Astronomers may have spotted the first signs of a moon orbiting a planet outside of our solar system. If observations of this “exomoon” are confirmed, it would be the first time a moon has ever been seen circling an exoplanet.
However, it’s just as likely that what they spotted is a planet orbiting a small star. And the thing is, it’s impossible to confirm. "We won't have a chance to observe the exomoon candidate again," lead author David Bennett of the University of Notre Dame says in a news release. "But we can expect more unexpected finds like this."
In any case, the possible exomoon was discovered by researchers with the joint Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork (PLANET) programs using ground-based telescopes in New Zealand and Tasmania.
They used a technique called gravitational microlensing, which takes advantage of chance alignments between stars -- known as brightening events. When a star passes between us and a more distant star, the closer one acts like a magnifying glass that focuses and brightens the light of the more distant star in the background. And if the foreground star -- also referred to as the lens or passing object -- has a planet orbiting it, the planet becomes a second lens to dim or brighten the light more.
These sorts of measurements help researchers figure out the mass of foreground objects relative to the smaller body orbiting it. Now, if the closer object is a planet, and not a star, researchers can measure the mass of the planet relative to its orbiting object, which would be a moon.
And that’s what happened here, except the foreground (lensing) object isn’t clear. The researchers were able to tell that the ratio of the larger body to its smaller companion is 2,000 to 1. Potentially, the pair could be a small star circled by a planet. In that case, the planet is about 18 times the mass of Earth. Or, it could be a planet more massive than Jupiter coupled with its moon, which would weigh less than Earth.
But there’s no way to tell which one is right, since these brightening events last only about a month and are generally encountered by chance. The researchers think that the lensing system here -- which they’ve dubbed MOA-2011-BLG-262 -- is a free-floating planet and its moon. They suspect the planet was ejected from a young planetary system but kept its moon in tow.
For future brightening events, researchers hope to combine microlensing with the parallax-distance technique. That’s when two telescopes far away from each other view the same distant star, and this effect happens where the star seems to “move.” (It’s like looking at your finger with your right eye only and then your left eye only.) That alters how the starlight is magnified.
The findings were published in the Astrophysical Journal.