Extrasolar Planets 2012 Mid-Year Statistics

by Dr. Tahir Yaqoob on July 9, 2012

We are halfway through the year, and it is a good time to see how the basic numbers pertaining to exoplanet observations have changed. As of July 6, 2012, according to the Extrasolar Planets Encyclopedia, the total confirmed planet count was nearly 800 (777), breaking down into 623 unique planetary systems, 101 of which harbored more than 1 planet. The graphs below are updated versions of those shown in Exoplanets and Alien Solar Systems which included data only up to 8 October 2011.

The first graph shows the number of exoplanets discovered per year, and the cumulative number discovered by the end of each year (or by 6 July 2012 in final year). The year 2011 still holds the record for the greatest number of planets discovered (189) and the number so far in 2012 is only 61 so twice as many in the second half of 2012 will have to discovered compared to the first half if the 2011 numbers are to be matched. As for the Kepler mission exoplanets, in July 2012, there were 2,321 candidates, and 74 confirmed planets.

Number and cumulative number of exoplanets discovered by year

Number and cumulative number of exoplanets discovered by year as of 6 July 2012

Mass and radii estimates are two of the most important derived parameters that we would like to know about exoplanets. Over 98.5&#37 of confirmed exoplanets (766/777) have either a mass lower limit, upper limit, or mass estimate and this is illustrated in the histogram below.

Exoplanets mass distribution

Exoplanets mass distribution;
Me=Mercury;Ma=Mars; V=Venus; S=Saturn; J=Jupiter;U=Uranus; N=Neptune.

Our solar system planets are shown for direct comparison. The basic form of the mass distribution has not changed compared to October 2011: it has two broad peaks, one around Jupiter’s mass, and the other around Uranus/Neptune mass. The origin of this distribution is not understood and it cannot be due to observational biases alone (see Exoplanets and Alien Solar Systems for details). There is still a painful deficit around a couple of Earth masses and all masses below that. The planet (KOI-55c) with the lowest mass (that does not orbit a pulsar) has a mass lower limit of about two-thirds of Earth’s mass and has a radius about 80&#37 of that of Earth. However, the next two planets in an ascending mass-ordered list are Kepler-42d and Kepler-42c which have mass upper limits of about the mass of Earth, and twice the mass of Earth respectively, so they could be less massive than KOI-55c. However, all three of these planets have orbital periods of less than 2 (Earth) days so they are very close-in to their host star. I will not give the so-called equilibrium temperatures here since they are misleading because without knowledge of details of the surfaces and atmospheres of the planets, their “equilibrium temperatures” may be completely different from their actual temperatures (as is the case for Venus). Definitive Earth analogs (however they may be defined) in a habitable zone have yet to be announced, one of the main factors for delay being that at least 3 orbits must be confirmed and for Earthlike planets that means at least 3 years of observations.

In the 6 July 2012 sample, 252 out of 777 exoplanets has radius (i.e., size) estimates, corresponding to about 32&#37, which is a modest improvement on 27&#37 in October 2011. Below is the latest radius versus mass diagram for the July 2012 sample, showing the positions of our solar-system planets for direct comparison.

Exoplanets radius versus mass data and relation

Exoplanets radius versus mass data and relation; Me, V, E, Ma, J, S, U, N, correspond to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, respectively.

Compared to the corresponding plot for the October 2011 sample shown in Exoplanets and Alien Solar Systems, there is a striking new development. Aside from the space between Earth-mass and Neptune-mass planets becoming more filled out, there appears to be a new region being populated that corresponds to Uranus and Neptune-sized planets (about 3-4 times Earth size), but with masses covering the wide range of about twice Earth-mass to 4000 Earth-masses (or more than 12 Jupiter masses).

There are a lot more updates which will be posted in the future but here I will just end with the orbital period versus mass diagram for those exoplanets that have estimates of both (740 out of 777). Remember that the masses are mostly lower limits or upper limits, however.

Exoplanets period versus mass data and relation.

Exoplanets radius versus mass data and relation; Me, V, E, Ma, J, S, U, N,; correspond to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, respectively.

The results are essentially unchanged compared to the October 2011 results. If anything, the three “patches” that are heavily populated in the diagram are reinforced. The interpretations of the period versus mass diagram have been discussed at length in Exoplanets and Alien Solar Systems, including observational biases. Those discussions are still relevant. In particular, the pile-up of hot-Jupiters seen in the lower right-hand-side patch remains unexplained: no convincing explanation has yet been offered in the literature. Another thing that remains fascinating is that all of our solar-system are still way offset from the extrasolar mass period versus mass populated regions.

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