This article was updated at 9:00 a.m. ET on Friday, July 11, to include new information.
After defending his PhD thesis on modeling interstellar objects on Monday, June 30, Matthew Hopkins intended to take some well-deserved time off. It would appear the universe had other plans for him.
The very next day, the Atlas Terrestrial-impact Last Alert System (ATLAS) telescope in Rio Hurtado, Chile, detected a mysterious object whizzing toward the Sun. Early observations suggested that it came from outside our solar system, potentially marking the third discovery of an interstellar object in history. Astronomers across the globe—including Hopkins—leapt into action, racing to gather as much data on this wandering space rock as they could. By Thursday, July 3, the International Astronomical Union’s Minor Planet Center confirmed that an interstellar object was, indeed, traveling through our cosmic neighborhood, naming it 3I/ATLAS. Now, Hopkins and his colleagues have determined that it may be the oldest comet ever discovered, hailing from a region of the galaxy filled with stars far older than our 4.6 billion-year-old Sun.
“It was incredibly exciting!” Hopkins, an astrophysics graduate student at the University of Oxford, told Gizmodo in an email. “I woke up to messages such as ‘3I!!!!!!!!!!’ and spent the entire day comparing my models to the data as it was collected in real time. I’ve been waiting for this opportunity my entire PhD, of course I was going to dive straight in.”
Through preliminary observations, astronomers have learned a lot about 3I/ATLAS. It’s the largest and brightest interstellar object yet, according to Space.com. Experts are fairly confident that it’s a comet zipping through our solar system at incredibly high speeds. Upon its discovery, the object was traveling 137,000 miles per hour (221,000 kilometers per hour), and it will speed up as it approaches the Sun. 3I/ATLAS appears to be much bigger than the two interstellar objects that came before it: ‘Oumuamua and Comet 2I/Borisov. Early size estimates suggest it could be 6 to 19 miles (10 to 30 kilometers) wide. Don’t worry, there’s no chance of this space rock hitting our planet, but it will safely make its closest approach to Earth on October 30.
Initial spectroscopy suggests 3I/ATLAS is redder in color than typical solar system comets, similar to some trans-Neptunian objects—minor planets that orbit the Sun beyond Neptune’s orbit—or tiny, icy celestial bodies located between Jupiter and Neptune called Centaurs. Another study made photometric observations of the object that found it to be similar in color to some solar system comets but markedly bluer than ‘Oumuamua and 2I/Borisov.
Together, these early findings paint a vivid picture of our solar system’s latest guest, but figuring out where this comet came from is equally as important as figuring out what it looks like. Interstellar objects like this one are “pristine, primordial remnants from the planet formation process in other planetary systems,” Darryl Seligman, an associate professor of physics and astronomy at Michigan State University, wrote in an article for The Conversation. Understanding 3I/ATLAS’s origin would allow astronomers to extrapolate information about how planets form in this foreign star system.
To that end, Hopkins and his colleagues analyzed 3I/ATLAS using the Otautahi-Oxford model. This new model integrates data from the European Space Agency’s (ESA) Gaia observatory with models of protoplanetary disk chemistry and galactic dynamics to predict the distributions of velocities, ages, and compositions across the interstellar object population in our cosmic neighborhood. Hopkins and several of his co-authors contributed to the development of Otautahi-Oxford.
Their findings, which are currently available on the preprint server arXiv and have yet to be peer-reviewed, suggest 3I/ATLAS may be incredibly old—more than 7 billion years old, to be exact. That’s roughly 2 billion years older than our solar system. Based on its trajectory, this ancient comet originated from a part of the Milky Way’s galactic disk. This is the disk-like component of our galaxy that contains stars, gas, and dust and rotates in circular coplanar motion around the center of the galaxy. According to Hopkins’s study, 3I/ATLAS likely came from the thick part of the disk, which contains about 10% of the stellar mass of the Milky Way. These stars are generally much older than those located in the galaxy’s thin disk.
“Studying its chemistry and dynamics will add to our understanding of how the processes of planetesimal formation and evolution happen across the Milky Way’s disk, and how such objects respond to the Milky Way’s potential,” the study states. Planetesimals are diminutive celestial bodies—like asteroids or comets—that serve as the building blocks of planets.
“[Comet 3I/ATLAS] is probably from an old star in the thick disc, and we think that it’s likely that this thing’s been out there for longer than the age of the Solar System,” co-author Chris Lintott, a professor of astrophysics at Oxford, told IFLScience. “There’s a two-thirds chance that it’s older than 7 billion [years], and that would explain the color.” If confirmed, it would mark the first known interstellar object to visit from that distinct galactic population.
To test their hypothesis, the researchers will keep a close eye on 3I/ATLAS as it approaches the Sun. Older stars tend to produce water-rich interstellar objects, Lintott explained. If this comet comes from the thick disk, it should produce a lot of cometary activity, which occurs when surface ice heats up and transitions directly from a solid to a gas.
Later this year, the newly launched Vera Rubin Observatory in Chile will begin its Legacy Survey of Space and Time (LSST). This 10-year astronomical survey of the southern sky should find a lot more interstellar objects, Hopkins said. Prior to 3I/ATLAS, he and his colleagues predicted LSST would find between five and 50, “but the discovery of a third one before that survey even starts boosts our predictions to the optimistic end of that!” he added.