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. This usually results in the star being physically larger, but this rule breaks down once the star dies.
Stars die once they can no longer fuse elements to create energy. What happens next depends on the mass of the star. The most massive stars form either black holes or neutron stars, while stars similar to the Sun in mass form white dwarfs. All these remnants are incredibly dense, giving them a high mass compressed into a tiny radius.
In addition to being a different flavor of stellar remnant, there is another feature that separates the discovery of WD 1856 b from its dead star orbiting predecessors. This planet likely formed while the white dwarf was a young star, in the same fashion to the planets of our own solar system. It then survived the death of its star to continue orbiting once the star become a white dwarf. This gives us a peek at a possible future for our solar system.
When massive stars end their life to become neutron stars or black holes, they expel material in a colossal explosion known as a supernova. Any orbiting planet would be unlikely to survive such an event. Planets orbiting neutron stars are therefore thought to be a second generation of planet formation that begins after the neutron star was created.
But a white dwarf does not first undergo a supernova explosion. However, that is not to say it is a trivial experience for the orbiting worlds of a dying sun. As a star like our sun nears the end of its life, it will swell to become a red giant.
Cases do exist in binary star systems where the expanding radius of the red giant has enveloped its stellar sibling. Dragged on by the enveloping stellar material, the second star rapidly spirals inwards. As the distance between the stars decreases, gravitational potential energy is released. This is the same energy that is released when a penny is dropped into a well. The potential energy of the penny is turned into speed as it moves closer to the Earth’s centre. For binary stars, this energy helps to blow away the outer layers of the red giant to leave the white dwarf. If the assistance boosts the dispersal of the outer layers sufficiently, the binary pair still exist but on a very tight orbit that lasts between hours and days.
However, such a scenario does not fit the evidence for WD 1856 b. Firstly, the planet’s orbit is not super short, taking about 1.4 days. This gives the system a lower mass and longer orbital period than any similar example. Secondly, the potential energy released by the inward spiral of a planet is too small to quickly disperse the outer layers of the red giant. The planet would therefore crash into the forming white dwarf before the enveloping layers could be ejected and remove the drag.
The more likely scenario is that WD 1856 b was orbiting far enough away from the star to avoid the red giant envelope, perhaps in a similar location to our own gas giants. But as the mass was lost from the red giant as its outer layers gradually dispersed, the planet’s orbit could have become unstable. If the resulting path was an elliptical orbit that took the planet close to the white dwarf, the varying gravitational tug from the dead star would have steadily dragged the planet onto a closer orbit.
This discovery suggests that the outer planets of our own solar system may survive the sun’s demise. In truth, there is not a lot of hope for the Earth. The estimated radius of the sun’s red giant phase hovers just around the Earth’s orbit, suggesting the Earth will either be enveloped or sit incredibly close to the sun’s bloated surface. Either way, it will be good day to be off-world.
An interesting twist might be the fate of the hypothetical ‘Planet 9’ that has been suggested to exist beyond Neptune. Simulations of the end of the solar system propose scenarios in which Planet 9 is scatted onto a closer orbit around the ex-sun white dwarf. The prospect of such frozen worlds acquiring a second phase of evolution on a possibly more temperate orbit is intriguing, and perhaps future discoveries around white dwarfs may reveal evidence for the formation of these distant planets.
†: The first discovered exoplanets are usually described to be orbiting a pulsar. A pulsar is a type of neutron star whose high energy jets are orientated such that they pass over the Earth to give rapid pulses as the neutron star rotates.
