Forget moons — the asteroid 10199 Chariklo is stepping up decorative accessories for small bodies. It has two rings. This tiny rock in the outer Solar System joins the gas giants Jupiter, Saturn, Uranus, and Neptune as the fifth and by far smallest object with an observed ring system.
Artist impression of the view from Chariklo.
Chariklo is minuscule, only about 250 kilometers diameter orbiting more than a billion kilometers farther out than Earth. It's small enough that an especially fast car (like a Bugatti Veyron 16.4 or McLaren F1) could potentially hit the escape velocity of only 220 mph (350 kilometers per hour). It poses no threat to Earth, and meets none of the requirements for exploration via the Asteroid Redirection Mission. That means the only way we're going to learn anything about it is from long-distance observations.
The best time to observe an asteroid is when it passes in front of a star, occulting it. By carefully measuring any changes in light, we can get an idea of their size, shapes, and even if they have moons. Conceptually, measuring changes in a star's brightness because of objects between us and the star is in the same family of techniques as that are used by Kepler when hunting for exoplanets.
A sky survey turned up the possibility that Chariklo would occult the star UCAC4 248-108672 on 3 June 2013. The occultation would be visible across South America, so seven different telescopes coordinated observing the occultation. The observations worked, with an extra bonus surprise:
The video shows the light dipping when the rings pass in front of the star, then gets blocked completely when the asteroid occulted it. Looking at that in more detail in the static figure from the research article, the brightness has a small dip, recovers, then drops more severely. Each dip is a ring around Chariklo:
Measurement of the star's light dimming predictably by the passage of Chariklo, and quite surprisingly by the passage of Chariklo's two rings.
This was, to put it bluntly, a surprise. In the words of coordinating astronomer Felipe Braga-Ribas of the Observatório Nacional, "We weren't looking for a ring and didn't think small bodies like Chariklo had them at all, so the discovery — and the amazing amount of detail we saw in the system — came as a complete surprise!"
Why is Braga-Ribas dodging calling Chariklo an asteroid? Because technically speaking, it isn't an asteroid. Chariklo is a centaur, a small body that mixes the characteristics of asteroids and comets. Centaurs are small and rocky like asteroids, but have unstable orbits like comets from being frequently perturbed by the giant planets. Chariklo is the largest of the centaurs, with an orbit that passes between Saturn and Uranus.
So, how did this little centaur get its rings? The debris was probably formed by a collision, which was potentially shepherded into the dense, narrow rings by some yet-unobserved mini-moons. Like Saturn's iconic rings, these rings are probably a temporary feature struggling to collapse into satellites while pulled apart by Chariklo's tiny tides. For now, they're going by the nicknames Oiapoque and Chuí, a pair of Brazilian rivers.
Chariklo is pocket-sized, and its rings are adorably dainty to match, but don't get too carried away with the diminutives:
Astronomer Uffe Gråe Jørgensen from the Niels Bohr Institute at the University of Copenhagen in Denmark captures the beauty and wonder this discovery provokes:
For me, it was quite amazing to realise that we were able not only to detect a ring system, but also pinpoint that it consists of two clearly distinct rings. I try to imagine how it would be to stand on the surface of this icy object — small enough that a fast sports car could reach escape velocity and drive off into space and stare up at a 20-kilometre wide ring system 1000 times closer than the Moon.
It's lovely that this week is full of discoveries closer to home in our own Solar System. Go outside. Look up. The universe is beautiful.
All images and video courtesy of the European Southern Observatory. Don Davis has an alternate visualization of the centaur and its rings.