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Possible “Exomoon” Found

By David Reneke

Astronomers find a possible “exomoon” and a dead galaxy orbiting the Milky Way.

Titan, Europa, Io and Phobos are just a few members of our solar system’s pantheon of moons. Are there others out there, orbiting planets beyond our sun? It appears so!

NASA-funded researchers have spotted the first signs of an "exomoon", and say the finding is a tantalising first step toward locating others. The discovery was made by watching a chance encounter of objects in our galaxy, which can be witnessed only once.

The international study is led by the Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork (PLANET) programs using telescopes in New Zealand and Tasmania. Their technique, called gravitational microlensing, 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. 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 scrutinising these brightening events, astronomers 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. Researchers might then be able to measure the mass of the planet relative to its orbiting companion – a moon.

In the new study, the nature of the foreground lensing object is not clear. The pair could be either a small, faint star circled by a planet about 18 times the mass of Earth, or a planet more massive than Jupiter coupled with a moon weighing less than Earth. The problem is that astronomers have no way of telling which of these two scenarios is correct.

If the lensed object in fact turns out to be a planet and its moon, it would herald the discovery of a totally new type of system. The planet may have been ejected from the dusty confines of a young planetary system while keeping its companion moon in tow.

Dead Galaxy Orbits the Milky Way

Astronomers have spotted a tiny galaxy floating on the edge of the Milky Way that could be the first ever formed in the universe. Segue 1 is believed to be a fossil left over from the early days of the universe. A recent study has found it contains fewer heavy elements (e.g. metals) that are abundant in all known galaxies.

Researchers at MIT found that not only is Segue 1 extremely small – it is made up of a few hundred stars compared with the hundreds of billions that galaxies usually create – but the lack of metals such as iron indicates it may have stopped evolving more than 13 billion years ago.

“Segue 1 is so ridiculously metal-poor that we suspect at least a couple of the stars are direct descendants of the first stars ever to blow up in the universe,” says Evan Kirby of the University of California, who worked on the study.

The unusual size of Segue 1 is also helping to shed light on galaxy formation and evolution. The findings suggest it went through one brief bout of star formation long ago before stopping forever. Why did it stop? A galaxy like this should have been able to make a million more stars.

While it is possible it was once a much larger body but was stripped of its metal-rich stars by the neighbouring Milky Way, some believe it was actually formed this small. This is something not though possible before, and could mean there are more galaxies like Segue 1 hiding in space.

If there is no barrier to such puny galaxies forming in the first place, and it would seem there isn’t, then mini-galaxies like Segue 1 could be plentiful, but unseen.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at