The Sun has a new neighbor that was hiding in plain twilight. An asteroid
that orbits the Sun in just 113 days—the shortest known orbital period for
an asteroid and second shortest for any object in our Solar System after
Mercury—was discovered by Carnegie’s Scott S. Sheppard in evening twilight
images taken by Brown University’s Ian Dell’Antonio and Shenming Fu.
The newfound asteroid, called 2021 PH27, is about 1 kilometer in size and is
on an unstable orbit that crosses that of Mercury and Venus. This means that
within a few million years it will likely be destroyed in a collision with
one of these planets or the Sun, or it will be ejected from its current
position.
Studying objects like this can help scientists understand where asteroids
originated and the forces that shaped our Solar System’s architecture. “Most
likely 2021 PH27 was dislodged from the Main Asteroid Belt between Jupiter
and Mars and the gravity of the inner planets shaped its orbit into its
current configuration,” Sheppard said. “Although, based on its large angle
of inclination of 32 degrees, it is possible that 2021 PH27 is an extinct
comet from the outer Solar System that ventured too close to one of the
planets as the path of its voyage brought it into proximity with the inner
Solar System.”
Because 2021 PH27 is so close to the Sun’s massive gravitational field, it
experiences the largest General Relativistic effects of any known Solar
System object. This is seen in a slight angular deviation in its elliptical
orbit over time, a movement called precession, which occurs at about one
arcminute per century. Observation of Mercury’s precession puzzled
scientists until Einstein’s Theory of General Relativity explained its
orbital adjustments over time. 2021 PH27’s precession is even faster than
Mercury’s. “2021 PH27 gets so close to the Sun that its surface temperature
gets to around 900 degrees Fahrenheit at closest approach, hot enough to
melt lead,” Sheppard said.
Future observations of this object will shed more light on its origins.
Comparing 2021 PH27 to objects that orbit beyond Earth will improve
researchers’ knowledge of its composition and the materials that enable its
survival under these extreme conditions. An object like 2021 PH27
experiences tremendous thermal and internal stresses due to its proximity to
the Sun.
A census of asteroids near and inside of Earth’s orbit is crucial for
identifying those that could potentially impact our planet, but are
difficult to spot because they approach Earth during daylight. These types
of asteroids are not easily detected by most surveys, which usually observe
at night. The asteroid will soon pass behind the Sun and be unobservable
from Earth until early next year, at which time observers will be able to
refine its orbit to the precision needed to give it an official name.
The only efficient method for spotting asteroids that move around the Sun in
orbits closer than Earth’s own is to take images as the Sun sets or rises,
which Dell’Antonio and Fu did with the Dark Energy Camera on the National
Science Foundation’s Blanco 4-meter telescope in Chile. Their main research
is part of the Local Volume Complete Cluster Survey, which is observing most
of the massive galaxy clusters in the nearby universe with increased detail.
In collaboration with Sheppard, Dell’Antonio and Fu switched from focusing
on some of the most distant objects in the universe to some of the nearest,
using the first few minutes of evening twilight on August 13 to take images
in which Sheppard was able to find 2021 PH27 a few hours later.
“Because the object was already in the Sun’s glare and moving more toward
it, it was imperative that we determine the object’s orbit before it was
lost behind our central star,” explained Dave Tholen of the University of
Hawaii, who measured the fast-moving asteroid’s position on the sky and
predicted where it would be the night after the initial discovery. “I
surmised that for an asteroid this size to remain hidden for so long, it
must have an orbit that keeps it so near to the Sun that it is difficult to
detect from Earth’s position.”
Additional images were obtained the following night using the Magellan
telescopes at Carnegie’s Las Campanas Observatory in Chile as well as again
with NSF’s 4-meter Blanco telescope. A third night of follow-up observations
were needed to determine the new asteroid’s orbit before it was lost, but
cloudy weather in Chile elicited a trek around the world to South Africa
thanks to the activation of the Las Cumbres Observatory’s extensive network
of global 1-meter telescopes.
“Although telescope time is very precious, the international nature and love
of the unknown makes astronomers very willing to override their own science
and observations to follow-up new interesting discoveries like this,” said
Sheppard. “We are so grateful for all of our collaborators who enabled us to
act quickly on this discovery.”
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