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Pulsar Planets

Published on Jul 20, 2006 at 5:12 am. No Comments.
Filed under extrasolar planets, planets.

For a long time, astronomers have believed that planets form as a consequence of star formation. The accretion disk feeding the protostar becomes a disk of material orbiting the young star. This disk, called a proplyd, is the site of forming planets. Otto Struve even proposed that planets are a natural consequence of star formation, not just an occasional thing. So, an intensive search was on to try to find planets around other stars. Several claims of extrasolar planets came forth, but were eventually disproved. Finally, in 1995, Michel Mayor and Didier Queloz, of Switzerlandâ€™s Geneva Observatory, presented the first conclusive evidence of an extrasolar planet orbiting the star 51 Pegasii. But, this wasnâ€™t really the first extrasolar planet found. It was merely the first normal extrasolar planet. What was an utter and complete shock to astronomers, and really blew my socks off, was the discovery in 1992 of two planets around the pulsar PSR 1257+12. Now, let me explain why this was so shocking.First, letâ€™s discuss pulsars. When a massive star, about eight or so times the mass of the Sun or more, dies, it does so in an amazingly violent manner. The core of this star collapses from something about the size of the Earth to an object about the size of a large city. This is an object so compressed that the atoms themselves are destroyed, and even most of the protons are transformed into neutrons. In fact, most of the object is neutrons, so we call it a neutron star. The energy released in forming this object then blows the rest of the star into space in a monstrous explosion. This explosion, called a supernova is unbelievably violent. A supernova momentary is so bright that it is as bright as an entire galaxy worth of stars. The radiation and blast is pretty much enough to blow up anything nearby. About all that can survive is the core of another star. I might say more about supernovae in a later post. For now, we are interested in the thing left behind: the neutron star. This neutron star has a massive magnetic field, and this magnetic field will likely be offset somewhat from the rotational axis, like so many rotating bodies with magnetic fields. The magnetic fieldâ€™s poles then serve as radiation sources, as radiation and particles stream outward. As the rotating neutron star spins, these beams sweep around the sky. If you happen to be somewhere in space where these beam sweep, then you will see a flash of radiation (mostly radio) at regular intervals. This is a pulsar. In other words, a pulsar is a neutron star whose rotation carries these beams of radiation past Earth on a regular basis.

So, a pulsar is a neutron star. A neutron star is the byproduct of a supernova. A supernova is an amazingly powerful and violent explosion. Such an explosion should totally disrupt any planets that might be orbiting a star. So what the heck is a planet doing orbiting a neutron star???? Well, to make it even more confusing, Aleksander Wolszczan and Dale Frail, the discoverers of the pulsar planets around PSR 1257+12 announced the discover of two planets orbiting the pulsar! Adding even more confusion, in 1994, the same team of astronomers announced a third planet around this pulsar. Then, in 2002, they announced yet another object orbiting PSR 1257+12. All of these objects were found by the tiny gravitational effect that they have on the pulsar. As the objects orbit the pulsar, they cause it to shift back and forth slightly. The pulsar pulses at very regular intervals. But, if the pulsar moves back and forth by a little bit, careful measurements can detect the tiny shift in arrival times of the pulses. These pulses move at the speed of light. The speed of light is FAST, but is it not infinite. So, if the pulsar is slightly farther away, then the pulses arrive slightly late, and if it is slightly closer, then they arrive earlier. Carefully studying the timing of these pulses shows the slight regular shift back and forth, revealing the presence of the planets.

