Pedagoguery

Over the past several years, astronomers have discovered dozens of planets outside of our solar system. With the Kepler satellite currently in orbit, that number will only grow (Kepler has discovered 5 exoplanets so far as of this writing). However, some of the stranger planetary systems have been discovered not by Kepler or other conventional means, but by the Spitzer infrared telescope. Through Spitzer and other means, we have found planets around some of the more unlikely stars: neutron stars, white dwarfs, and brown dwarfs.

Many people do not realize that the first exoplanets discovered were actually found orbiting the neutron star PSR 1257+12. As you might guess by its designation, PSR 1257+12 is a pulsar, which was instrumental in facilitating the exoplanets' discovery. You see, as the planets orbit the neutron star, their gravity tugs the neutron star back and forth, causing a noticeable irregularity in the pulses of the neutron star. From this irregularity, scientists were able to deduce three planets, located at 0.19, 0.36, and 0.46 AU around the star. However, neutron stars are formed when a massive star explodes in a supernova. How could planets survive such an event? The answer is they didn't. The original star probably had a radius greater than 1 AU, so none of the current planets date from that time. What happened is this: the supernova ejects most of the mass of the outer envelope, but some of it remains gravitationally bound to the collapsed core. This matter falls inward and forms an accretion disk. It is from this accretion disk that the planets formed. Additional evidence for this theory is found in the neutron star 4U 0142+61, which shows evidence of an accretion disk. The neutron star itself is only 100,000 years old, and it is believed that planets take several million years to form, so the age is right. In addition, the diameter of the accretion dsik is about 1 AU, which is consistent with the orbits of the planets around PSR 1257+12.

The situation in white dwarf systems is very different. The death of a star like our sun is much less violent than a supernova, so some of the planets could conceivably survive. For instance in the case of our solar system, when the sun dies, Mercury and Venus will probably be destroyed, and the Earth stands a fifty-fifty chance of survival. Planets like Jupiter and Saturn may end up losing part or all of their outer atmospheres.

An example of a white dwarf system is G29-38. Evidence as early as 1987 showed that the white dwarf has an accretion disk. Accretion disks around white dwarfs are not unusual, but most of them occur when the white dwarf in question has a stellar companion and G29-38 did not. In addition, spectral analysis of the star showed lines of calcium and iron, heavy elements which should be drawn into the interior of the star by gravity. The only explanation is that these elements continue to rain down on the star from the accretion disk, and therefore the disk itself must be made up of such heavy elements, probably from a left over asteroid that wandered too close and was pulled apart by the star's gravity. More recent analysis of the disk by Spitzer has confirmed that the disk is comprised mostly of silicates. What's more, the silicates match planetary composition rather than interstellar composition indicating that it is in fact the remains of planetary bodies.

Spitzer has also observed what is likely the remains of a Kuiper belt around a very young white dwarf: WD2226-210, which lies at the center of the Helix nebula, a popular target for amateur astronomers. This indicates that distant planets and asteroids can indeed survive the death of a star like our sun.

The final type of unusual planetary system is the brown dwarf. Brown drarfs are the almost-stars – bodies that lack enough mass to generate core temperatures necessary to initiate the fusion of hydrogen into helium. As such, they form much like regular star, and so there is no reason why they couldn't have protoplanetary disks like regular stars. Hundreds of such stars have been found in various astronomical surveys, and many of them are found to have disks. One such system, OTS 44 has enough mass in its disk for a planet the size of Uranus or Neptune to form.

All of this leads us to the knowledge that planets may be much more common than we originally thought.

Next time, black stars.