Telescopes only picked it up a week ago, but it's likely been traveling through interstellar space for millions of years.
For centuries, skywatchers have chronicled the comings and goings of thousands of comets. Every one of them has come from someplace in our own solar system, either the Kuiper Belt beyond Neptune or the much more distant Oort Cloud at the fringes of the Sun's realm.
But an object swept up just a week ago by observers using the PanSTARRS 1telescope atop Haleakala on Maui has an extreme orbit — it's on a hyperbolic trajectory that doesn't appear to be bound to the Sun. Preliminary findings, published earlier today by the International Astronomical Union's Minor Planet Center (MPC), suggest that we are witnessing a comet that escaped from another star.
"If further observations confirm the unusual nature of this orbit," notes Gareth Williams, the MPC's associate director, "this object may be the first clear case of an interstellar comet."
Designated C/2017 U1, Comet PanSTARRS was a dim, 20th-magnitude blip when first spotted on October 18th, after having zipped within 37,600,000 km (23,400,000 miles) of the Sun on September 9th. Such a close approach to the Sun's searing heat would ordinarily spell doom for a small comet. Based on its apparent brightness, dynamicist Bill Gray calculates that it would have a diameter of about 160 meters (525 feet) if it were a rock with a surface reflectivity of 10%. "It went past the Sun really fast," Gray notes, "and may not have had time to heat up enough to break apart."
Now it's headed out of the solar system, never to return. It passed closest to Earth on October 14th at a distance of about 24,000,000 km (15,000,000 miles), and astronomers worldwide have been tracking it in the hopes of divining its true nature — especially whether it's displaying any cometary activity.
What gives C/2017 U1 away as an interstellar visitor wasn't its very high inclination (122°) with respect to Earth's orbit, which isn't particularly rare, but more critically its extreme hyperbolic eccentricity (1.19). Check out the comet's pass through the inner solar system using JPL's interactive Horizons app (requires Java).
Dynamicists had previously calculated how often comets and asteroids from other stars should be in our midst. However, the only other comet suspected to have an interstellar origin was Comet Bowell (C/1980 E1), which had an eccentricity near 1.05. However, notes S&T Senior Contributing Editor Roger Sinnott, "Comet Bowell apparently was not hyperbolic on the way in, but only as it left" because that object passed within 35,000,000 km (0.23 a.u.) of Jupiter, whose gravity gave it a boost in speed.
According to Gray, Comet PanSTARRS appears to have entered the solar system from the direction of the constellation Lyra, within a couple of degrees of right ascension 18h 50m, declination +35° 13′. That's tantalizingly close to Vega — and eerily reminiscent of the plot of the movie Contact — but its exact path doesn't (yet) appear to link any particular star.
This object entered the solar system moving at 26 km (16 miles) per second. At that speed, in 10 million years it would traverse 8,200,000,000,000,000 km — more than 850 light-years.
Scientists Spot First Alien Space Rock In Our Solar System
Astronomers have spotted some kind of outer space rock that's the first visitor from outside of our solar system that they've ever observed.
The discovery has set off a mad scramble to point telescopes at this fast-moving object to try to learn as much as possible before it zips out of sight.
"Now we finally have a sample of something from another solar system, and I think that's really neat, " says Karen Meech, an astronomer at the University of Hawaii Institute for Astronomy, "and so you'd love to see if it looks like stuff in our solar system."
It's long been assumed that an interstellar object like this one should be out there, because giant planets in forming solar systems are thought to toss out bits of space crud that haven't yet glommed into anything. But this is the first time scientists have actually found one.
The mysterious object is small — less than a quarter mile in diameter — and seems to have come from the general direction of the constellation Lyra, moving through interstellar space at 15.8 miles per second, or 56,880 miles per hour.
"The orbit is very convincing. It is going so fast that it clearly came from outside the solar system," says Paul Chodas, manager of NASA's Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in Pasadena, Calif. "It's whipping around the Sun, it has already gone around the Sun, and it has actually gone past the Earth on its way out."
The asteroid was discovered on October 19 by Hawaii's Pan-STARRS 1 telescope, which searches the sky for near-Earth objects.
"It became clear that it didn't move like asteroids and comets normally do," says Robert Weryk of the University of Hawaii, who contacted a colleague to acquire follow-up images using another telescope owned by the European Space Agency. The combined observations made it clear that this was an interstellar visitor.
The asteroid is now speeding toward the constellation Pegasus and is fading out of sight fast. "We might have, for moderately large telescopes, another handful of days, maybe a couple of weeks. So we don't have much time to study it," says Meech, who wants to know what its shape is and what its chemical composition might be. She says the Hubble Space Telescope should spy on this object as well, in the coming weeks.
She notes that different stars have different chemical compositions, so she'd like to know if the solar system this came from produces material similar to the planet-making stuff in our own.
"We've been expecting this for decades, really," says Chodas. "We don't know enough about how much material is floating around between the stars. And so this will give us the first data point. We hope to find more of this stuff."