Astronomers Spot First-Known Interstellar “Comet”

Telescopes only picked it up a week ago, but it's likely been traveling through interstellar space for millions of years.

An update on this story, posted November 10th, is available here.

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.

Orbit of interstellar object A/2017 U1

The new PanSTARRS object A/2017 U1 raced within 0.25 astronomical unit of the Sun in late September and passed relatively close to Earth a month later. Based on its extreme orbit, astronomers believe it arrived here from interstellar space.
NASA / JPL / Horizons

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 body 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."

Initially designated C/2017 U1, this interloper was a dim, 20th-magnitude blip when first spotted on October 19th, 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."

Pan-STARRS in dawn's light

Dawn's light envelops the first Pan-STARRS telescope atop Haleakala on Maui. In the distance is Mauna Kea.
Rob Ratkowski

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.

Its true character became clearer after Karen Meech (University of Hawaii) recorded a series of images with the Very Large Telescope that, when stacked, showed a perfectly starlike object. So it's a deep-space asteroid, not a comet, and consequently officials at the MPC changed its designation to A/2017 U1.

At least two astronomers managed to take crude spectra of the object: Alan Fitzsimmons (Queens University Belfast) with the 4.2-m William Herschel Telescope and Joseph Masiero (Jet Propulsion Laboratory) with the 5-m Hale reflector. Both show A/2017 U1 to be somewhat reddish but otherwise undistinguished.

What gave this object away as an interstellar visitor wasn't its spectrum or its very high inclination (122°) with respect to Earth's orbit, which isn't particularly rare, but more critically its extremely hyperbolic eccentricity (1.19). This value, well in excess of 1, means the object's orbit is not bound to the Sun. Check out its pass through the inner solar system using JPL's interactive Horizons app (requires Java).

Comet PanSTARRS on Oct 21st

Here's how the PanSTARRS "comet" (A/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.
Paulo Holvorcem & Michael Schwartz (NASA grant #NNX15AE89G)

Dynamicists had previously calculated how often comets and asteroids from other stars should be in our midst. However, the only other object 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.

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.

According to Gray, the PanSTARRS "comet" appears to have entered the solar system from the direction of the constellation Lyra, within ½° of right ascension 18h 44m, declination +34° 40′. That's 4½° from Vega — tantalizingly close and eerily reminiscent of the plot of the movie Contact. Given that Vega lies 25 light-years away, a hypothetical escapee would have taken nearly 300,000 years to get here. However, due to its combined radial and proper motion, back then Vega itself would have been nowhere near where it is now.

More intriguing is the fact that A/2017 U1 is coming from a spot only 6° from the solar apex, the direction that our Sun is moving (at about 20 km/s) through its interstellar neighborhood and thus, statistically, the most likely incoming direction for an interstellar visitor.


15 thoughts on “Astronomers Spot First-Known Interstellar “Comet”

  1. Ron

    Could an artificial craft gather enough interstellar dust on it during a 100 million year trip to look like a comet? And then not break up when it went past a sun because, well, you know.

  2. Edward Schaefer

    I am finding it very unlikely that an extrasolar comet would come into the inner Solar System like that. This is not impossible, but still must be a very rare occurrence.

    My guess is that this is more likely a signpost for Planet X. See if the orbit of this comet goes past the projected orbit area for Planet X. If so, then my guess is that it was recently perturbed by Planet X; and in that case you can say where Planet X was at a given time, and can narrow the search for Planet X based on that.

  3. Tom-Fleming

    I had occasion to visit the MPC website to examine comet eccentricities a while back. I was surprised at the number of comets exceeding a value of 1. In the basic school of math, a value greater than 1 is like being ‘ a little bit pregnant’… you’re hyperbolic. My uninformed opinion is that the comets listed in the MPC site have their orbital elements pretty well nailed down. However this discussion above would seem to introduce a shade of gray to the pregnancy test. Could you enlighten me (or us) to this seeming discrepancy?

    1. Kevin Heider

      As a result of planetary perturbations, you need to solve the orbit for a long period comet (ones with orbits greater than hundreds of years) when the comet is outside of the planetary region (say 50AU from the Sun). The solutions you see when the comet is near perihelion are generic one-size fits all solutions that are only good for around perihelion passage. Planetary perturbations can can increase or decrease the eccentricity. Barycentric solutions that account for the gravity of the Sun+Jupiter are better yet.

