Planet Hunting Toolkit V: Pulsar Timing April 2, 2009Posted by CosmicThespian in Toolkit.
The first planets discovered outside of our Solar System were found in what is probably the least likely place astronomers would have expected to find them: in orbit around the fading embers of a now dead star.
The most massive stars in the Universe do not fade away quietly. These stars, about eight or more times the mass of our Sun, end their lives in one of the most violent events in the Universe: a supernova! A supernova occurs when a massive star can no longer fuse elements in its core and provide the energy necessary to support its own bulk. When that happens, the star begins to implode. The core is crushed to a hyperdense state and the infalling material effectively bounces and sends a supersonic shockwave rippling back up through the star. The result is a cataclysmic explosion. The energy released in a supernova is roughly equal to the total amount of energy our Sun will radiate in its entire 10 billion year lifetime. Seen from across the Universe, a supernova can temporarily outshine its host galaxy. Imagine a single star shining with the brilliance of a hundred billion stars for nearly a month!
For stars less than about twenty times the mass of our Sun, the stellar core is left behind to become what’s called a neutron star. The densities and temperatures inside the remnant core are so high that protons and electrons actually fuse together to form neutrons. The state of the matter beneath the surface isn’t well understood – neutron stars stretch the limits of our understanding of physics. What we do know is that a neutron star has roughly twice the mass of the Sun but is only about 15 miles in diameter which means it would fit comfortably within the limits of a small city. The gravity around a neutron star is so strong that an object dropped from just one meter above the surface will strike the star at 4.3 million miles per hour!! The density is such that one teaspoon of material from the surface would weigh the same as 15 times the world human population. It’s no understatement to say that neutron stars are among the most exotic objects we know of.
But, what does this have to do with finding planets?
Well, as the core collapses to form a neutron star, the conservation of angular momentum demands that it increase its rotation speed. This is exactly the same principle that causes a figure skater to spin faster as she brings her arms closer to her body. Additionally, the increased magnetic fields surrounding the star can form jets of highly energetic charged particles to shoot out from the magnetic poles. As the star spins around, this beam of particles sweeps out an arc through space, much like a lighthouse. If the Earth happens to lie in the path of the beam, the star will appear to us to be “pulsing”. Hence, astronomers call these objects pulsars.
Pulsars can spin incredibly fast. The fastest pulsars have a rotation period of just a couple of milliseconds (the so-called millisecond pulsars). Imagine a star the size of a city spinning roughly a thousand times a second! But what’s of greater interest to us is how steady pulsars are. The accuracy of the pulsed beams coming from these stars can rival that of atomic clocks on the Earth. In fact, when the first pulsar was discovered, the steadiness of its radio pulses caused some to speculate (half-jokingly) that it could be a radio beacon from extraterrestrials. Hence, the object was nicknamed LGM-1, an acronym which stands for “Little Green Men”.
Because the pulsars are so steady in their timing, periodic deviations could signify the presence of an unseen body in orbit. Much like a star’s absorption spectrum will be doppler shifted by an orbiting world, a similar thing can happen to the pulse train from a pulsar. As the pulsar is tugged towards the Earth, the time between pulses will shorten slightly; as it moves away, the pulse train gets “stretched out” causing the pulsar timing to get longer. If this happens with a fixed period, that means the pulsar is moving back and forth in space which indicates that something – a planet – is pulling it around.
The very first planets ever discovered were found in orbit around a pulsar. In 1992, astronomer using the Arecibo Radio Telescope in Puerta Rico discovered just such a periodic variation in the timing of the pulsar PSR B1257+12, a millisecond pulsar 980 light-years away in the constellation Virgo. Analysis of the variations indicated that not one, but two planets were in orbit around the pulsar! Further study eventually revealed a third world in this exotic system.
Only four planetary systems have been found around pulsars. Their existence is something of a mystery. It was surprising to astronomers to find planets in orbit around a neutron star; one would think that any planets wouldn’t have survived the supernova explosion. Either they were able to withstand the shockwave of the supernova or they formed from the material that was blown off after the explosion; either hypothesis is equally intriguing.
One of the most fascinating findings in the past fifteen years of exoplanetary research is that planets continuously turn up in places we would never expect to find them!