PLANETS AROUND PULSARS

The first planets orbiting a star other than the Sun were discovered around an old, rapidly spinning neutron star, PSR B1257+12, during a large search for pulsars conducted in 1990 with the giant, 305-m Arecibo radiotelescope. Neutron stars are often observable as the so-called pulsars because of their highly beamed, regularly pulsing radio emission. They are extremely small and dense leftovers from supernova explosions that mark the deaths of massive, normal stars.

Because of their exotic physics and very attractive applications as probes of various processes in physics and astrophysics, pulsars have been routinely searched for since the time of their discovery in 1968. But how does one detect planets around such an object? The answer has to do with the fact that pulsars, especially those with a very rapid spin, represent the most precise natural clocks in the Universe. The rotating beams of radio emission, rigidly attached to the star create geometry, which is analogous to that of a beam of light emerging from the lighthouse. As a result, the observer records regular flashes or pulses of radio emission appearing periodically as dictated by the neutron star spin. These periodic pulses can, in fact, be favorably compared to the ticking of the best atomic clocks on Earth!

Imagine now, that a rapidly spinning pulsar is orbited by a planet that makes it wobble around the center of mass of the system. Because the pulsar does not stand still in space, its pulses arrive at the telescope with a tiny, variable delay that perturbs the ticking of our pulsar clock in a measurable way. Millisecond departures from the clock's regular behavior can be caused by Earth-sized planets and are easy to detect with an atomic comparison clock. In fact, even microsecond clock irregularities can be measured with the pulse timing technique, which means that it has a capability to detect large asteroids!

The three initially discovered pulsar planets have masses of 0.02, 4.3, and 3.9 Earth masses and their orbits are inclined at ~50^◦ with respect to the plane of the sky. Compared to our solar system, the three planets would fit within the orbit of Mercury with the respective orbital periods of 25, 66 and 98 days. There is some evidence that the pulsar may have an asteroid belt that appears to be located well beyond the orbit of Mars, just like it happens in our solar system. In addition, gravitational perturbations between the two larger planets have been detected and then used to measure true masses and orbital inclinations of these objects, as described above. The fact that the planets have almost coplanar orbits represents a convincing evidence that they have evolved from a protoplanetary disk in the process that was probably similar to the one that created planets around our Sun.

The pulsar planet system represents not only the first one detected since Tombaugh's discovery of Pluto in 1930, but it continues to serve as a dramatic illustration of possibilities to extract information about alien planets given a sufficiently powerful observing method. Above all, it has served as a convincing demonstration that, if the planet formation process is robust enough to make and retain planets around a pulsar, it should be even more efficient in making planets around normal stars. This prediction has found its spectacular confirmation in 1995, three years after the announcement of the pulsar planet discovery, when a Jupiter-mass planet was discovered in a tight, 4.2-day orbit around a Sun-like star, 51 Pegasi. In the following years, more than a 100 giant planets have been detected around nearby solar-type stars.