Copied Right: How astronomers identified the first visitor from another solar system

The Economist explains
How astronomers identified the first visitor from another solar system

Neither bird, nor plane, this is A/2017 U1

Nov 3rd 2017
| by A.B.

ON October 19th Rob Weryk of the University of Hawaii saw something
rather strange. In pictures produced by Pan-STARRS 1, a telescope on
Haleakala, he identified an unusually fast-moving, faint object that
he concluded could not have originated in Earth’s solar system. It
was travelling at more than 25km per second. That is too fast for it
to have a closed, elliptical orbit around the Sun. Nor could its
velocity have been the result of the extra gravitational kick
provided by an encounter with a planet, since it arrived from well
above the ecliptic plane near which all the Sun’s planets orbit.
Indeed, after swinging around the Sun, it passed about 25km below
Earth, before speeding back above the ecliptic plane.

Observations from other telescopes have now confirmed that Dr Weryk’s
object is the first extrasolar object to be spied by astronomers.
The object was originally classified as a comet and thus named
C/2017 U1 (the “C” stands for comet). But it lacked the tail of gas
and dust produced when these icy rocks fly close to the Sun.
Furthermore, an analysis of the sunlight it reflected suggested that
the surface is mostly rock. So it has now been classified as an
asteroid, A/2017 U1, which, judging from its brightness, is about 400
metres wide.

This is puzzling. Comets are formed on the cold periphery of distant
solar systems. Asteroids reside within such systems’ interiors, where
any comet-like volatiles will have been driven off by the heat of
their parent stars. Models of planet formation suggest that
interstellar objects such as A/2017 U1 are more usually comets, as
they can be more easily dislodged from their orbits than asteroids.

One explanation is that over many millennia cosmic rays have
transformed the icy, volatile chemicals that would be expected to
stream off a comet into more stable compounds. Another is that the
Sun is not the first star A/2017 U1 has chanced upon, and its
volatile materials were boiled off by previous stellar encounters.
Or it could indeed be that the object was rocky to begin with—
perhaps once orbiting its parent star in an equivalent of our
solar system’s asteroid belt, before its ejection by an encounter
with a Jupiter-like planet.

Why, then, has nothing like A/2017 U1 been seen before? Those planet-
formation theories suggest such GULLIVER objects should be a
reasonably common sight. Perhaps the theories are wrong. Or perhaps
these interstellar visitors have been overlooked in the past,
and A/2017 U1 will now inspire a spate of such sightings in future.

Sadly for astronomers, A/2017 U1 may not be visible long enough for
these questions to be resolved decisively. It is now charging out of
the solar system towards the constellation of Pegasus—at 44km per
second. Small uncertainties in the calculation of its trajectory
may mean that where exactly it came from and where it is heading
will remain a mystery.

3 Responses

  1. The headline of the story promised so much more.

    Like

  2. Suddenly, the left isn’t fond of identity politics.

    Weird.

    Like

  3. Good, it’s their land.

    Like

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