False color image of Uranus and its moons taken by Hubble in 2003. Credit: NASA/Erich Karkoschka (Univ. Arizona)
As a planet, Uranus is out there. Way out there. It orbits the sun at a distance so great that it takes 84 years for the lopsided gas giant to make a full circle around the sun. At such a distance from our prying eyes, it makes sense that there are still things about the seventh planet that we still don’t know. But we’re learning new things about our universe every day.
Case in point, researchers from the University of Idaho announced in a recently posted paper that Uranus may be orbited by two previously undiscovered moons. The scientists spotted the moons after taking a closer look at images and data collected by the Voyager 2 probe in 1986, 30 years ago. The adorably small moons aren’t alone in Uranus’ orbit. They’ve got plenty of company up there, with over 11 rings and 27 other moons orbiting the gas giant.
Uranus in true and false color, taken by the Voyager 2 probe. Credit: NASA/JPL
So why haven’t we noticed these moons in the past three decades? Well, for one thing, the moons are tiny, with radii only measuring between two and seven kilometers (1.2 to 4.3 miles). So small that, in the publication, the researchers refer to the moons as “moonlets.” Another reason we’re only seeing them now is because our imaging technology has advanced considerably since Voyager 2’s 1980’s heyday. The astronomers suspect that the moons’ signals are so dim, they may have gotten lost in the background image noise.
“We haven’t seen the moons yet, but the idea is the size of the moons needed to make these features is quite small, and they could have easily been missed,” Matt Hedman, a co-author of the paper said. “The Voyager images weren’t sensitive enough to easily see these moons.”
The two moons might explain another mystery about Uranus. The planet’s rings are much narrower than those of other gas giants, and it might be because these moonlets are shepherding the debris of the rings into narrower bands than normal.
Of course, seeing is believing, and astronomers will now face the challenge of trying to find additional proof that the moonlets are, in fact, in orbit around the planet. But hey, at least now we know what to look for!