When astronomers discovered the first worlds orbiting other stars thirty years ago, they also began to accept what might be called galactic planetary census, counting the number and types of exoplanets in the Milky Way. While it is impossible to carefully study all the hundreds of billions of stars in our galaxy, a representative sample of them can provide important information. By studying the planetary population of such a sample, researchers hope to learn which worlds are most common and which are rare, and how our own Earth and solar system relate to them.
But there are several different ways to find planets, and each works best on different types of worlds, which can lead to skewed results. The currently dominant methods 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. The orbital “years” of such worlds are only a few days or weeks, and they do not exist in the solar system. In contrast, direct viewing of planets—so-called direct imaging—needs to distinguish them from the blinding brilliance of a star, which is easiest to do for giant planets on the outskirts of the system. If such orbits revolved around our own Sun, most of these planets would be located far beyond Pluto.
Fortunately, new methods and larger datasets now allow scientists to bridge the gap between these extremes by combining results from multiple planet-searching methods to get a clearer picture of the Milky Way’s true planetary population. A new study published in The science is one of the first successes in this synergistic approach, which gives not only a newfound “in the middle of the road” planet, but also a broader strategy for finding and exploring many others. The largest and brightest of those planets to be discovered could also be good candidates for future direct studies, potentially allowing astronomers to discern their atmosphere and climate.
“When we unite [motion and imagery] together we get all three key properties of a planet — its orbit, its mass, and its atmosphere — so we learn a lot more,” says Thane Curry, planetary researcher at NASA’s Ames Research Center and lead author of the study.
catch a star
Curry and colleagues found their new planet, a giant world called HIP 99770 b, by comparing its star motion data collected in 2021 by the European Space Agency’s Gaia spacecraft with similar but less accurate measurements taken in the early 1990s. predecessor of Gaia. , an ESA Hipparcos satellite. Gaia and Hipparcos were intended to map the stars of the Milky Way (not its planets) using a technique called astrometry to accurately track stellar positions, distances, and movements. But astrometry can also reveal planets: a planet orbiting a star can cause a cyclic, very slight shift in the star’s position, oscillating back and forth in the plane of the sky. By determining the size and frequency of this shift, astronomers can determine the mass and orbit of the invisible planet.
The initial discovery of the planet and subsequent photographic observations were only possible thanks to several decades of Gaia-Hipparcos data, which made it possible to detect the long orbit of HIP 99770 b. The unified catalog itself has been in the making for years. After the first release of the Gaia data in 2016, Timothy Brandt, an astronomer at the University of California at Santa Barbara and co-author of the new study, published a list of tens of thousands of stars, reconciled and augmented by earlier Hipparcos data. observations, updating them again in 2021 after the last release of Gaia data. The result was an approximately 25-year window into how these stars moved across the sky.
Several teams have begun deepening a new database of stellar companions, “each in their own way determining exactly what information to consider when choosing a target,” says Caroline Morley, a researcher who studies exoplanet atmospheres at the University of Texas at Austin. part of a new study.
In the case of HIP 99770 b, the Gaia-Hipparcos data showed it to be a gas giant world orbiting its star at a distance slightly greater than Uranus from the Sun—large, bright, and far enough away from its stellar host to be within range of . reach of direct visualization. Subsequent observations with the SCExAO Direct Imaging Instrument on the Subaru Telescope at Mauna Kea in Hawaii confirmed these suspicions, revealing the planet as a dot obscured by molecules of water vapor and carbon monoxide. Climate models suggest that the temperature on the planet is between 1300 and 1400 kelvins (1880 to 2060 Fahrenheit). Although HIP 99770 b is clearly unearthly, its overall properties make it a relatively close relative of Earth.
“This is the first [finding from this database] it can actually state, “It’s probably a planetary mass,” says Beth Biller, who was not part of the research team. Biller, an astronomer at the University of Edinburgh in Scotland, further noted that the heavy world is in the gray zone between a planet and a brown dwarf, and that some may object to classifying it as a planet. Regardless, “this is definitely the least mass object detected by this method,” she says.
Worth a thousand words
Results like these could help fill in the remaining 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—10 to 100 million years old—and still glowing with the heat left over from their formation. The cumulative result of all these previous polls, according to Biller, was important but still underwhelming. “We found that [hot, young, wide-orbiting] giant planets are quite rare,” she says.
While many stars are expected to have some kind of planet in orbit, direct imaging studies have shown that far fewer stars have giant planets on their edges. Infrared images allow a better understanding of the atmosphere of these worlds, and models give an estimate of their mass. Of the dozens of exoplanets captured by direct images, astronomers have been able to more accurately narrow down the mass of only two using follow-up measurements with indirect planet detection techniques. Part of the problem is the pre-existing preference for observing young planets, which accordingly have young host stars that are much more active than more mature stars and therefore more damaging to stellar measurements of the companion’s mass.
“If you have a directly imaged planet, there is a certain amount of guesswork in confirming its physical properties,” says Brandt. Combining astrometry and direct imaging not only opens the door to detecting more targets; it also eliminates some of that guesswork by revealing the orbit and mass of each new planet, along with its atmosphere.
While Gaia targets two billion stars, Hipparcos has only studied 100,000, all relatively bright and close to Earth. Curry estimates that about a third of the stars studied in the combined catalog have companions, most of which are low-mass. If only one in 100 cataloged stars with satellites has a planet that can be photographed, a new confluence of planet detection methods should greatly increase the total number of worlds that astronomers will soon be able to see directly. The researchers say that by the end of their decade-long study, Gaia will be able to identify up to 100 additional planets as candidates for direct imaging with current instruments — more than four times as many directly imaged worlds as have been identified to date. 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 could get if we just did a blind search,” says Curry, “and the information we get is much richer than if we just did direct images.”
