When it comes to detecting planets around other stars, called exoplanets, astronomers have a variety of methods at their disposal. These techniques fall into two broad categories: direct and indirect. Both have advantages and disadvantages.

Historically, most exoplanets were discovered by indirect methods. This means that planets are assumed to exist because of the effect they have on their parent star. Whereas in direct imaging, a telescope actually sees the planet.

While astronomers have detected more than 5,000 exoplanets using indirect means, only about 20 have been imaged directly. Indeed, for planets to be visible with our current level of technology, they must be widely separated from their parent star and be much more massive than Jupiter, the largest planet in our solar system.

Because nature doesn’t make many of these types of planets, astronomers would like to know exactly where to look. Most direct imaging searches are “blind”, meaning they simply target stars based on their age and distance and hope that a planet will be seen. Of hundreds of stars studied in this way, only a handful have produced planets.

“We wanted a different strategy,” says Thayne Currie, of the National Astronomical Observatory of Japan (NAOJ), Hilo, Hawaii and the University of Texas-San Antonio. In his attempt to draw the dice in favor of success, he and his colleagues turned to the Gaia mission to search for stars that were literally flickering in the sky.

In particular, they used the Hipparcos-Gaia catalog of accelerations. This catalog combines data from Gaia with data from ESA’s previous star-mapping mission, Hipparcos, to provide a 25-year baseline for comparing precise star positions. Measuring the position of a star in the sky is called astrometry. From this database, the team identified a number of stars that appeared to change position in the night sky in a way that suggested they were each orbiting a giant planet.

Then they turned to the Subaru Telescope at NAOJ on Mauna Kea, Hawaii, and took observations in July and September 2020, as well as May and October 2021. They used the Subaru Coronagraphic Extreme Adaptive Instrument Optics (SCExAO) of the telescope coupled with the High Resolution Coronagraphic Imager and Spectrograph (CHARIS) – and they quickly captured an exoplanet.

The newly discovered planet is called HIP 99770 b. It is about 16 times the mass of Jupiter in our own solar system and orbits a star nearly twice as massive as the Sun. Even though the planet’s orbit is more than three times larger than Jupiter’s orbit around the Sun, it receives almost the same amount of light as Jupiter because its host star is much brighter than the Sun.

The success of the discovery of this planet also has wider implications.

“This opens up a new avenue for discovering more exoplanets and characterizing them in a much more holistic way than we were able to do before,” Thayne says.

Indeed, direct and indirect detection methods provide different information about a planet. Direct imaging can provide excellent constraints on a planet’s temperature and composition. Meanwhile, indirect methods provide excellent measurements of a planet’s mass and orbital characteristics, especially when combined with measurements of the planet’s position by direct imaging.

Combining data from Gaia with images from Subaru gives astronomers the best of both worlds. And that’s just the beginning.

Now that astronomers know the planet is there and visible, other telescopes can take on the task of further analyzing its light. “The discovery of this planet will spawn dozens of follow-up studies,” says Thayne.

And there will be more discoveries to come from this method. HIP 99770 was one of the first stars observed among Gaia candidates. Currently, Thayne and his colleagues are analyzing data from about 50 other stars and what they have seen promises that more discoveries are on the way.

“[HIP 99770 b] is a proof of concept for this new strategy to find imageable planets that will improve a lot over the next five years,” he says.

This method of targeting stars for the discovery of exoplanets will accelerate. Indeed, Gaia’s fourth data release (DR4), which will be based on 5.5 years of data (nearly double the baseline for DR3) will make it much easier to spot flickering stars.

Eventually, this combined approach will allow us to target other Earths. Finding a planet like ours remains an ultimate goal for astronomers. Such a planet will be much closer to its star and will therefore spend a lot of time in front of or behind that star, making it impossible to see.

“It’s kind of a test 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 a direct imagery. So I think that’s really exciting,” says Thayne.

Direct imaging and astrometric detection of a gas giant planet orbiting an accelerating star by Thayne Currie is published online April 13, 2023 and in the print issue of Science April 14, 2023.