interested in. Image Credit: NASA
But to find the rarer planets that are more massive than Jupiter and orbit their stars at a great distance—so great that it could take hundreds of years for a transit to occur—astronomers need a more targeted way to find them. Casting a wide net isn’t effective, and that’s where Currie and his co-authors sought a different solution.
“We wanted a different strategy,” said lead author Thayne Currie.
Their effort to develop a different strategy led them to the Hipparcos-Gaia Catalogue of Accelerations (HGCA). The HGCA is a cross-collaboration of Gaia and Hipparcos data that highlights astrometrically accelerating stars. The catalogue’s measure of proper motions “… provide a powerful tool to measure the masses and orbits of faint, massive companions to nearby stars,” according to the catalogue’s introduction.
Even though the planet type in question orbits its star at a great distance, its mass is large enough to tug on the star, creating a wobble. Measuring the wobble a planet induces in its star is part of astrometry.
Astrometry detects a star’s motion through the sky by taking precise measurements of its position over time. It can also measure the tiny, almost imperceptible wobble caused by orbiting exoplanets, even when we can’t directly detect the planet. As exemplified by this new research, Gaia is creating a massive astrometric catalogue of stars, including their exoplanet-induced wobbles. Image Credit: ESA, CC BY-SA 3.0 IGO
Of course, those planets weren’t sitting there in the data plain for anyone to see. Currie and his colleagues still had to find them. After working with the HGCA, they found what they were looking for. The team identified a number of candidates that could be massive planets tugging on their stars from a distance.
Next, they turned to the NAOJ’s Subaru Telescope. The telescope has a large, 8-meter mirror. But perhaps more importantly, it has two powerful instruments: the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument and the Coronagraphic High-Resolution Imager and Spectrograph (CHARIS) instrument. The team used the telescope and instruments in July and September 2020 and May and October 2021. With those observations, the team found what they were looking for; a giant exoplanet on a wide orbit.
These images from the research letter are the two best direct images of HIP 99770 b. HIP 99770 b is shown by the white circle, and the white arrow in the right-hand image shows the direction of the planet’s orbit. Image Credit: Currie et al. 2023.
The new planet is called HIP 99770 b, with HIP referring to the catalogue of data from Hipparchos. HIP 99770 b is about 14 – 16 times more massive than Jupiter and orbits a star about twice as massive as the Sun. Its orbit is three times larger than Jupiter’s orbit around the Sun.
This graphic from the ESA helps explain the new research. Not only did the researchers find the giant exoplanet, but they also identified a dusty disk in the distant solar system. Image Credit: ESA.
The researchers are excited about finding the first exoplanet with their method and hopeful that this is just the beginning.
“It provides a new path forward to discovering more exoplanets and characterizing them in a far more holistic way than we could do before,” says Currie.
Combining direct measurements and indirect measurements is a holistic approach to exoplanet science that will only grow in the future. The combination is effective and makes sense. Each type of measurement contributes something different to our understanding of an exoplanet.
Direct measurements are effective at constraining a planet’s temperature and composition, and indirect measurements are effective at measuring a planet’s mass and orbit. When astronomers combine direct measurements of a planet’s position with indirect measurements of its mass and orbit, a more complete picture of the planet emerges.
For the exoplanet HIP 99770 b, this is just the beginning. Now that astronomers know it’s there, there will be follow-up observations to deepen our understanding of it. “The discovery of this planet will spawn dozens of follow-on studies,” says Currie.
This method has proven successful once, and it’ll no doubt be refined and employed to find other giant planets. The team identified a number of candidate stars in the Hipparchos-Gaia catalogue that could host giant planets on wide orbits, and the star HIP 99770 was one of the first ones they looked at. That bodes well for the rest of the candidates they extracted from the catalogue.
“HIP 99770 b is a proof of concept of this new strategy for finding imageable planets that will get far better in the next five years,” Currie says.
Gaia’s next data release will be its fourth. It’ll be its most complete data set because it has a longer baseline, nearly 5.5 years. All that data will make it even easier to spot other giant planets on wide orbits.
Giant planets on wide orbits are interesting anomalies that can eventually tell astronomers a lot about solar system evolution and architecture. But the ultimate goal behind exoplanet research is to find another planet similar to ours. The holistic approach the team developed can eventually be used in the search for a so-called Earth 2.0. An Earth-like planet will be much closer to its star, which means it’ll spend a lot of time behind or in front of the star. That makes it very difficult to image directly, though transits could be detected if the planet orbits on the right plane from our vantage point. But this combined approach holds a lot of promise.
“This is sort of a test run for the kind of strategy we need to be able to image an Earth. It demonstrates that an indirect method sensitive to a planet’s gravitational pull can tell you where to look and exactly when to look for direct imaging. So I think that’s really exciting,” says Thayne.