In a paper that will be featured in the Astrophysical Journal, the first exomoon candidate has been discovered. Using telescopes based in New Zealand, in June of 2011 a brief brightening in the Sagittarius constellation occurred as a rare phenomenon called microlensing, when a celestial object passes between earth and a distant star. NASA-funded researchers, including the lead author from the University of Notre Dame David Bennett, have reported that while observing the gravitational magnification of the starlight, astronomers hypothesized that it was either a small star or brown dwarf and a Neptune-sized planet about 19 times the mass of earth, or a planet larger than Jupiter with an orbiting moon smaller than earth.
The possible exomoon was observed during a joint study by the Probing Lensing Anomalies NETwork, or PLANET, and the Japan-New Zealand-American Microlensing Observations in Astrophysics, or MOA. The ratio of the large object to its small companion is 2,000 to one, according to NASA’s Jet Propulsion Laboratory at Caltech, but unfortunately the encounter was by chance and therefore cannot be viewed again to confirm their suspicions. If it is an exomoon, the chief scientist for NASA’s Exoplanet Exploration Program, Wes Traub, believes that the planet may have been ejected from another planetary system along with its companion moon.
Astronomers have discovered about 1,800 exoplanets, but they have yet to find an exomoon. Microlensing has been key, and when an object passes between earth and a distant star, the brightening effect of the focused light can create events lasting around a month. The exomoon discovery is suggested by the effect created by a smaller orbiting body around the celestial object called the lens. By measuring the brightening events, NASA is capable of identifying the mass of the foreground star in relation to its planet, but if the celestial object is a wandering planet and not a star, then astronomers will be able to measure the planet in relation to its moon.
Though researchers are pointing to the probability of it being a moon, detractors say that because the event can never be observed again that it is impossible to tell, and that the potential of either occurrence in nature leans towards a star instead. Either way, the mystery revolves around the distance to the companions, because a lower-mass pair close to earth would produce the same microlensing effect as a larger pair farther away.
When the telescope in New Zealand first spotted the event, the Keck telescope in Hawaii and others in Tasmania and the southern hemisphere turned to measure the change in the object’s brightness over time, a deduction according to Bennett that resulted in his conclusion that it was a moon. With NASA’s hopeful discovery of the first exomoon, whatever the two objects happen to be they are floating through deep space alone, and at least one microlensing study has suggested that perhaps billions of rogue planets are roaming in a similar way without a star. If it is a moon, it would be a new class of system that is previously unknown.