An exoplanet – short for extrasolar planet – is a planet existing outside our solar system, orbiting a different star to our Sun. Whilst imaginative (and remarkably prescient) works of science fiction like Star Trek have long envisaged a galaxy in which planetary systems, indeed those with life forms, are ubiquitous, in reality, the first confirmed detection of an exoplanet orbiting a main-sequence star like our Sun was surprisingly recent: in 1995, astronomers Michel Mayor and Didier Queloz used spectroscopic measurements to observe the ‘wobble’ of the star 51 Pegasi due to the gravitational tug of a planet in orbit. Since then, over six thousand exoplanets have been discovered in our galaxy. But long before this, a mysterious, overlooked observational oddity may have been the first clue of the ‘exoplanet revolution’ to come.
Back in 1917, astronomer Adriaan van Maanen observed a curious – and, at first glance, seemingly unrelated – phenomenon. With the aid of the 60-inch telescope of the Mount Wilson Observatory in California, at the time the largest operational telescope in the world, van Maanen viewed the spectrum (the full range of light of all wavelengths or colours) of what he believed to be an F-type star – slightly more massive and bright than our Sun, with a presence of calcium, amongst other elements, in its outer atmosphere, or photosphere. However, he later calculated that this would have to be the faintest F-type star ever discovered. It is now understood that this peculiar entity was a different type of star altogether – a 'white dwarf' – and, unbeknownst to van Maanen, the perplexing spectrum he developed may well have been the first ever observational evidence for the existence of exoplanets, 78 years prior to Queloz’s discovery. Yet, it remained misunderstood for nearly a century.
"White dwarfs are extremely dense: just one teaspoon of van Maanen’s star would weigh about as much as an African elephant"
White dwarfs are the tiny, dim remnants of stars like our Sun that have exhausted their final supplies of fuel and can no longer sustain the process of nuclear fusion in their cores. White dwarfs are extremely dense: just one teaspoon of van Maanen’s star would weigh about as much as an African elephant. This immense density means that elements heavier than hydrogen or helium, should they be produced by some chemical process within the star, would sink very rapidly under the immense gravitational force and should never be found occurring in the upper atmosphere. So, what van Maanen saw – a spectrum 'polluted' with such exotic heavy elements as calcium – was a physical impossibility.
But as more and more of these incomprehensibly polluted white dwarfs continued to be discovered in the first half of the 20th century, a prevailing theory emerged that these heavy elements must have been drawn in from the surrounding interstellar medium. Nonetheless, there were discrepancies: at a recent talk in Cambridge, visiting professor Benjamin Zuckerman explained that the distribution of elements inferred from the absorption spectra of the white dwarfs never quite matched the known abundances of elements in the interstellar medium. Back in 1987, Zuckerman observed a mysterious preponderance of infrared light coming from the regions around a white dwarf, which was interpreted at the time to be due to an orbiting brown dwarf – a 'failed star'. Within a decade, astronomers finally began to conflate the facts and truly solve the mystery.
"What van Maanen saw—a spectrum 'polluted' with such exotic heavy elements as calcium – was a physical impossibility"
As a star nears the end of its main-sequence lifetime and swells into a red giant, it will begin to expel a significant amount of mass, ultimately collapsing to form a white dwarf. As this happens, orbiting exoplanets, no longer as strongly bound by the gravitational force of the star, begin to spiral outwards. These migrating exoplanets can behave in a rather tyrannical manner, driving gravitational instabilities that can push smaller rocky objects like asteroids so close to the host star that they are eventually torn apart by its monstrous gravitational tidal forces and spun into a dusty accretion disc, whereafter the pulverised remains eventually settle onto the star. This dust would contain the chemical remnants of that bygone rocky world, including, of course, all of its heavy elements. This planetary debris is what had polluted van Maanen’s star. White dwarf pollution, so it turns out, is a signal of exoplanetary systems, though little did van Maanen realise it in 1917.
As this planetary debris disc is heated, it begins to emit infrared light, which explains Zuckerman’s curious 1987 observation. That said, to scientists, either outcome of the behaviour of nature is fascinating – especially if at first it does not so neatly fit into the grand scientific picture we have painted hitherto. Professor Amy Bonsor from the Institute of Astronomy remarked that “the system would have been equally exciting had it turned out to have a close-in brown dwarf companion. We have so few similar systems and they turn out to be really important for studying the effects of irradiation on atmospheres for exoplanets.”
"The world of science requires detectives brave enough to pursue such formidable cases"
In a 2007 publication, Zuckerman and colleagues reported the abundances of a staggering 17 polluting elements in the atmosphere of the white dwarf GD 362 and deduced that this material must have come from planetary matter with a composition not unlike that of our Earth–Moon system. Still, many big, alluring mysteries remain. Could there exist rocky terrestrial worlds around white dwarfs that are in the so-called habitable zone? When our Sun swells into a red giant in about 5 billion years’ time, it will engulf Mercury and Venus – and if so, how exactly did an enormous Jupiter-like planet end up in an orbit much closer to the white dwarf WD 1856+534 than Mercury’s around our Sun? Could an Earth-like planet survive long enough to one day trace out a stable orbit around a white dwarf Sun? And what about its moon? The world of science requires detectives brave enough to pursue such formidable cases.
It is all too tempting to dismiss white dwarf systems as merely the bleak, eternal fates of once-magnificent planetary systems such as ours. But if this scientific detective story should teach us anything, it is that we should not dismiss relics of the past, of any form, as so archaic as to be surely irrelevant. Just as there is as much to be learnt about the beautiful and intricate workings of nature on Earth from studying fossil records as there is from living creatures, so are white dwarfs the wise old sages of the stellar kingdom, who alone can tell us enchanting tales about once thriving exoplanetary systems. They are the sentinels of the night sky, unfailing in their candor so long as we ask the right questions.
It stands to be wondered: what other observations that are mystifying to us today – perhaps even a touch idiosyncratic, so that they may be relegated to our archives as curious but apparently disconnected occurrences – will we look back upon in 78 years’ time as but the first hint of a most rich and wondrous phenomenon? The responsibility is left to us to never discard any of the evidence we accrue along the way, no matter how puzzling now, as we continue our journey in solving the mysteries of the Universe.
