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Feasibility and benefits of pulsar planet characterization
Abstract
Planets orbiting neutron stars seem to be rare, but all the more interesting for science due to their origins. Characterizing the composition of pulsar planets could elucidate processes involved in supernova fallback disks, accretion of companion star material, potential survival of planetary cores in the post-MS phase of their stars, and more. However, the small size and unusual spectral distribution of neutron stars make any spectroscopic measurements very difficult if not impossible in the near future. In this work, we set to estimate the feasibility of spectroscopy of planets orbiting specifically pulsars, and to review other possible methods of characterization of the planets, such as emissions from aurorae.
1. Introduction
The first ever confirmed extrasolar planets were discovered by Alexander Wolszczan and Dale Frail in 1992 around the stellar remnant PSR B1257+12. The existence of planets around such an extreme stellar object caused much surprise, and yet they have received comparably little attention after discoveries of planets around main sequence (MS) stars, starting with 51 Pegasi b (Mayor & Queloz 1995). Two more pulsar systems have been discovered up to date (Sigurdssson et al. 2003, Bailes et al. 2011), but pulsar planets remain rather understudied. This is understandable due to the apparent scarcity of these systems, the difficulty of learning anything but the mass of the planet(s) through pulsar timing, and the absence of direct relevance for search for Earth-like planets and conditions for life. However, their indirect importance may be high (not speaking of direct relevance for many other fields), and the formation, evolution and characteristics of pulsar planetary systems may prove relevant even for the popular topic of searching for “Earth 2.0”.
To start with their formation, there are five basic ways how a pulsar may acquire planets: i) they could be remnants of planetary cores of objects formed in-situ, ii) they could be objects formed in-situ from the fallback debris after a supernova explosion, iii) they could be objects formed in-situ from a debris disk from a merger of two white dwarfs, which also gave existence to the pulsar, iv) they could be remnants of a stellar companion that lost most of its mass either to the pulsar, or during the supernova explosion, or v) they could be captured objects, most likely from a companion star, less likely rogue planets. Podsiadlowski (1993) used finer criteria to describe different formation processes, and summarized the following options: planet survival; fallback disk origin; WD-WD/WD-NS merger; disrupted companion forming a disk; planet capture; evaporation of a stellar companion; ablation of a close binary during the supernova explosion; Be binary model (where a massive disk is accreted from a massive companion); massive binary model (where a circumbinary planet is dragged in during the Thorne-Żytkov phase); TŻO deflation (where the TŻO envelope deflates to form a massive disk); protostellar disk capture by a millisecond pulsar; planets surviving around an overmassive WD. We refer the reader to study the possible frequency and predictions of the individual models in the Podsiadlowski (1993) paper.
Considering the PSR B1257+12 system (Wolszczan & Frail 1992, Wolszczan 1994), Podsiadlowski (1993) concluded that a white dwarf merger is the likeliest scenario. Margalit and Metzger (2016) proposed a formation by tidal disruption of a C/O white dwarf companion by the pulsar, specifying a more general companion disruption scenario (Yan et al., 2013) and providing valuable scenarios of disk evolution for both WD-NS merger disks and supernova fallback disks.
PSR B1620-26 b is a circumbinary planet orbiting a pulsar and a white dwarf, and likely formed around the white dwarf precursor, with its system later captured by the pulsar, giving rise to a binary, while the pulsar’s original stellar companion was ejected (Sigurdssson et al. 2003). In a globular cluster with high star density, where this system is present, such an event is more likely than in the galactic disk. Finally, the PSR J1719-1438 system contains most likely a remnant of a disrupted WD companion that narrowly avoided its complete destruction, based on its minimum density (Bailes et al. 2011).
The nearly coplanar orbits of the first three discovered pulsar planets around PSR B1257+12 (REF) suggest formation in-situ. The possibility of an in-situ formation, especially by a WD-WD/NS-WD merger or companion disruption, is further supported by the discovery of a circumstellar disk of the magnetar 4U 0142+61 (Wang et al. 2006), and a tentative asteroid belt around the millisecond pulsar B1937+21 (Shannon et al. 2013). There is also evidence of an asteroid or in-falling debris around PSR J0738−4042, a middle-aged, isolated radio pulsar (Brook et al. 2013).
But we cannot completely discount the option of planetary cores surviving a supernova explosion, however unlikely it seems, and although it’s not applicable to planets of recycled pulsars. Podsiadlowski (1993) notes that in case of an asymmetric explosion, if the neutron star receives a kick of the order of the orbital velocity of the planet (and in a direction similar to the planet’s motion during the supernova explosion), the stellar remnant may retain the planet, which would otherwise become gravitationally unbound in case of a symmetric explosion. In this scenario, the planets’ composition would likely be heavily altered by the event. Not only would likely only cores of massive and preferably distant planets survive, but the conditions during a supernova explosion, especially the strong neutrino flow, could change the core’s chemical make-up as well, but a detailed model of the compositional changes is out of the scope of this study.
Planets formed from the supernova fallback material – if possible despite its low angular momentum – would also exhibit likely very distinct properties; we could expect metal-rich composition and a variety of short-lived isotopes. Planets arising from WD disruption disks can be expected to have a predominantly carbonaceous composition – essentially to be “diamond planets” (Margalit & Metzger 2016, Kuchner & Seager 2005). On the other hand, planets captured after the explosion would not possess the above-described distinct properties. Finally, planets arising directly from WDs would be recognizable by their extreme density.
Recently, another formation possibility has been mentioned by Greaves & Holland (2017), based on observation of the Geminga pulsar’s interaction with the interstellar medium (ISM). ISM dust grains seemed to be able to penetrate into the pulsar wind nebula. With enough infalling dust, a disk around the pulsar might form. While ISM in general is composed mainly of hydrogen and helium, the dust particles contain a lot of oxygen, iron, magnesium and other heavier elements (Pinto et al. 2013), so it is conceivable that such a disk would provide a planetary-forming environment. These planets would likely manifest properties akin to those orbiting MS stars.
Could pulsar planets possess atmospheres? The answer to that question depends on the star-planet distance and the formation mechanism. Detailed models that are out of the scope of this paper are needed to answer it more reliably; what follows in the rest of the paragraph remains pure speculation insofar. We expect that captured planets could hold onto their atmospheres, if their orbital separation is sufficiently large. The chance of atmospheres on cores surviving the supernova explosion or formed around the pulsar seems low, even if there is sufficient fallback material. Planets originating from a companion disruption disk might be able to form an atmosphere, most likely CO-dominated (Kuchner and Seager, 2006). But since most of pulsars’ spin-down energy is released in the form of relativistic particles – pulsar wind –, any atmosphere might be quickly eroded unless the planet also possessed a magnetic field. This particular case would increase the chances of characterization.
Patruno and Kama (2017) have recently investigated the survival of pulsar planets’ atmospheres and the potential habitability of these worlds. __________________(doplnit)
Do the great distance of known pulsar systems and the faintness of the light source present insurmountable obstacles for now and the near future, or could they be resolved? We try to provide estimates for expected planetary characteristics and future observation in the next sections of our paper.