Confirmed: a planet orbiting a white dwarf star

For the first time, a small planet has been confirmed orbiting a white dwarf star.

White dwarfs represent the end of the line for main-sequence stars such as the sun. At the conclusion of their nuclear-burning stage, healthy stars shed most of their outer material, leaving only a dense hot core. A typical white dwarf has a mass equivalent to that of the sun, but is only about the size of the Earth.

Astronomers have long theorised that it was possible for planets to survive the explosive degeneration of a main-sequence star into a dwarf, but evidence has been thin. 

There have been several sets of observations from which researchers have deduced the existence of rubble and gas clouds – both assumed to be the remnants of destroyed planets – orbiting white dwarfs.{%recommended 555%} 

Until now, however, there has only been one tentative identification of intact small planet, or planetesimal, orbiting such a star. That involves regular light-curve variations observed at a white dwarf called WD 1145+017, which astronomers have inferred are the product of dust clouds being thrown off a planetesimal orbiting every four-or-so hours

Now, however, Christopher Manser from the UK’s University of Warwick, and colleagues, present what they believe is much firmer evidence, in the matter of a white dwarf known as SDSS J122859.93+104032.9.

In a study published in the journal Science the researchers detail variation in gas emission lines that occur every 123.4 minutes. Their results were gained using the Gran Telescopio Canarias (GTC) on La Palma, Canary Islands, in two observation sessions roughly a year apart.

The variations were unchanged between the first and second sessions, they report, meaning that they must also have been happening for the approximately 4400 orbital cycles that took place in between.

Manser and colleagues estimate the planetesimal is around 600 kilometres in diameter, and may lie just 200 kilometres above the star’s surface. Its orbit, they suggest, appears to be eccentric – a not unexpected situation, given that at some stage it weathered some enormous explosions as its host entered its final stages of stellar existence.