|Image Credit: Swinburne Astronomy Productions, Swinburne University of Technology|
First, let's go over what the astronomers observed. The team, headed by Professor Matthew Bailes of Swinburne University of Technology in Australia, was looking at a millisecond pulsar. Pulsar are the remains of very massive stars that exploded as supernovae at the end of the stars' lives. Pulsars typically contain about 1.3 times the mass of our sun squeezed into a sphere only a dozen miles across. This is so dense that ordinary atoms cannot exist, and most of the protons and electrons that made up the atoms in the original star merge to form neutrons. We therefore call these very dense remains of massive stars neutron stars.
Many neutron stars have very strong magnetic fields. These fields cause electrons and other particles in space to spiral around at speeds near the speed of light, a process that creates light and other radiation. As the star spins, the poles of the magnet sweep around. Every time a magnetic pole on the star sweeps across our line of sight, we see a flash, or pulse, of light. Neutron stars that are seen to flash are therefore called pulsars.
Just like an ice skater spins faster when he/she pulls her arms in close, stars that are squeezed down to the size of the sun spin faster and faster. New-born neutron stars often spin several times a second! The pulsar in the Crab Nebula (the remains of a star that exploded in 1054) spins at 30 times a second. Over time, as the pulsar's magnetic field interacts with gases in space, the pulsar slows its spinning. We can measure the Crab Pulsar slowing down, and we see pulsars that spin much more slowly than the Crab pulsar.
However, there are a class of very old pulsars which, though they should be spinning very slowly, are spinning at hundreds of times a second! Why are these pulsars spinning so fast? We think it is because these pulsars have companion stars. As the companion stars age, they swell up and get big enough that the pulsar's gravity can pull in the companion star's gas. As this stellar cannibalism continues, the pulsar's spin gets faster (because material is coming from far away and being moved very close to the center of the pulsar -- exactly what caused the pulsar to spin fast in the first place). Eventually, the pulsar will consume or blow away most of the companion star's mass, and we expect a small white dwarf to be left behind.
This idea of how to get pulsars spinning at hundreds of times a second predicts that these fast spinning pulsars should have companions, and most of those should be white dwarfs that have been whittled away to almost nothing. These companions should be detectable. Although they are small in both mass and radius, they still will have gravity that will pull on the pulsar. This causes the pulsar to move in a small orbit. This further means that the pulsar will be closer to Earth some times, and further away other times. When the pulsar is closer to Earth, its light reaches us a few seconds earlier than when it is further away. And since the pulsar spinning is often more reliable and constant than
atomic clocks, it is easy to see if pulses are arriving earlier, then later, then earlier again, then later again, as the pulsar traces out its tiny orbit.
Which brings us to yesterday's press release. Professor Bailes's group was studying one of these fast-spinning pulsars. They were looking for evidence of the companion star that sped up the pulsar's spin by looking for the changing arrival times of the pulsar's radio flashes. And they indeed saw that change. By measuring how early and late the pulses arrived and using some basic laws of gravity and orbits, the team was able to estimate how massive the companion is. The answer: it could be only as massive as our planet Jupiter!
Moreover, this pulsar companion orbits the pulsar every 2.2 hours, which means that it is only about half a million miles away from the pulsar. If we replaced our sun by the pulsar and its companion, the companion's orbit would be about the same size as the sun. This is shown in the artist's impression of the system at the top of this post, where you can see the pulsar (with squiggly lines representing the light coming from its magnetic poles), the companion, its orbit (the dotted oval), and our sun's size on the same scale (the golden circle).
If a companion is that close to a pulsar, the pulsar's gravity will try to steal material from the companion. If the companion is small enough, its gravity will be strong enough to hold on to its material. If it is too big, its gravity will be weak and material will get pulled onto the pulsar. We don't see any signs of any transfer of matter, so it must be pretty small. Remember that this companion could have about the same mass as the planet Jupiter. If we were to put Jupiter in the same orbit as this object, Jupiter's atmosphere would get pulled onto the pulsar. So the pulsar companion has to be smaller than Jupiter. Much smaller, in fact.