      Most comets listed with an eccentricity just over 1 are actually bound to the Sun if you account for the barycenter (Sun+Jupiter system) and calculate the orbit 20 years before/after perihelion passage. C/2017 U1 is around 1.18 coming in and out.

  4. LuxorionLuxorion

    The Oort cloud is really huge, extending up to 3 light-years with a maximum close to 0.8 l.y. So before claiming that this comet comes from the interstellar space, and believe in that rumor, knowing that we do not even know bodies evolving in the KB and in the Oort cloud, I trust in the author of the article who concludes in agreement with the more careful astronomers “a comet on such an extreme path doesn’t necessarily have to come from interstellar space.”

  5. foratwoohonesevenuone

    Hello All,
    Very exciting event! Lots of fuel for my imagination.
    Nothing on the MSM here in Canada.
    Only non-scientific published news story I found online was in the Maui Newspaper.
    I spent a night on top of Haleakala ten years ago a few hundred meters away from all the hardware.
    Brought lots of extra clothes…still froze.
    One more observation:
    The calculations of time and distance here have been based on a constant velocity.
    A little presumptuous maybe?
    Wishing you all well……

  6. PalomarJack

    Please excuse my ignorance, but I think I’ll take the word of people with doctorates covering orbital mechanics long before those of Coast to Coast AM guests. Now, even if it was caused by a mythical (nonexistent) planet, it still would not have the trajectory presented. How difficult is that to understand? I know it’s not exiting or dramatic that it’s just a tiny interstellar lump of rock and ice and not a derelict UFO, but I’m sorry, sometimes science is a little more boring than science fiction. Or is it, ponder where this object may have come from, where it’s been and someday soon we’ll be able to intercept one. Now there is a dramatic story.

  7. millerf1

    I checked the preliminary comet orbital elements looking to work on a future research proposa,l and I found the
    RA/DEC did not match anywhere close to Vega. Does anyone have reliable element set that will work in SkyX?

    1. Kelly BeattyKelly Beatty Post author

      hi, millerf1 . . . the current sky position isn’t what you want — it’s the direction the object had as it entered the solar system, and that is indeed a location in Lyra. (as an aside, nearby-star expert Eric Mamajek has looked for possible links with stars within 25pc radius, and finds none: as for getting an element set that works with TheSky X, try the JPL Horizons site:

      1. millerf1

        Thank You for the clarification and element set. Yes, it is close enough. The papers all assume unpowered flight and only indicate, not a local natural origin. Last year there was a proposal for a micro-probe using a solar sail. If we have neighbors such a probe could be expected to arrive now since we have been broadcasting for 100 years. At 25 LY Vega is almost the right distance for such a probe to arrive now. I advised elsewhere about a year ago to be on the lookout for such. A flyby probe and closed sail would be big enough to observe…

  8. Martian-BachelorMartian-Bachelor

    > Dynamicists had previously calculated how often comets and asteroids from other stars should be in our midst.

    Perhaps someone with access to the full paper could explain more fully exactly what the dynamicists did, since some of the abstract linked to is rather impenetrable, particularly this part:

    “We first generated a quasi-realistic ISO population to ~750 au from the Sun and propagated it forward in time to generate a steady state population of ISOs with heliocentric distance <50 au."

    If every star is presumed to have an Oort cloud like the sun's ranging from ~10^4 AU out to ~10^5 AU, then on average for every incipient comet knocked out of the Oort cloud and into the inner solar system (where we observe them with e's very near but slightly less than parabolic) there should be approximately one knocked out in the other direction (since they are so loosely bound to the sun), a process that would slowly fill up interstellar space with such objects over the life of the galaxy.

    Since the sun has a velocity of order 10 km/sec with respect to the LSR (Local Standard of Rest in the Milky Way) it should be possible to come up with an estimate of the rate at which we 'plow' through this population of objects, and thus the rate at which we'd expect to turn one up.

    Is this more or less what they did, or did they take an entirely different approach?

  9. Robert-CaseyRobert-Casey

    I suppose it’s heading out too fast for a space probe to ever catch up to visit it. It’d be interesting to see what an interstellar asteroid/comet looks like up close.

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