Telescopes only picked it up a week ago, but it's likely been traveling through interstellar space for millions of years.
Comet PanSTARRS (C/2017 U1)
© NASA/JPL/Horizons
Comet PanSTARRS (C/2017 U1) raced within about 0.25 astronomical unit of the Sun in early September and is now relatively close to Earth. Based on its extreme orbit, astronomers believe it arrived here from interstellar space.
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 1 telescope 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).
Comet PanSTARRS (C/2017 U1)
© Paulo Holvorcem & Michael Schwartz (NASA grant #NNX15AE89G)
Here's how Comet PanSTARRS (C/2017 U1) looked on October 21st as recorded at Tenagra Observatory near Rio Rico, Arizona. The images span 9 minutes, during which time the telescope tracked the object's motion, so background stars appear trailed. Each field is 3 arcminutes wide with north up.
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.