Astronomers have found another dozen extremely rare exoplanets orbiting neutron pulsars: it is still not clear how they orbit such extreme objects.
The first exoplanet discovered in the early 1990s orbited a pulsar, a neutron star sending regular, short radio pulses to Earth. That was a stroke of luck: planets near pulsars are extremely rare, and it is still not clear how they can appear in such extreme conditions. New results from scientists at the University of Manchester point to this. Liliana Nițu will be presenting them at the National Astronomers Meeting (NAM 2022) taking place these days in the UK. A short summary of the work is given in a press release from the NAM 2022 organising committee.
The biggest stars at the end of their lives turn into black holes, and those with insufficient mass to do so turn into neutron stars. These objects are of incredible density: with a mass comparable to that of an average star, they may be only a couple of tens of kilometres in diameter. Some are spinning extremely fast and emit strong, narrow radio bursts from their poles. At times when this beam is directed towards Earth, we see the star as a burst of light, a periodic pulsar.
One such pulsar is PSR 1257+12, 2300 light-years from the Sun, which emits with a frequency of about 160 hertz. It was the first planet detected outside the Solar System, the first confirmed system of B and C exoplanets, in 1992. Today it is known that there are three planets there and they are all in the super-Earth class, having a rocky surface and several times the mass of the Earth. However, it is still not clear how they could have come from such an extreme object as a pulsar.
In any case, it is very rare: of the several thousand exoplanets known today, only about half a dozen are in pulsar systems. And only 0.5 per cent of pulsars have discovered planets. A new search for such worlds has been conducted by Liliana Nitu and her colleagues, identifying ten possible candidates that can be confirmed by additional observations. The most interesting of these was the pulsar system PSR J2007+3120, which appears to contain at least two exoplanets. Both are several times more massive than Earth and make a complete revolution around PSR J2007+3120 in 1.9 and 3.6 years.
The search has covered possible planets of up to 100 Earth masses and with orbital periods of 20 days to 17 years. At the same time among the found candidates it is not possible to single out a "preference" in one direction or the other. The only peculiarity of the planets at the pulsars, which astronomers have noticed, are their strongly elongated orbits. Both the Sun and the vast majority of other systems have planets that follow circular orbits for the most part, while the pulsars have highly elliptical orbits. This may help solve the mystery of their origin.
Scientists are currently considering three hypotheses for the appearance of planets near pulsars. It is possible that such worlds could be the remnants of a nearby star that died in the same supernova explosion that produced the pulsar itself. Alternatively, they may be formed from the dust and debris of a neighbour destroyed in an explosion. Finally, pulsars can sometimes pick up passing planets, capturing them by their gravity.