When Was the First Exoplanet Discovered?

The answer to the question, “When was the first exoplanet discovered?” is actually quite complex. The problem is that detections of exoplanets have been claimed on various occassions, and usually those claims have subsequently been disproven. However, in two cases, claims of exoplanet detection were “satisfactorily” verified and supported years after the initial claims. So how do you date the discovery?

Do you adopt the claim date as the discovery date or do you claim the verification date as the discovery date? In my opinion it should be the confirmation date, because up until that point, the detection could have been false (and in most cases it was). Wikipedia takes the other point of view and adopts the exoplanet gamma Cephei b as the first-discovered exoplanet, citing the date 1988, which is the date of the claim, not of confirmation (scientific paper: Campbell et al. 1988.) Officially, the accepted date of the confirmation is 2003 (scientific paper: Hatzes et al. 2003). What is fascinating is that despite the official 2003 “acceptance,” some subsequent publications remain extremely cautious about the planetary interpretation. For example, Neuh\"{a}ser et al. (2007) use the word “sub-stellar object” instead of planet in the title of their paper, and in the abstract they say, “...and a substellar companion with $M_{p} \sin{i} = 1.7 M_{\rm Jup}$ that is most likely a planet.” (In other words they think it could still be what is known as a subbrown dwarf, a very low mass stellar-type object; here $M_{p}$ is the mass of the smaller object and $M_{\rm Jup}$ is the mass of Jupiter.)

The Extrasolar Planets Encyclopedia does not agree with the above interpretation of history and lists the planet gamma Cephei b as being discovered in 2003. Instead it lists the exoplanet HD 114762 b as the first discovered, in 1989. A paper by Kane et al. (2011) gives the subsequent history as: discovery paper: Latham et al. (1989), and confirmation by Cochran et al. (1991).

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Both of the above planets are hot Jupiters. The planet gamma Cephei b is listed in the Extrasolar Planets Encyclopedia as having a minimum mass just over 2 Jupiter masses, an orbital period of about 903 days, and no radius measurement. It is also listed as being detected by the radial velocity method. The planet HD 114762 b is listed as having a minimum mass that is nearly 11 Jupiter masses, an orbital period of nearly 84 days, and no radius measurement. It is also listed as being detected by the radial velocity method.

After the messy beginnings described above, the subsequent history is somewhat simpler. The next exoplanet discoveries came in 1992, with three planets found around pulsars. Pulsars are remnants of stars that have exhausted their fuel and taken a specific evolutionary path. They do not therefore radiate like regular stars, which sets apart the exoplanets associated with pulsars. They are exoplanets nevertheless. The next exoplanet discoveries came in 1995, using the velocimetry method, after which the field really took off. The first of these discoveries, 51 Pegasi b, is perhaps the best-known. It is a hot Jupiter, with a minimum mass of about half that of Jupiter, and an orbital period of about 4.2 days, but no radius measurement. The first transiting exoplanets were discovered in 1999.

If you would like to invesigate the first exoplanet discoveries for yourself, a good place to start is the Extrasolar Planets Encyclopedia catalog. Click on the button in the header of the table that says “All fields” and then click on the column called Discovery. When you click on it, the data in the table should be sorted in order of numerical value in that column (i.e., by the discovery year). An arrow just to the right of the word Discovery tells you whether the numerical order is ascending or descending (the arrow will point upwards for an ascending order). If you click the column header again, the numerical ordering will flip from ascending to descending or vice versa. So, to list the earliest discovered exoplanets you should sort the discovery year by ascending order.

Read more about exoplanets in Exoplanets and Alien Solar Systems.

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File under: When was the first exoplanet discovered and what was it? History of exoplanet discoveries. The first hot Jupiter discovered. The first transiting planet discovered.