A Batch of More Than 60 New Planets and Development of the Mass-Period Diagram

by Dr. Tahir Yaqoob on October 7, 2012

Apologies for the long delay since the last post – funding deadlines and unforseen circumstances this month have consumed a lot of time. Still, I endeavor to bring you exclusive content that is not easily found elsewhere. (One of these days I’ll report on some of the keyword phrases in search engines that bring people to this blog, and these are actually interesting probes of what is not out there in internet land, because this website is certainly not in any major league.)

Anyone who closely follows the number of confirmed exoplanets will have noticed that there was a long stagnant period in the middle of the year in which the number was stuck at 777 or so for weeks. Then, all of a sudden, a batch of over 60 new planets added in a short space of time quickly brought the running total of confirmed exoplanets to just about 160 shy of 1000.

So how does this new batch of exoplanets compare to the database that existed before? What are they like? Anything different?

One way to do a quick, “bird’s eye” comparison, which will at least begin to reveal the picture, is the mass versus orbital period diagram. Although this is just a two-dimensional slice of a multidimensional and complex set of properties, it goes a long way. Remember that even now, for two-thirds of confirmed exoplanets, the radius or size is unknown. On the other hand, right now over 92.3% of confirmed exoplanets have both a mass and period estimate, so a mass-period diagram is actually one of the most inclusive two-dimensional slices you can make on the data.

Below is one version of the mass-period diagram. The blue crosses represent members of the whole sample that have both mass and period estimates, and the red overlaid squares are the new exoplanets that were recently added in a batch of over 60. We can see that they cover a large part of the diagram, reinforcing some of the properties that were already established. In other words, they fit right in with what was going on before and do not represent any substantial “pushing the envelope” on previous boundaries.

exoplanets mass orbital period diagram october 2012

Let’s be a bit more specific about this. The mass-period diagram for exoplanets is turning out to be really quite fascinating for a number of reasons, and the fact that there is no unique and convincing explanation for its features makes it even more compelling. Looking at the above figure you can see that there seem to be three fairly distinct “islands”. The group at the top left of the diagram contains the by now well-known hot Jupiters: short orbital periods of less than 10 days, with some less than a day, and masses similar to Jupiter, but with a wide range covering a few times less and more massive than Jupiter. Then on the top right of the diagram we have the “cold” Jupiters, with orbital periods in the range of 100 to more than 10,000 Earth days. Our own Jupiter falls in this group. On the bottom left we have the short-period super-Earths and hot Neptunes. Basically, planets with masses in the range of Earth-mass to about 30 Earth-masses (or roughly twice Neptune-mass). The more than 60 newly added planets fall mostly in the two short-period groups.

How have things changed in the past year? The figure below compares the mass-period diagram for a snapshot on 5 October, 2012 (black points; when the total number of confirmed planets was 839), with a snapshot on 8 October 2011 (red points; when there were only 692 confirmed planets). This time the mass and period axes have been switched, the masses are in units of Earth masses, and the solar system planet positions have been overlaid (indicated by the first one or two letters of the solar system planet’s name). It is quite remarkable that even with the addition of 148 new planets, the basic form of the diagram has not changed. If anything, the three groupings appear to be reinforced. The unexplained gap spanning about 10 to 100 Earth days is still as conspicuous as ever, and there also appears to be a paucity in the mass domain (a vertical gap in the diagram) between about 30 and 70 Earth masses. Whilst the vast unoccupied region to the left and top of the diagram can be at least partially explained by observational biases, the gaps between the groups cannot be easily explained. Certainly, theories have been advanced in the literature but all them invoke, in some way or another, ad hoc assumptions that are speculative. There is no “natural” explanation.

exoplanets orbital period mass diagram comparing 2011 and 2012

Notwithstanding the fact that the planet masses are mostly only lower or upper limits, what is really intriguing is the possibility that there are now enough data points for the mass-period diagram to reveal evolutionary tracks, but we are just not seeing them yet. (In other words, is there information here analogous to a “Hertzsprung-Russell diagram” for stars?) I can certainly imagine drawing various tracks on the exoplanet mass-period diagram, and I’m sure you can too. But that is a dangerous game to play. On the one hand, it’s frustrating to know that either we have to wait for more data or we have to figure out the message that is already written in the diagram. On the other hand it’s nice to know that we are witnessing this happening in real time and we are not just presented with something in a textbook that has already been figured out.

To emphasize how much things have not changed in the mass-period diagram with the addition of a year’s worth of newly discovered planets, below are histograms of mass and period distributions (i.e., the mass-period diagram collapsed into one dimension, in two ways). The black lines correspond to all the data (5 October, 2012 snapshot), and the red lines correspond to a snapshot from a year ago (8 October, 2011). (Again, the solar system planets are marked on these diagrams in the usual way.) As you can see, in each case the black (new) and red (old) distributions are hard to distinguish from each other.

exoplanets mass distribution compare 2011 with 2012
exoplanets orbital period distribution compare 2011 2012

I’ve mentioned on previous occasions the fact that our solar system planets closer than Saturn are way off from pretty much all of the confirmed exoplanets on the mass-period diagram. Although observational bias plays a role, the big question is, when those biases are finally overcome, how will the diagram fill out exactly? Will there be more islands? If so, will they be offshoots from the existing patches in the other parts of the diagram? Or will they be more uniform? Just how special or not is our solar system? We still have no idea really.

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