This artist’s concept shows interstellar object
1I/2017 U1 (‘Oumuamua) after its discovery in 2017. While itself not a dark
comet, ‘Oumuamua’s motion through the solar system has helped researchers
better understand the nature of the 14 dark comets discovered so far.
European Southern Observatory / M. Kornmesser
These celestial objects look like asteroids but act like comets now come in
two flavors.
The first dark comet — a celestial
object that looks like an asteroid but moves through space like a comet — was
reported less than two years ago. Soon after, another six were found. In a new
paper, researchers announce the discovery of seven more, doubling the number of
known dark comets, and find that they fall into two distinct populations:
larger ones that reside in the outer solar system and smaller ones in the inner
solar system, with various other traits that set them apart.
The findings were published on
Monday, Dec. 9, in the Proceedings of the National Academy of Sciences.
Scientists got their first inkling
that dark comets exist when they noted in a March 2016 study that the trajectory of “asteroid” 2003 RM had moved ever so slightly
from its expected orbit. That deviation couldn’t be explained by the typical
accelerations of asteroids, like the small acceleration known as the Yarkovsky effect.
“When you see that kind of
perturbation on a celestial object, it usually means it’s a comet, with
volatile material outgassing from its surface giving it a little thrust,” said
study coauthor Davide Farnocchia of NASA’s Jet Propulsion Laboratory in Southern
California. “But try as we might, we couldn’t find any signs of a comet’s tail.
It looked like any other asteroid — just a pinpoint of light. So, for a short
while, we had this one weird celestial object that we couldn’t fully figure
out.”
Weird
Celestial Objects
Farnocchia and the astronomical
community didn’t have to wait long for another piece of the puzzle. The next
year, in 2017, a NASA-sponsored telescope discovered history’s first documented
celestial object that originated outside our solar system. Not only did 1I/2017 U1 (‘Oumuamua) appear as a single point of light, like an
asteroid, its trajectory changed as if it were outgassing volatile material
from its surface, like a comet.
“‘Oumuamua was surprising in
several ways,” said Farnocchia. “The fact that the first object we discovered
from interstellar space exhibited similar behaviors to 2003 RM made 2003 RM
even more intriguing.”
By 2023, researchers had identified
seven solar system objects that looked like asteroids but acted like comets.
That was enough for the astronomical community to bestow upon them their own
celestial object category: “dark comets.” Now, with the finding of seven more of these objects, researchers could
start on a new set of questions.
“We had a big enough number of dark
comets that we could begin asking if there was anything that would
differentiate them,” said Darryl Seligman, a postdoctoral fellow in the
department of Physics at Michigan State University, East Lansing, and lead author
of the new paper. “By analyzing the reflectivity,” or albedo, “and the orbits,
we found that our solar system contains two different types of dark comets.”
Two Kinds of
Dark Comets
The study’s authors found that one
kind, which they call outer dark comets, have similar characteristics to Jupiter-family comets: They have highly eccentric (or elliptical) orbits and are on the larger
side (hundreds of meters or more across).
The second group, inner dark
comets, reside in the inner solar system (which includes Mercury, Venus, Earth,
and Mars), travel in nearly circular orbits, and are on the smaller side (tens
of meters or less).
Like so many astronomical
discoveries, Seligman and Farnocchia’s research not only expands on our
knowledge of dark comets, but it also raises several additional questions:
Where did dark comets originate? What causes their anomalous acceleration?
Could they contain ice?
“Dark comets are a new potential
source for having delivered the materials to Earth that were necessary for the
development of life,” said Seligman. “The more we can learn about them, the
better we can understand their role in our planet’s origin.”
For more information about
asteroids and comets, visit:
https://www.jpl.nasa.gov/topics/asteroids/
No comments:
Post a Comment