In 2019 Michel Mayor and Didier Queloz received the Nobel Prize for their 1995 discovery of an exoplanet, which is just a planet orbiting about a start that isn’t our sun. Since then, exoplanet discovery has boomed with us now knowing of over 4000 different exoplanets. Most of these exoplanets, and their corresponding data, are located in individual journal papers which are distributed and hard to access in mass. Solving this problem is the NASA Exoplanet Archive, which brings together most of this data, vets it using a team of astronomers, and stores it in an easy to access format. You should check it out! The rest of the post will be me playing around with the dataset and answering some questions I had about exoplanets that this dataset helped resolve.
Artists Representation of the Trappist System, showing relative scale. Source: JPL/NASA
Exoplanet Period
Astronomers have discovered 4000, exoplanets, but their methods for doing so, and confirming that they are actual signal and not just noise, biases their search towards exoplanets with shorter periods.
NOTE: when I say bias, it does not have a negative connotation, just that there is a prejudice in favor of a certain outcome. Exoplanateers are doing a hard job and are often limited by their equipment and experimental biases that make certain types of exoplanets harder to find than others.
The question I had when starting was how much of a bias towards short period exoplanets is there? From the dataset we find that the median period is a little under 12 days. This means that of the 4000 exoplanets, half have a period shorter than this! The shortest period planet has a period of just 2.16 hours! Because of Kepler’s third law, we know that the planetary period is proportional to its distance from the primary body, in this case, its star. Now, appealing to Kepler’s 3rd law is a bit of a stretch as it’s formulated using Newtonian gravity. Any planet orbiting this close to the massive gravity well provided by a star is bound to be experiencing massive relativistic effects.
Just to put this massive bias towards short periods in perspective, the period of Mercury, the planet in our solar system with the shortest period, at around 88 earth days, is longer than 81% of the exoplanets in the database. Here’s a histogram of all the planetary periods from 0 to 70 days just to show what the distribution looks like.
Eccentricity vs # of Planets
If we look at the number of planets in the system versus the orbital eccentricity we see a nice relationship. The more planets there are in a system, the more likely the planets are in circular orbits, and the variance of eccentricities in the orbit decreases. Both of these make sense because a planet with a non-zero eccentricity is more likely to interact unstably with other planets in the system. Here’s a nice article on Exoplanet orbital eccentricity.
Planetary Radius vs Period
Returning to this idea of biases, there’s another bias in how we discover exoplanets, our telescopes. If we plot the planetary period vs its radius and differentiate between the Kepler space telescope and most other telescopes we see two major families appear. the first with a planetary radius of around 1 Jupiter radius and a period of around 1 to 2 days. The second’s much larger and is for planets that are a 10’th the radius of Jupiter and a much larger range of periods. This second family is made up mostly of exoplanets discovered by the Kepler telescope.
Exoplanets Found By Telescope
As was seen in the last gif, a majority of the exoplanets were found by the Kepler telescope. Overall the database is made up of contributions of 72 different telescopes. Now, that’s a lot of telescopes, but most only discovered a handful of exoplanets. I’ve grouped any telescope with less than 27 discovered exoplanets into the Misc category and then provided a histogram of the number of exoplanets discovered by every telescope. It’s on a log scale because the Kepler telescope has discovered almost an order of magnitude more exo-planets than most other telescopes.
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