Phalanx of instruments scrutinize 3I/ATLAS for clues about planet formation elsewhere in the galaxy
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| On 1 July, astronomers discovered 3I/ATLAS, just the third interstellar object seen passing through our Solar System. Telescopes around the world are scrambling to observe the object, including the European Southern Observatory’s Very Large Telescope in Chile, which recently obtained this 13-minute time lapse of it moving against a background of distant stars.O. Hainaut/European Southern Observatory |
3I/ATLAS, named for the instrument that found it, the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope, is just the third object of its kind ever spotted. Like its predecessors, 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019, it is a stray leftover from the formation of exoplanets around other stars and will vanish back into interstellar space within months. As a result, astronomers around the world are swiveling ground- and space-based telescopes toward the object, hoping to catch it while it remains visible, and trying to make sense of early clues. “I couldn’t really work on Thursday,” says University of Cologne astronomer Susanne Pfalzner, a theoretician specializing in stellar and planetary formation. “I was just jumping about.”
Beyond a few basics, ATLAS remains mostly a mystery, but astronomers have already learned enough to intrigue them. For one, the object is conspicuously bright. ATLAS was discovered while it was still 670 million kilometers from the Sun, out near the orbit of Jupiter—more than five times farther than ‘Oumuamua was when it was first spotted. This suggests ATLAS might be as large as 20 kilometers across, much larger than ‘Oumuamua (thought to be a cigar-shaped chunk 400 meters long) and Borisov (975 meters wide). However, estimates of the object’s size come with an asterisk. If ATLAS is outgassing like a comet, it might be surrounded by a haze that makes it appear larger than it really is.
What is certain is ATLAS’s unusually wide curve through the Solar System. To describe how much an object’s orbit deviates from a perfect circle, astronomers calculate a metric known as eccentricity. All planets, asteroids, and comets within the Solar System have orbits with an eccentricity between zero and one, meaning they trace out an ellipse around the Sun. ‘Oumuamua’s eccentricity was 1.2—a hyperbolic arc. Borisov’s was 3.3. With an eccentricity above six, ATLAS is blowing both out of the water.
According to Auburn University astronomer John Noonan, ATLAS’s trajectory could mean the object has been drifting through the Galaxy for longer than ‘Oumuamua and Borisov. However, pinning down exactly where ATLAS came from remains challenging, as the gravity of dust clouds or stars that it passed could have bent its path through the Milky Way. At least one team has used high-precision data on the positions of our Galaxy’s stars to suggest ATLAS may have originated in the Milky Way’s sparse outer disk.
As the object enters the inner Solar System, astronomers are scrambling to get as many eyes on it as possible. “It’s about as ‘code red’ as astronomy can get,” says MSU astronomer Adina Feinstein, who studies young stellar systems. Time is short. By the beginning of October, when 3I/ATLAS will have entered the orbit of Mars, it will be too close to the Sun to be observed with ground-based telescopes. At that point scientists hope to enlist spacecraft like NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s (ESA’s) Jupiter Icy Moons Explorer, which is outbound to the gas giant, to study the object. ATLAS then should reappear for Earth observation by early December and remain visible until May 2026.
The main thing scientists want to figure out is 3I/ATLAS’s composition. Comets and asteroids represent the leftovers of planet formation—making ATLAS a discarded sample of alien worlds, flung out from the dusty disk surrounding a distant star. By comparing its ratios of things such as carbon monoxide, water, ammonia, and salts with those of our local comets and asteroids, researchers hope to learn whether other planetary systems have ingredients like our own. “If we see that this building block looks like all the building blocks in our Solar System, that’s good news,” says Karen Meech, an astronomer at the University of Hawaii at Manoa. “That’s encouraging for the potential for life” elsewhere in the Galaxy.
Already ATLAS appears to have a reddish color, which means its surface probably contains organic materials such as methane, typical for both comets in our Solar System and ‘Oumuamua and Borisov. More detail is likely to come as larger instruments such as the Hubble Space Telescope and JWST swing into action. A team of astronomers has submitted a request to target the object with JWST next month. Noonan has sent a similar proposal to Hubble, with the goal of observing 3I/ATLAS near the beginning of December.
Future interstellar findings are likely to come at a more regular pace. The powerful Vera C. Rubin Observatory on Cerro Pachón in Chile, which recently began to catalog the entire southern night sky every 3 days, is expected to find between six and 51 additional visitors over the coming decade. Upcoming telescopes designed to find near-Earth asteroids, such as NASA’s space-based NEO Surveyor satellite or ESA’s Flyeye instruments, should also detect a few interstellar objects once they become operational.
As the field grows, the exhilaration over each individual discovery will probably decrease, but the wonder these fleeting objects evoke won’t fade. Astronomers expect they will one day be studying entire populations of interstellar interlopers—yielding insights into the cosmos that were once the stuff of dreams. “We get these tiny memories of solar systems past zipping through,” Noonan says. “And we have months to characterize them as best as we can before they’re gone again.”