An interstellar object is cruising through the solar system

Hindustan Times

10-07-2025

ON THE NIGHT of July 1st, in a remote corner of Chile, a small robotic telescope noticed something in the sky. What at first seemed a routine detection of an object travelling through the solar system soon turned out to be anything but. The object's trajectory revealed it to be a much rarer visitor than first thought. Formed around a distant star elsewhere in the Milky Way, it is an interstellar wanderer, not a merely interplanetary one.
That realisation sparked a scramble. 'It's been full gas for the past week,' says John Noonan, an astronomer at Auburn University in Alabama. 3I/ATLAS—named after the Asteroid Terrestrial-impact Last Alert System, the project that discovered it—is only the third such interloper ever spotted. Dr Noonan is one of the authors of a quickly written paper that tries to establish some basic facts about 3I/ATLAS, including what it is (a comet); how big (perhaps 10km across); how fast it is moving (around 60km/sec) and how far into the Solar System it will come (well inside the orbit of Mars).
Most excitingly, 3I/ATLAS offers a chance to test some early theories of interstellar-object-ology, a fledging branch of astronomy (though one in need of a snappier name) that began to receive serious attention only after the detection of 1I/'Oumuamua in 2017, the first interstellar object discovered. 'We think these are the most common macro-scale objects in the galaxy,' says Chris Lintott, an astronomer at the University of Oxford. 'In hindsight it's odd that people hadn't been thinking more about them before.' The current thinking is that interstellar objects (ISOs) are leftover bits of protoplanetary discs, the doughnuts of dust and ice that surround young stars and from which their planets condense. Some 90% of the asteroids and comets formed this way might be ejected from their parent star systems by gravitational interactions with bigger objects.
One paper, published in 2018, concluded that there might be around 1026 ISOs in the Milky Way, a million billion times more than the number of stars. Others have modelled how they spread through the galaxy (in braided streams, it seems); or worked out that, because of the Sun's orbit around the galactic core, they should come more often from certain directions. A study from 2019 proposed that ISOs could help explain planet formation: a tiny fraction of ISOs might get captured by young stars and act as nuclei around which full-size planets can grow.
In a preprint published on July 9th, Dr Lintott and his colleagues apply some of these new theories to 3I/ATLAS. They conclude that there is a two-thirds probability that it is more than 7bn years old—around half the age of the universe, and far older than the Sun. Its trajectory suggests it comes from a star somewhere in the Milky Way's 'thick disc', a group of old stars that sit above and below the central plane of the galaxy. If so, chemical differences between old and young stars mean it should contain more water than comets native to Earth's solar system. Such predictions will be checked as bigger telescopes catch sight of the comet. The James Webb Space Telescope, a powerful instrument launched in 2021, could make observations towards the end of July, when 3I/ATLAS comes into its field of view.
As 3I/ATLAS approaches the Sun, it will grow brighter and begin to shed parts of itself, making it easier to study. Frustratingly, it will disappear from earthly view in September before its closest approach to the Sun on October 30th. But it will remain visible from Mars. Plans are afoot to get probes there to take pictures of their own—though one called MAVEN, which carries an instrument capable of probing the comet's chemical composition, may be decommissioned on October 1st as part of big cuts planned to NASA's budget. 'We'll be taking our glasses off right before the fireworks,' Dr Noonan says, glumly.
Still, the future of ISO-ology looks bright. The Vera Rubin telescope, which saw its first light on April 15th, could spot dozens of ISOs over the next ten years. At that point, says Dr Lintott, 'we'll have a proper population of these things, which will be transformative.' It may even be possible to send a probe to look at one up close. The European Space Agency's Comet Interceptor mission, due to launch in 2029, will sit in a parking orbit waiting for a comet to chase after. But it could also run down an ISO, if a suitable one presents itself. Astronomers have dreamed of building an interstellar probe for decades. But why go to the trouble of flying all the way to an alien star system when the alien star system can come to you?
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