White dwarf stars and their entourage of planets and debris

Most stars in the galaxy will end their lives going through a phase of stellar evolution where they become a white dwarf. When stars exhaust their supplies of hydrogen fuel, they undergo a core collapse and shed their outer layers as a supernova. Low to medium mass stars then become white dwarfs, essentially dead stars. White dwarfs are the burnt out remnant cores of stars up to ~8x the mass of the Sun, made up of extremely dense and compact carbon and oxygen. These cores are so dense, they can have the equivalent of the mass of the Sun, squeezed into a volume the size of the Earth. It is thought that around 97% of stars in the galaxy will end their lives in this way. As an interesting side note, while massive stars are responsible for the majority of the brightness in the galaxy, they are much rarer and harder to make than low mass stars, which are far more numerous, but contribute the vast majority of the stellar mass in the galaxy. Given how common white dwarfs are, there is growing interest in researching the properties of these stellar endpoints, and furthermore, the debris discs and even planets that may surround them. Almost all known exoplanets (planets orbiting stars other than our Sun, so outside our solar system) orbit stars that will end their lives as white dwarfs.

White Dwarf star Stein 2051 B and a smaller star, which appear close neighbours. In reality, the white dwarf is a mere 17 ly from Earth, while the small one is 5000 ly away. Credit: NASA/ESA/Hubble

Along with carbon and oxygen, white dwarfs can be decorated with the occasional smattering of heavier elements like magnesium, silicon or calcium, which can appear on their surfaces due to the accretion (collecting) of rocky bodies, possibly planetary debris, landing on their surfaces. Heavy metals present on the surface of a white dwarf would sink very quickly towards the core, in the same way that a rock will sink through water, due to their different densities. For this reason, astronomers would not expect to see any metals on the surface of a white dwarf unless the white dwarf was actively in the process of gobbling something from its surroundings, which I like to call the entourage which accompanies some white dwarfs. This 'something' being eaten may be rocky debris, comets, asteroids, or even the shredded remnants of a planet as it cascades inwards. These are called 'polluted' white dwarfs which are extremely helpful in this regard, as they give astronomers a rare opportunity to probe the interiors of distant planets, which would otherwise not be possible. In addition to the studies of planetary atmospheres, studying planet interiors helps form a fuller picture of their formation and evolution.

To date, the entourage that has been detected around white dwarfs includes giant planets, minor planets (asteroids, moons, comets), debris discs and fragments of all of the above. Entourage is a word that has long amused me, as it conjures up images of celebrities, particularly rappers, in sunglasses (despite often being indoors) surrounded by private security and paparazzi. Almost all known exoplanets orbit stars that will end their lives as white dwarfs. Throughout this stage of stellar evolution, the planets may either survive or be destroyed throughout the transformation. Planetary systems around white dwarfs have now been observed at various stages of destruction, ranging from fully intact planets to those shredded down to their constituent chemical elements. To illustrate just how diverse the planets around white dwarfs can be, my favourite discoveries range from an ice giant (WD J0914+1914 b) orbiting a mere 0.07 AU from its host white dwarf star, to a gas giant (WD 0806-661 b) orbiting at a whopping 2500 AU from its host white dwarf star. Note, 1AU is the distance from Earth to the Sun in our Solar System, which gives an idea how just how diverse these systems can be in both planetary characteristics and system architecture.

Clumps of debris from a disrupted planetesimal are irregularly spaced on a long and eccentric orbit around the white dwarf. Individual clouds of rubble intermittently pass in front of the white dwarf, blocking some of its light. Because of the various sizes of the fragments in these clumps, the brightness of the white dwarf flickers in a chaotic way. Credit: Dr Mark Garlick/University of Warwick

It is a theme I covered during my MPhil project at Cambridge last year, where we were researching the possibility of life emerging and persisting on planets around white dwarfs, and then delving into the challenge of could we detect these habitable planets. White dwarfs can range in temperature, with the cooler end of the spectrum being more common. This has implications on where the Habitable Zone of planets around white dwarfs lies, where this zone characterises the range of distances from a star in which liquid water can survive on a planetary surface, which is determined from the temperature. Too close to its host star, and the high temperatures mean any liquid water evaporates away, along with it many hopes of finding life. Too far away, and the cold means that liquid water turns to ice, which while not ruling out the possibility of life, is currently thought to make it less likely. This is another advantage of white dwarfs- owing to their relative coolness, and steady temperatures over a long period of time, their Habitable Zones are fairly static and close in, about 100x closer than in our own solar system meaning liquid water could survive on planets around 0.01 AU from the coolest white dwarfs, where it would have long since boiled around stars as hot as the Sun. This is again advantageous as it makes detections of transit observations significantly easier (transits are where the planet passes in front of its star causing a blocking of starlight and temporary dip in the star's brightness). Hence, owing to their ubiquity and advantageous observing characteristics, the study of white dwarfs and their surrounding planets is a buzzing and rapidly growing field.

Indeed, our own star, the Sun, will exit its 'Main Sequence' phase of hydrogen burning in about 5 billion years, and become a Red Giant. Another 2 billion years later, its outer layers will be blown off as a supernova, and a white dwarf remnant will be left behind. In a funny way, studies of white dwarf stars are like glimpses of the distant future for our own solar system. For this reason, some people find the study of astrophysics almost philosophical, as stellar archeology can trace the history of past stellar systems, and offer an insight into the future of others. Either way, much food for thought.