It has been an exciting month for planets. Just days after the announcement of a detection of phosphine in the clouds of Venus, another first in planet discoveries was declared. The new find is the first planet observed to be orbiting a white dwarf; a dead star that is much smaller than the planet it hosts.
Planet WD 1856+534 b was first spotted by the NASA’s Transiting Exoplanet Survey Satellite (TESS) and confirmed with a series of observations from ground-based telescopes. The results showed the light from a white dwarf being periodically dimmed by a staggering 56% for brief 8 minutes.
For comparison, one of the easiest exoplanet types to detect is a hot Jupiter that would typically cause a 1% dip in brightness of its star over a period of a few hours.
This suggested a Jupiter-sized planet was closely orbiting a white dwarf that was similar in size to the Earth. Light from the white dwarf is obscured each time the planet passes in front of (or transits) the dead star’s surface on its orbit. Interestingly, the light dip is shaped like the letter V, showing a gentle gradient decreasing and rising from the maximum occultation. The lack of a sharp drop in brightness implied the planet’s orbit was slightly inclined so that it grazed the white dwarf’s surface and only obscured part of the much smaller star.
Although certainly unusual, WD 1856 b is not the first planet known to orbit a smaller star. The first extrasolar planets to be discovered orbit another type of stellar remnant known as a neutron star†. While white dwarfs typically have sizes similar to a terrestrial planet, neutron stars have city-sized diameters of order 10 km.
The fact both these cases involve dead stars is no coincidence. In order to orbit, the mass of the planet must be much less than that of the star.… Read more