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The (Unusual) Making of a Planet

Paul Kohlmiller


When a star of sufficient size explodes in a nova, the remnant can be a pulsar. Pulsars are neutron stars, stars that are so dense that atoms are crushed to the point that only neutrons exist. Imagine shrinking the size of the sun to the size of the city of San Jose. The pulses come from beams that come out from the magnetic poles. These beams of (typically) X-ray or radio electromagnetic radiation sweep around as the star rotates. So from Earth they appear to be flashes.

Pulsars are so consistent they are as accurate as atomic time clocks. This means that when something perturbs them, like another object orbiting the pulsar, it is easy to characterize what must be causing the blips in the data.

Of course, many stars are binary stars. What happens to the companion star? Well, one outcome is that the mass of the second star gets pulled bit-by-bit onto the neutron star. These are called “Black Widow” pulsars. Now, think about what that kind of ablation means. The companion star gets smaller and smaller. It might have started out as a sun-like star but it loses mass. Eventually it will no longer have the mass necessary to continue nuclear fusion. Then it gets even smaller until it becomes ... Jupiter! Or at least something about that size.

In Science magazine (Sept. 23, 2011), researchers describe finding a planet around a pulsar (PSR J1719-1438) that is spinning very fast - one rotation in 6 milliseconds or less. Its companion can be detected by perturbations in the pulses that can be detected on Earth. As a star the companion probably got to be a carbon white dwarf.

Planets around pulsars are not new. The very first exoplanets were detected around pulsar PSR B1257+12. They are often considered to be “not real planets” because of the strange nature of their planetary system. On the other hand, these strange exoplanets are the lowest mass exoplanets discovered to date, as small as the Moon.

Although the planetary companion of PSRJ1719-1438 is about the size of Jupiter, its mass is about 20 times that of Jupiter. This suggests it is not made up of hydrogen and helium, the best guess is carbon. At these pressures, the carbon would be in a crystallized form, hence this planet’s nickname “The Diamond Planet”.

In Science magazine, one article is a summarization of a more technical paper. In this case the technical paper is written by M. Bailes et. al. (where the “al” consists of 17 coauthors). A question raised in the technical paper is why should some millisecond pulsars (MSP) have companions and some do not. The distinction, as you might expect, is how close the companion is when it starts losing mass to the larger star. If it is far enough away, say several solar radii, the companion will survive.

But should this pulsar companion be considered a planet at all. You will recall the 2006 attempt to restrict the definition of “planet’ which excluded Pluto, to the horror of school kids around the world. The article for Science was written by Frederic Rasio from Northwestern. He says “The lesson here is perhaps that, as our rapidly improving astronomical instruments make it possible to detect objects of smaller and smaller mass in distant and, sometimes, very exotic environments, any narrow definition of planet will soon become obsolete.”


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