This XKCD whatif talks about circumbinary and [the other kind of] exoplanets. How many such exoplanets have actually been discovered?
I tried to look it up at exoplanets.org, but didn't find a column mentioning if the star is a binary.
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1$\begingroup$ Have you checked the NASA Exoplanet Archive? They have fields for number of stars in the system and a flag for circumbinary planets (you may need to add them as extra columns). $\endgroup$– user24157Dec 5, 2019 at 16:18
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1$\begingroup$ @antispinwards I looked at exoplants.org, but didn't find any column indicating the number of stars in the system. I'll check out the NASA archive $\endgroup$– usernumberDec 5, 2019 at 16:19
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2 Answers
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Go to the exoplanets.org database.
Top left hand dropdown menu - select "Stars"
You get a table listing all the known stars with exoplanets.
Click on the big plus sign in the top right hand of the table and select "Binary Flag"
This will now show a boolean flag that indicates if the star is known to be part of a binary (or multiple) system.
In the FILTER window at the top of the page, just type "BINARY" and the table will just show binary stars.
It is possible to export the table as a csv file using the "export" button at the top right. Counting the number of lines with the BINARY flag indicates that as of December 2019, there are 511 known exoplanets around binary stars.
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$\begingroup$ Does the database also indicate which planets are circumbinary and which planets are the other kind? $\endgroup$ Feb 28, 2020 at 10:55
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A recent paper suggests that there are possibly more planets around binary stars than we have been able to detect because of observational biases. This means that there are more planets in binary systems than what we have discovered. From exoplanet.eu
until recently, observational surveys, especially those relying on the radial velocity method, had been strongly biased against binaries
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for many cases, the binarity of the system was not known at the time of the exoplanet's discovery and was established by later observational campaigns. This means that there should still be a potentially large population of exoplanet-hosting "single" stars that are in fact members of a (yet undetected) multiple system.
However, the formation of planets is impeded, and any planets are more likely to be thrown out of the system when the separation between both stars is less than 100 AU. So the fraction of binary systems with planets should be smaller than the fraction of single stars with planets.
Another recent paper addresses the question of the frequency of planets in binary star systems based on data from Gaia's second release. Around close binaries, there are very few planets. However, there seem to be many planets around binaries with a white dwarf. So how many planets are around binary stars seems to depend on the geometry of the binary star system.
On a side note, the "other kind" of planet in the xkcd strip is referred to as a "circumstellar planet".
answered Feb 28, 2020 at 13:08