Sky & Telescope - January 2022 - 18

Outer Solar System
Discovering outer-solar-system objects requires more than a single night of
observations. Objects must be tracked over months or years.
not well explained by just these ices. Astronomers are actively
trying to understand the drivers of cometary activity in Centaurs,
using both ground-based and space-based observations
as well as modeling and laboratory work.
Another startling discovery is the presence of rings around
at least one Centaur. In 2014 Felipe Braga-Ribas (now at Paris
Observatory) and others announced the detection of a ring
system around 10199 Chariklo. Astronomers usually estimate
the size and shape of an object based on how much sunlight
it reflects. Yet these estimates are uncertain because they
rely on assumptions about the albedo, or reflectivity, of the
surface, which varies due to composition and other factors.
Braga-Ribas's team, on the other hand, waited for Chariklo to
pass between a background star and us, briefly blocking out
the light from that star in what's known as a stellar occultation.
If we know the orbital position and speed of the small
body, we can measure its physical size by timing how long it
blocks out the star. If we measure this " shadow " from multiple
positions, we can even measure its shape (as was done for
the KBO Arrokoth prior to its visit from the New Horizons
spacecraft). For Chariklo, the occultation showed equally
Ingress
Ring 2
Egress
Ring 2
spaced dips in starlight on either side of the central body's
shadow created by its ring system, the first to be detected
around a solar system object that isn't a giant planet!
There is some evidence that Chiron also possesses a ring
system. Researchers are still debating how such rings could
form, and how old they are. Is formation more likely early
in the solar system's history, meaning that rings can survive
the trip into the Centaur region from the Kuiper Belt? (We've
observed a ring around another KBO dwarf planet called
Haumea, for example.) Or did the rings form after the objects
became Centaurs, perhaps due to their activity?
Road Trip?
There are undoubtedly many more discoveries waiting to be
made about Centaurs. Even at a very basic population level,
there are a lot of things we simply don't know. Centaurs' wide
range of heliocentric distances (from 5 to 30 astronomical
units) has made them a challenging group of targets. Many
of them are too distant and therefore faint for existing all-sky
surveys to spot. Even the brighter Centaurs move too slowly
against the background stars to be easily picked up by surveys
optimized for faster-moving near-Earth objects - though
archival searches through these surveys have yielded some
distant detections.
At the opposite end, KBO surveys don't detect Centaurs
Ring 1
Ring 1
-10
-5
Time (seconds)
5
10
15
easily either because they're designed to catch the slowermoving
Kuiper Belt populations. Discovering outer-solar-system
objects requires more than a single night of observations.
Objects must be tracked over months or years so that their
orbital path (and thus the population to which they belong)
becomes apparent. But Kuiper Belt surveys often cover small
sky areas, meaning faster-moving Centaurs can move out of
the search fields before observers can determine their orbits if
special efforts are not made to track them.
It is thus quite difficult to tell if our current Centaur
RINGS Top: When the Centaur Chariklo passed in front of a
12.4-magnitude star, observers in South America watched the star
wink out (central dip). But they also saw a pair of much smaller fades
both before and after Chariklo's, indicating that the small object has
two rings. Above: A 3D model of what Chariklo and its rings might
look like. Chariklo is roughly 300 km wide.
18 JANUARY 2022 * SKY & TELESCOPE
census accurately represents the whole population, especially
in terms of how many objects there are of different sizes.
(We have to know what we can and can't detect in order to
make accurate estimates!) Measuring the size distribution
of Centaurs would help us bridge the gap between observed
JFCs - which have typical diameters less than 10 km - and
observed KBOs, which are typically at least 50-100 km in
diameter. This, combined with a better understanding of
activity in Centaurs, will provide critical insights into the
physical evolution of cometary nuclei from their primordial
start to their end states.
In the coming years, the Vera Rubin Observatory's Legacy
Survey of Space and Time (LSST) should dramatically improve
our observational census of Centaurs. Over a 10-year period,
LSST will cover a large percentage of the sky down to limiting
magnitudes much deeper than previous all-sky surveys. It
Brightness
CHART: F. BRAGA-RIBAS ET AL. / NATURE 2014; CHARIKLO WITH ITS
RINGS: M. KORNMESSER AND L. CALÇADA / ESO
Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022
