Astronomers using telescopes in New Zealand and Tasmania say they have observed an extrasolar moon candidate orbiting an object called MOA-2011-BLG-262.
Using a technique called microlensing, astronomers discovered what could be either a moon and a planet – or a planet and a star. This artist’s conception depicts the planet-moon scenario. Image credit: NASA / JPL-Caltech.
The scientists used an innovative technique called gravitational microlensing.
The technique takes advantage of chance alignments between stars: when a foreground star passes between us and a more distant star, the closer star can act like a magnifying glass to focus and brighten the light of the more distant one. These brightening events usually last about a month.
If the foreground star has a planet circling around it, the planet will act as a second lens to brighten or dim the light even more.
By carefully scrutinizing these brightening events, scientists can figure out the mass of the foreground star relative to its planet.
In some cases, however, the foreground object could be a free-floating planet, not a star.
Astronomers might then be able to measure the mass of the planet relative to its orbiting companion: a moon.
In the new study, published in the Astrophysical Journal (arXiv.org version), the nature of the lensing object is not clear. The ratio of the larger body to its smaller companion is 2,000 to 1.
If the planet-star scenario is true, the exoplanet would be about 18 times the mass of Earth.
These images from the VISTA 4-m-telescope show the exoplanet-exomoon system MOA-2011-BLG-262. Image credit: D. P. Bennett et al.
If the moon-planet scenario is true, the moon, labeled MOA-2011-BLG-262Lb, would have a sub-Earth mass, the planet would have a mass of about 4 Jupiter masses.
The problem is that astronomers have no way of telling which of these two scenarios is correct.
Dr Wes Traub of NASA’s Jet Propulsion Laboratory in Pasadena, who was not involved in the study, explained: “one possibility is for the lensing system to be a planet and its moon, which if true, would be a spectacular discovery of a totally new type of system.”
“The researchers’ models point to the moon solution, but if you simply look at what scenario is more likely in nature, the star solution wins.”
The answer to the mystery lies in learning the distance to the circling duo.
A lower-mass pair closer to Earth will produce the same kind of brightening event as a more massive pair located farther away. But once a brightening event is over, it’s very difficult to take additional measurements of the lensing system and determine the distance.
The true identity of the system MOA-2011-BLG-262 will remain unknown.
“We won’t have a chance to observe the exomoon candidate again. But we can expect more unexpected finds like this,” explained study lead author Dr David Bennett from the University of Notre Dame.
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D. P. Bennett et al. 2014. MOA-2011-BLG-262Lb: A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge. ApJ 785, 155; doi: 10.1088/0004-637X/785/2/155
