“Just So Science” – The Cosmos Wanderers

By Elf Eldridge 19/05/2011 2

So the big astronomy news of today is the announcement of “Unbound planetary mass objects” around stars towards the centre of our galaxy. Now before i get too excited, I should probably mention that we have discovered large free-floating planets before (maybe) but these have been confined to large clouds of dust and gas near star forming regions – this is are the first time that they have been reported elsewhere. Plus, Kiwi’s helped find them!

Astrophysics in action
Astrophysics in action!

Published in a letter to Nature, astronomical groups from all over the world participated in this, the largest contributors being from MOA (The Microlensing Observations in Astrophysics Collaboration) and OGLE (the Optical Gravitational Lensing Experiment Collaboration) – which also win the prize for most entertaining acronyms. From the name it’s pretty obvious that they used gravitational microlensing to detect these planets – but what they actually did is even more interesting. They autonomously watched 50 MILLION individual stars, for 2 years (50 million!) and carefully monitored their intensity. In ten cases (out of 50 million) they were able to detect increases in the intensity of the starlight because a massive object passed between us and the star – acting pretty much like a magnifying glass – but this is pretty standard stuff in astrophysics. Here’s the kicker, what made these 10 events different is that the were no “brightening events” near these that would indicate a ‘host’ star nearby – hence we believe that these must be free-floating objects.

One of the other notable bits from the paper was that these are common: roughly 2 of these free planets for every major star. “But Elf! (I hear you cry in the narcissistic confines of my skull) 10 events out of 50 million really isn’t that common!” True, but microlensing events aren’t common as they rely on the object you want to detect passing almost directly between earth and one of the stars being observed – which is rare! That’s why they looked towards the centre of the galaxy (due east after dark at this time of the year) where there are lots of stars and so the chances of one of these free floating planets passing in front of them are higher.

Artists impression of possible exoplanets
Artists Impression of possible Exoplanets

The size of these is also surprising, and leads us the authors to believe that they form differently from normal stars or Brown dwarfs (low mass stars), but exactly how remains to be seen. Regardless, this is a truly epic discovery for both knowledge it provides us and the technical challenges posed by the observation, collaboration and interpretation of the huge amount of data – that’s the spirit of science right there!

As a final comment – if you haven’t seen the image for this on the front page of the Dominion Post today it’s well worth a look here, if just for a laugh.  Looks like someone used  Microsoft Paint to put a green circle on a stock photo of the Milky Way (N.B. they didn’t, this is the result of an expensive computer simulation – plus a couple of artistic ‘liberties’ that include the planet being ‘lit’ even though it’s not really near any stars….)


T.Sumi et al “Unbound or distant planetary mass population detected by gravitational microlensing”473, 349-352 (May 2011)  doi:10.1038/nature10092

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