Astronomers search for planets around pulsars
The team of astronomers comes from the University of Manchester's Jodrell Bank Center for Astrophysics. Jodrell Bank has a group working on Pulsars and time domain astrophysics. Pulsars are objects of interest for a number of different reasons, and Jodrell Bank monitors 800 pulsars as part of their work.
The team is presenting the results in a paper titled “A search for planetary companions around 800 pulsars from the Jodrell Bank pulsar timing program ”. The first author of the article is Iuliana Niţu and the article will be published in the monthly announcements of the Royal Astronomical Society.
Jocelyn Bell Burnell, Northern Ireland astrophysicist, discovered the first pulsar in 1967. It took some time for her and another astrophysicist to figure out what they were. There was the usual speculation about alien sources, but once other pulsars were discovered and studied, it became clear that they were natural objects.
A problem with the transit method is its inherent selection bias. It is much easier to detect large planets because they block more star light. It is also easier to find planets orbiting close to their stars because they orbit faster and cause dips in starlight more frequently.
This new effort to find exoplanets around 800 pulsars is unlike other hunting efforts. planets. This effort is not a new survey or monitoring program. Instead, it relies on researching existing pulsar data at the Jodrell Bank Center. “The dataset used in this work is composed of observations of approximately 800 pulsars from the Jodrell Bank pulsar timing database,” explain the authors.
But what is the probability of finding more exoplanets around the pulsars? Pulsars are extreme objects with long histories punctuated by episodic catastrophes. "The apparent rarity of systems such as that of PSR B1257 + 12 could be a consequence of the extreme conditions in which pulsars are formed," the authors write.
Pulsars are neutron stars and neutron stars have calamitous origins. They start out as massive stars between about 10 and 25 solar masses. At the end of their regular fusion life, these stars explode as supernovae and then collapse into ultra-dense neutron stars made of neutron degenerate matter. It is highly unlikely that any planet will survive this.
"The resulting system would consist of a normal pulsar with planetary companions in eccentric orbits," they write, although these types of planets would be very rare. A second scenario may be more likely. In this case, the supernova ejects a huge amount of material when it explodes, causing it to explode into space at high speed. But some of the material may not escape the gravity of the remaining neutron star. Instead of dispersing, it would form a protoplanetary disk and the planets would form through accretion. In this case, “… a normal pulsar is expected, surrounded by planets of relatively small mass in circular orbits,” explain the authors.
A third scenario is also possible. In this case, a planet would actually be a remnant of a neutron star in a binary pair of neutron stars. One of the neutron stars interrupts the other or partially evaporates the other. The remaining core would thus become a planet, made almost entirely of diamonds. These are just three of the possibilities for planet formation around pulsars. One of the rationale behind the search for multiple pulsar planets is to narrow these possibilities into a better understood picture. "Overall, there are a large number of proposed formation pathways of planets around pulsars, and thus large-scale searches of planetary mass companions and their orbital parameters are crucial in limiting and determining the feasibility of various models," explain the authors.
Despite the accuracy of pulsar timing, there are still some problems. One type of noise can creep into the measurements. “… The detectability of the planets around the pulsars is also limited by the presence of the so-called 'timing noise' which manifests itself as a long-term red noise process in the pulsar rotation. This poses an additional challenge in the search for planetary companions, as it can not only mask binary signatures, but also mimic them, ”the authors write.
Before the team could get the results, they had to model the effect a planet has on a pulsar. "When a pulsar is part of a binary system (with a star or a planet), it rotates around the system's center of mass, moving relative to the observer on Earth," they explain. This movement creates a slight delay in the signal reaching the Earth. This delay is called the Rømer delay.
The team of researchers used these factors and many others to develop their analytical method. There are limits needed in jobs like this, and the most important is the masses of exoplanets. "We have placed limits on the projected masses of any planetary companion, which reaches a minimum of 1/100 of the mass of the moon (about 10-4 Earth masses)." While this is a limitation, it is an extraordinarily small planet to be detected.
"We have found that it is highly unlikely that two-thirds of our pulsars will host companions of mass greater than 2 ~ 8 Earth masses," says the team. "Our results imply that less than 0.5% of pulsars could host terrestrial planets as large as those known to orbit PSR B1257 + 12 (about 4 Earth masses)." PSR B1257 + 12 is the first pulsar around which planets were found in 1992. It serves as a kind of reference point for pulsar planet systems.
There is at least one caveat to these findings though, and concerns the low-mass planets. "... however, the smallest planet in this system (about 0.02 Earth masses) would not be detectable in 95% of our sample, hidden by both instrumental and intrinsic noise processes ..." The team also points out that it is not clear whether small planets like this might exist in isolation.
15 of the pulsars in the sample showed some irregularities, but they weren't necessarily planets. The team explains that the severe magnetosphere around pulsars can cause irregular periodicities. "We have detected significant periodicities in 15 pulsars, however, we have found that intrinsic quasi-periodic magnetospheric effects can mimic the influence of a planet and, for most of these cases, we believe that this is the origin of the detected periodicity."
In their final analysis, pulsar planets appear to be very rare. Only a single pulsar among 800 is a likely candidate for hosting planets. “We believe that the most plausible candidate for the planetary companions in our sample is PSR J2007 + 3120.”
PSR J2007 + 3120 could host a pair of planets. The evidence for the second planet isn't as strong and may just be noise, but it can't rule out a population of much smaller planets. Some of these planets may be hidden in the noise. "We, therefore, confirm the hypothesis that the formation of planets around pulsars is rare, and PSR B1257 + 12 is a special case", they conclude. For now, it remains the only known pulsar to host Earth-sized planets.
As for habitability, it is extremely unlikely. The region around the pulsars is extremely harsh. Powerful magnetic fields could devastate any planet nearby. And pulsars are neutron stars, so there's no fusion going on. They are little more than ash, although they can still be extremely hot. Some of the planets found around pulsars are nothing more than the exploded remnants of a pulsar's stellar companion and may be made of pure diamond. Others are captured objects. But this study has never been about habitability. It aims to probe some of the most unusual objects in the Universe.
