The first confirmed discovery of an extrasolar planet took place in 1992 with the detection of planets orbiting the pulsar known as PSR 1257+12 followed in 1995 by the discovery of the first extrasolar planet orbiting a “normal” star, 51 Pegasi b. Since then, over 1,800 examples have been discovered with thousands more unconfirmed extrasolar planet candidates found using an increasing variety of direct and indirect detection techniques. And with these discoveries, astronomers have found out much about where and how planets have formed around a range of star types as well as how they affect their parent star.

While it has been two decades since the discovery of the first extrasolar planets, astronomer Benjamin Zuckerman (University of California – Los Angeles) considered the following question while preparing an invited conference paper in 2013: what was the first observational evidence that, with hindsight, indicated the presence of extrasolar planets? Dr. Zuckerman came up with an interesting answer which he presented at the 19th European White Dwarf Workshop held August 11 to 15, 2014 in Montreal, Canada.

Zuckerman’s answer involves white dwarf stars: the inert, hot cores of Sun-like stars left over after they can no longer sustain fusion when their hydrogen fuel is depleted at the end of their lives. Over the past decade, there have been observational and theoretical studies that have shown that heavy elements present in the spectra of white dwarfs comes from the accretion of rocky debris or even entire planets by these stars. No other explanation for this observed heavy element “pollution” in the atmospheres of white dwarfs exists.

Looking into the early history of white dwarf observations, Zuckerman found that the first observation of such a heavy element enhancement was made in 1917 in van Maanen’s Star by the star’s discoverer, Dutch-American astronomer, Adriaan van Maanen (1884-1946). Also known as “van Maanen 2”, van Maanen’s Star was only the third white dwarf discovered by astronomers after 40 Eridani B in 1783 and Sirius B in 1844 whose existence was initially inferred based on astrometric measurements but finally observed visually in 1862 (see “Sirius: The Search for Companions Continues“). Van Maanen’s star is 13.9 light years away in the constellation of Pisces and, despite the fact it has 0.68 times the mass of the Sun, it has a diameter of only about 15,000 kilometers with 0.00017 times the Sun’s luminosity.

What got van Maanen’s attention about this star was its high proper motion across the sky hinting that it was relatively close by. Subsequent investigation by van Maanen showed that this star was a hot F-type star (based on a spectrum obtained in 1917) but it had the lowest known luminosity for its spectral class (based on the apparent magnitude and an initial parallax measurement published in 1919). It would be over a third of a century before it was realized that white dwarfs like this were the cores of now dead Sun-like stars.

Based on Zuckerman’s research, Van Maanen’s Star was the first white dwarf found to have an enhancement of heavy elements in its atmosphere which is now taken as evidence that it is accreting debris from a planetary system it once had (although there are conflicting accounts about evidence that van Maanen’s Star currently possess substellar companions). And since this observation is based on a spectrum first obtained on October 24, 1917 and subsequently described in a paper published less than two months later, it has now been almost a century since the first observational evidence for extrasolar planets was obtained although the significance of that observation was not appreciated for another 90 years.

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Related Reading

“Sirius: The Search for Companions Continues”, Drew Ex Machina, September 8, 2015 [Post]

General References

A. van Maanen, “Two Faint Stars with Large Proper Motion”, Publications of the Astronomical Society of the Pacific, Vol. 29, No. 172, pp. 258-259, December 1917

A. van Maanen, “Stellar parallaxes derived from photographs made with the 60-inch reflector of the Mount Wilson Observatory”, Astronomical Journal, Vol. 32, No. 755, pp. 86-88, July 1919

B. Zuckerman, “Recognition of the First Observational Evidence of an Extrasolar Planetary System”, arXiv: 1410.2575 (To appear in the 2015 PASP Conference Proceedings of the 19th European White Dwarf Workshop), October 9, 2014 [Preprint]