Exoplanets in 2013 and an Overview of 2012

by Dr. Tahir Yaqoob on January 11, 2013

If we look back at 2012 and compare the number of new confirmed planets found, with the corresponding numbers in previous years, it turns out that a little surprisingly, 2011 was still more dramatic than 2012. In 2012, according to the Extrasolar Planet Encyclopedia, a total of 138 confirmed planets were added, compared to 189 in 2011. In terms of percentage increases, the 2011 numbers are even more dramatic of course, since at the end of 2010 the number of confirmed exoplanets was just over 500. The number of confirmed exoplanets at the end of 2012 stood at 854, and increase of about 19% relative to the same time last year (compared to a nearly 36% increase across 2011).

So how do things look for exoplanets in 2013 in terms of demographics? Many of the relations and properties discussed in previous posts haven’t changed much since the number of planets didn’t change much in the second half of 2012. However, we can take a look at roughly how the numbers break down into the usual classifications of jupiter-sized, neptune-sized planets, and super-Earths. Definitions (and associated problems and issues) of these classification have been discussed in previous posts so I won’t go through that again.

The first thing to note is that even though the fraction of exoplanets with size measurements remains at about a third of the total confirmed planet population, the absolute numbers are now getting serious. This is important because without a size there is no density estimate and other key properties remain unknown. At the time of writing there were 304 confirmed planets with radii estimates. Just under 10% of these are smaller than 2 Earth radii, and 24% are between 2 Earth radii and half a Jupiter radius. The majority, around 65% are therefore giant planets. However, because of the heterogeneity of the sample, and selection effects, these should not be taken as representative of general demographics. In particular this picture differs from, and is not to be confused with, the picture emerging from the Kepler candidates.

Aside from the fact that most of the Kepler candidates are not confirmed yet, the Kepler candidate planets are obtained from a small restricted region of the sky, which is “burned in” more and more as time goes by. So the number of candidates increases with time, and the proportion of the more difficult to observe smaller planets increases as the amount of data gathered increases. On top of that, the Kepler observations do not yield masses without follow-up, and give only radii in the first instance. To obtain demographics that are corrected for biases and other effects still requires adoption of a procedure (that is subject to assumptions), but nevertheless, such estimates have been done and the numbers have been recently updated (and this week announced at the American Astronomical Society) meeting.

An interesting histogram is shown on the Kepler website, which shows the numbers of exoplanet candidates in the different size categories. (The number of candidates is now 2,740, more than double the amount a couple of years ago, and of those 105 are confirmed.) Of the total 2,740 a huge proportion is in the form of Neptune-size or Super-Earth size planets (about 1,300 and 800 respectively). The hot jupiters are, in this sample, much scarcer, at about 10% of the total. The number of candidates that have a size less than 1.25 times that of Earth corresponds to about 13% of the total. This is a very different picture to that obtained from the confirmed sample (Kepler and non-Kepler) planets described above. The truth has yet to emerge.

The Kepler updates go on to talk about the percentage probabilities of a Sun-like star having a planet of various kinds, at various orbital distances. The qualifies “candidate” is dropped in most of the discussions, perhaps out of laziness, but it’s important to remember that, especially when reading statements such as, “it looks like practically all Sun-like stars have planets.” Still, it is interesting that Kepler finds only 5% of stars have a gas giant in an orbit that is smaller than 400 days, in sharp contrast to the apparent abundance of confirmed hot Jupiters in the non-Kepler sample.

The coming year promises to be interesting from the point of view of all of these data coming together to start to seriously rule out some variations of planet-formation and migration scenarios that have so far managed to survive. The multiplanet systems, of which there are 126 confirmed at present, will provide particularly important constraints. And of course, the end of 2013/beginning of 2014 will mark a time in our history when the number of other worlds that we have found reaches 1,000.

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