When astronomers discovered the first worlds orbiting other stars thirty years ago, they also began doing what might be called the galactic planetary census, counting the number and types of exoplanets in the Way milky. Although it is impossible to study in depth all the hundreds of billions of stars in our galaxy, a representative sample of them could provide important information. By studying the planetary populations of such a sample, researchers hope to discover which types of worlds are the most common or the rarest, and how our Earth and our solar system compare to them.
But there are several different ways to find planets, and each tends to work better for different types of worlds, leading to potentially skewed results. The dominant techniques to date infer the presence of a planet by looking for its subtle influence on its star, and they are most sensitive to giant planets very close to their stars. Such worlds have orbital “years” as small as days or weeks – and there are none in the solar system. By contrast, viewing the planets directly—called direct imaging—requires distinguishing them from the overwhelming glare of a star, which is easier to do for giant planets on the outskirts of a system. If such orbits were around our own sun, they would place most of these planets far beyond Pluto.
Fortunately, new methods and larger datasets are now allowing scientists to bridge the gap between these extremes, combining the results of several planet-hunting techniques to get better and clearer views of the planet’s true planetary population. Milky Way. A new study published in Science is one of the first successes of this synergistic approach, creating not only a new “middle of the road” planet, but also a broader strategy to find and study many more. The largest and brightest of these yet-to-be-discovered planets could also be good candidates for future direct imaging efforts, potentially allowing astronomers to discern their atmospheres and climates.
“When we combine [motion and imagery] together we get the three key properties of the planet – its orbit, mass and atmosphere – so we learn a lot more about it,” says Thayne Currie, planet hunter at NASA Ames Research Center and lead author of the study.
catch a star
Currie and his colleagues found their new planet, a giant world called HIP 99770 b, by comparing data on its star’s movements collected in 2021 by the European Space Agency’s Gaia spacecraft with similar but less precise measurements taken. in the early 1990s by Gaia’s predecessor. , ESA’s Hipparcos satellite. Both Gaia and Hipparcos were intended to map the stars of the Milky Way (rather than its planets) using a technique called astrometry to precisely track stellar positions, distances, and motions. But astrometry can also reveal planets: a planet orbiting a star can cause a very slight cyclic shift in the star’s position, oscillating back and forth in the plane of the sky. By determining the size and recurrence of this change, astronomers can determine the mass and orbit of an unseen planet.
The initial discovery of the planet and its photographic tracking were only possible thanks to Gaia-Hipparcos data spanning decades, which allowed the detection of the long orbit of HIP 99770 b. This combined catalog itself had taken years to prepare. After the first release of Gaia data in 2016, Timothy Brandt, an astronomer at the University of California, Santa Barbara, and co-author of the new study, published a list of tens of thousands of stars intersected and augmented by the previous Hipparcos. observations, updating them again in 2021 after Gaia’s last data release. The result was a roughly 25-year window into how these stars moved across the sky.
Several teams have begun dredging the new database for stellar companions, “each with their own spin on the exact information to take in to choose the target,” says Caroline Morley, a researcher who studies exoplanet atmospheres at the University of Texas at Austin and was not part of the new study.
In the case of HIP 99770 b, Gaia-Hipparcos data showed it to be a gas giant world orbiting its star at a distance somewhat further than Uranus from the sun – large enough, bright and moved away from its stellar host to be within direct imaging range. Follow-up observations made with the SCExAO direct imaging instrument at the Subaru Telescope on Mauna Kea in Hawaii confirmed these suspicions, revealing the planet as a speck clouded by molecules of water vapor and carbon monoxide. Climate models suggest that the planet has a temperature between 1,300 and 1,400 kelvins (between 1,880 and 2,060 Fahrenheit). Although distinctly supernatural, the properties of HIP 99770 b make it a relatively close cousin to Earth.
” It’s the first [finding from this database] who can really argue, “It’s probably planetary mass,” says Beth Biller, who wasn’t part of the research team. Biller, an astronomer at the University of Edinburgh in Scotland, went on to note that the heavy world is in the gray area between the planet and the brown dwarf and some might object to classifying it as a planet. Either way, “it’s definitely the lowest-mass object that’s been detected by this method,” she says.
Worth a thousand words
Findings like this can help fill in lingering gaps in the galactic planetary census. In addition to being limited to very large planets in very wide orbits, current direct imaging efforts work best for very young worlds – between 10 and 100 million years old – and still illuminated by the heat left behind by their training. The cumulative result of all these previous surveys, Biller says, was significant but still disappointing. “What we found is that [hot, young, wide-orbiting] giant planets are quite rare,” she says.
While many stars should have some sort of orbiting planet, direct imaging surveys have revealed that far fewer have a giant planet at their edges. Infrared images reveal information about the atmospheres of these worlds, and models provide an estimate of their mass. Of the dozens of exoplanets captured by direct imaging, astronomers were only able to more accurately reduce the masses by two, using tracking measurements with indirect planet-detection techniques. Part of the problem is the pre-existing observing preference for young planets, which have young host stars that are much more active than more mature stars, and therefore more disruptive to star-based measurements of the mass of a fellow.
“Once you have a directly imaged planet, there is a degree of guesswork in saving its physical properties,” says Brandt. The fusion of astrometry and direct imaging not only opens the door to finding more targets; it also eliminates some of that guesswork by revealing each new planet’s orbit and mass, as well as its atmosphere.
Although Gaia targets two billion stars, Hipparcos has only studied 100,000, all relatively bright and near-Earth. Currie estimates that about a third of the stars studied in the combined catalog have companions, most of them low-mass stars. If only one in 100 cataloged stars with companions has a photographable planet, the new fusion of planet detection methods should dramatically increase the total number of worlds astronomers will soon be able to see directly. At the end of its decade-long survey, the researchers say, Gaia could identify up to 100 additional planets as candidates for direct imaging with current instruments, more than four times as many directly imaged worlds as identified at this day. And it will expand our knowledge of planetary systems beyond the youngest and brightest, perhaps showing more worlds like ours.
“The yield of new discoveries is higher than what we would get if we just did a blind search,” says Currie, “and the information we get is much richer than what we would get if we just direct imagery.