The third planet found is actually the closest to the pulsar, and has a mass of only 2% that of Earth (about double that of our Moon). Such a tiny mass makes for a tiny shift in the pulsar, making it a very difficult thing to detect. The second and third planets are the ones first discovered. These planets are similar in size, the second one being 4.3 times the mass of the Earth, and the third one being 3.9 times the mass of the Earth. They are big enough and orbit close enough together that they have effects on each otherâ€™s orbits as the pass (the second planets orbits once every 66.5 days, and the third one once every 98.2 days). This provides proof positive that they are really there, and that they are not an artifact of measurement. The fourth object is really tough to call a planet, since it is a truly tiny thing, only 0.04% the mass of the Earth. That makes it even smaller the Pluto, probably closer to the size of a large asteroid like Ceres. This fourth object is also much farther out than the others. It is 2.7 AU from the pulsar, whereas the other planets are located at 0.19 AU, 0.36 AU, and 0.47 AU (AU stands for Astronomical Unit, the average distance between the Earth and the Sun). It is possible the this fourth object is the larger of a host of similar things located at this greater distance from the pulsar. If so, it may signify the presence of an asteroid belt or a comet cloud, similar to our Kuiper belt.

Finding one object would be remarkable, as it is hard to imagine how such an object could survive a supernova explosion. Four, or more, is just plain bizarre. So, is this simply an anomaly? After all, the universe is pretty big, and there are a lot of things in the universe. So much so, in fact, that if something can happen, even if it is highly improbable, there are enough chances for it to happen that somewhere it likely will happen. So, is PSR 1257+12 such an anomaly? Perhaps. But, perhaps not.

The problem with that interpretation is that another pulsar planet was found associated with pulsar PSR B1620-26 by Stephen Thorsett in 1993. This one is another weird system. It turns out that pulsar PSR B1620-26 was already known to be a binary system consisting of a pulsar and a white dwarf orbiting one another. A white dwarf is what you get when a star smaller than 8 solar masses dies. It is the collapsed core of a star, but it is only collapsed to about the size of the Earth. The outer parts of the star are pushed off into space, but in a rather gentle manner compared with a supernova. Thorsettâ€™s finding was of a third body, about 2.5 times the mass of Jupiter, orbiting the pair of stellar remnants. It is believed that this strange system results from the merger of two prior stellar systems, one of a neutron star and ordinary star, and a second system consisting of an ordinary star with a gas giant planet. When the stellar systems passed close to one another, the original companion of the neutron star was ejected, and the new star and neutron star settled into orbit around one another while the gas giant went into orbit around the pair of them. Eventually, the new star died and formed a white dwarf.

But, what about PSR 1257+12? The same thing didnâ€™t happen there. Several theories have been proposed. One is that perhaps the neutron star â€œstoleâ€ the planets from a passing normal star system. But, that canâ€™t really work, since it would be virtually impossible to steal a whole planetary system! Another proposal has been that the original star had originally had several gas giant planets with large rocky cores. The supernova stripped these gas giants of all but their cores, and these cores then spiraled close to the neutron star. That might work for one or two such objects. However, it doesnâ€™t explain the smallest objects like the one found in 2002. It definitely doesnâ€™t explain how a belt of such things might exist, which is what this smallest object might signify. This leads to the possibility that these are secondary planets, formed out of fallback from the debris cloud created by the supernova.

The suggestion that pulsar planets might be formed as secondary planets is strengthened by the announcement just a couple months ago by a team of astronomers let by MITâ€™s Deepto Chakrabarty of a dusty disk of material, similar to a proplyd, surrounding the pulsar 4U 0142+61. This disk is believed to hold about 10 or so times the mass of the Earth worth of material. If left alone, it may form one or more planets.

So, this then leads to a new question: How common are pulsar planets? They are hard to detect, because they will only be detectable if they orbit neutron stars whose radiation beams pass by Earth. But, most of the neutron stars will have radiation beams not too far from their rotational poles, so most motion of the pulsar due to planets will be side-to-side, which will not produce any detectable shift in pulse times. Only back-and-forth motion is readily detectable, and that only if the beams of radiation originate from points on the neutron star fairly far from the poles. So, this limits the number of possible cases where we could even hope to detect planets. That weâ€™ve found planets this way with PSR 1257+12 is amazing. PSR B1620-26 is another strange case. But, 4U 0142+61 is interesting because the dust disk was not detected by the shift of the pulsars, but rather by using the orbiting Spitzer infrared telescope. That we have found this disk suggests that perhaps planets may form fairly regularly around pulsars. Clearly, this is an area that needs lots more work.

-Astroprof