Some more calculations show that the largest the companion can be without transferring matter onto the pulsar is about 50,000 miles across. To still have the same mass as Jupiter, it would need to be twice as dense as lead, and even denser than platinum. And remember, this is the minimum density; if the companion is smaller it could be even denser.
So you can see why many people have called this companion a planet. It has about the same mass as Jupiter, it must be smaller in radius than Jupiter, and it is denser than our densest metals. But remember that, whatever this companion is, it had to transfer enough stuff onto the pulsar to make the pulsar spin hundreds of times a second. The amount of matter it takes to do that is far, far more matter than a planet that got too close would have. This companion used to be a star.
As I said above, fast-spinning pulsars are thought to form when the companion star is in the process of dying, because then it can swell up enough for the pulsar's gravity to begin pulling matter over. Most stars in the process of dying end up forming white dwarfs. White dwarfs are very dense (squeezing the sun into something the size of the Earth), and can be made out of either helium or carbon. Carbon white dwarfs are the most common kind. But white dwarfs tend to be about half the mass of the sun or more, not the mass of Jupiter. However, if the pulsar swallowed enough material and nearly swallowed the entire star, it is possible to whittle what was once a star like the sun down to a pile of ashes no bigger than Jupiter. And this object would be far more dense than platinum -- about 40,000 times more dense. So it seems reasonable to guess that the companion to the pulsar is a white dwarf made out of carbon. There are other, more complex arguments, too, and it is far from certain that this white dwarf is not made out of helium. But we cannot see the white dwarf directly, so we can't confirm what it is made out of.
Let's assume we indeed have a white dwarf made out of carbon that is 40,000 times the density of lead but only the mass of Jupiter. What would it be like?
When a white dwarf is first formed, it is hot. Hundreds of millions of degrees hot – this used to be the central fusion reactor of a star. Over time, it will cool off, and in general less massive white dwarfs cool off the fastest. So a Jupiter-mass white dwarf, only 1/500th the mass of a normal white dwarf. should cool off relatively quickly, astronomically speaking.
As a white dwarf gets cooler and cooler, it can begin to crystallize (think of it "freezing"). On earth, one form of crystallized carbon is what we call a diamond. So, we astronomers often say that crystallized white dwarfs are Earth-sized diamonds. In reality, it is not a diamond. Crystallized white dwarfs are a hundred thousand times denser than diamond, and the detailed atomic structure is very different from diamond, too. A ring with a gemstone of white dwarf "diamond" would weigh several hundred pounds. So, the "diamond" term is an analogy, used to explain a white dwarf in a way that paints a picture most people can understand. But is it really a diamond? No.
So, is it fair to call the whittled down, crystallized white dwarf that used to be a full-sized star but now is about the mass of Jupiter a "diamond planet". I think not. The term "diamond planet", to me, suggests something that was always planet sized but made out of diamond. It is catchy, though.
So, DeBeers doesn't need to invest in a rocket to protect their diamond monopoly, and there is no "diamond planet". Too bad. But reality, that we are seeing a whittled down chunk of superdense material, the mass of Jupiter but smaller in diameter, and that used to be a full-blown star perhaps similar to the sun, is just as cool as a diamond planet. Perhaps even cooler.
26 Aug 2011 11:37am CDT: Edited to correct broken link in citation.
M. Bailes, S. D. Bates, V. Bhalerao, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, N. D’Amico, S. Johnston, M. J. Keith, M. Kramer, S. R. Kulkarni, L. Levin, A. G. Lyne, S. Milia, A. Possenti, L. Spitler1, B. Stappers, & W. van Straten (2011). Transformation of a Star into a Planet in a Millisecond Pulsar Binary Science Express : doi 10.1126/science.1208890