Thursday, March 3, 2016

Exoplanets in the Milky Way: The view from above

Finally my figure of the positions of exoplanets in the Milky Way is finished. Again you see my artist impression of a view from above on the Milky Way, but this time I added the positions of all the known exoplanets for which I could find a distance measurement. The exoplanet data are coming from There are a little bit more than 1000 exoplanets in this map, which means we only have distances for about half of all the exoplanets we know today.

The methods used to detect the planets are indicated by different colors and symbols. Most of the planets in this plot come from the RV method (618) and not from transits (313). At first this might be unexpected, but on average stars observed in RV campaigns are closer to the Sun than transit host-stars because a spectrum needs more light than a brightness measurement. For closer stars the distance is usually easier to determine than for stars far away, for example when using the parallax method.

A quite large number of planets detected by transits orbit stars further away than several thousand light years. This is especially true for those regions in the sky that were observed intensely by transit surveys as for example Kepler. I marked the Kepler field-of-view in the map where several distant planets were found.

The most distant exoplanets, however, were found by microlensing surveys - with the exception of the SWEEPS transit survey. These distant microlensing planets are all located on a line pointing to the center of the Milky Way. Why? This is due to the way the microlensing method works: to see an event we do not only need a planet around a star, we also need a background star which gets 'lensed'. Because the density of stars is highest in the galactic center, the probability to get a lensing event is largest there. Since it is a method which only requires photometric observations, you can see events caused by very distant planetary systems as long as the lensed source is bright enough - or the lensing effect strong enough - to be seen by your telescope.

In case you wonder how scientist get the distances to planetary systems that are so  far away: This would lead too far in this post, but it is not by using the parallax. In these cases distances are usually estimated and, thus, the uncertainties are quite large. Because of the large uncertainty on the distance of the SWEEP exoplanets, one might argue that they possibly are not that far away.

I leave you with a list of names for the most distant planetary systems which are further away from the Sun than 20000 light years. The last two are transiting planets, the rest were all detected in microlensing events.

20961 ly  -  MOA-2010-BLG-353L b
21190 ly  -  OGLE-2005-390L b
22168 ly  -  OGLE-2008-BLG-355L b
22820 ly  -  OGLE-2008-BLG-092L b
23961 ly  -  MOA-2011-BLG-262L b
24058 ly  -  MOA-2011-BLG-028L b
25102 ly  -  MOA-2011-BLG-293L b
25232 ly  -  MOA-2011-BLG-322L b
26732 ly  -  KMT-2015-1 b
27710 ly  -  SWEEPS-4
27710 ly  -  SWEEPS-11