An international team of researchers led by the Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork (PLANET) programs used telescopes in Austrailia, Chile, Hawaii, New Zealand and Tasmania to perform a technique known as gravitational microlensing. This is done by taking advantage of alignments between stars. When a star passes between an observer and a more distant star, the relativistic effects of the closer star's gravity can bend light in the same manner as a magnifying glass, giving observers a more precise view of a distant star. Such events typically last for about a month, and almost never recur.
“We won't have a chance to observe the exomoon candidate again,” said David Bennett of the University of Notre Dame, Ind., lead author of a new paper on the findings appearing in the Astrophysical Journal. “But we can expect more unexpected finds like this.”
If the closer star has a planet in orbit, then that planet can act as a second lens. Astronomers can use this effect to calculate the mass ratio between the closer star and its planet. This technique may also reveal information about a rogue planet-moon system that passes in front of a distant star. A rogue planet is a planet that exists outside of a solar system.
In this case, it is not clear whether the lensing object is a star or a rogue planet. The mathematical modeling suggests the latter but the former is statistically more likely to occur. If it is a rogue planet and moon, it would be the first exomoon ever discovered. What is known is that the mass ratio of the larger body to the smaller body is 2,000 to 1. This would correspond to a red dwarf-hot Jupiter system or a large gas giant-small moon system.
While this brightening event is over and its mystery is unlikely to be resolved, it may be possible to use parallax to measure the distance to a future brightening event. This could be done with the Kepler and Spitzer space telescopes, which are far enough away from Earth to use parallax to measure distance. As a lower-mass pair closer to Earth will produce the same kind of brightening event as a more massive pair located farther away, measuring the distance to the source of a brightening event is a strong method for determining its nature.
A previous NASA-funded study, also led by the MOA team, was the first to find strong evidence for planets the size of Jupiter roaming alone in space. Such planets might form from interstellar dust and gas, but are far more likely to be kicked out of solar systems by collisions or gravitational effects of massive objects, such as black holes. The new exomoon candidate, if authentic, would orbit one such rogue planet.
Astronomers have discovered 1,780 exoplanets orbiting 1,103 stars to date, but they are still looking for their first confirmed exomoon.