Strange planets

(News & views on the paper by J. E. Horvath, RAA, 2012, vol.12, 813-816)

Ignazio Bombaci

Dipartimento di Fisica E. Fermi, Università di Pisa and INFN Sezione di Pisa, Pisa Italy bombaci@df.unipi.it

The query for the possible existence of ``other worlds'' (i.e. extrasolar planets using modern terminology) has been one of mankind's great questions for millennia. In Western civilization, this idea dates back to the ancient Greek philosophers, including the atomists Leuccippus (5th century BC), Democritus of Abdera ( 460--371 BC) and Epicurus of Samos (341--270 BC), as well as Aristarchus of Samos (310--230 BC) who is best known for being the first to propose a model that placed the Sun at the center of the Universe with the Earth revolving around it.

In the early modern period, the idea of ``other worlds'' was again proposed by the Italian philosopher Giordano Bruno (1548--1600). Bruno was one of the early supporters of the Copernican heliocentric model, but his cosmological speculations went beyond the Copernican model in proposing an infinite Universe (Bruno 1584) in which the fixed stars were not lights attached to the surface of a sphere surrounding a finite Universe, as Copernicus (and Ptolemy) thought them to be. Instead, they were suns and, like our Sun, they were encircled by planets.

The first extrasolar planets were discovered by Wolszczan & Frail (1992) around the millisecond pulsar PSR B1257+12, during a large search for pulsars conducted with the Arecibo radio telescope. Three years later, Mayor and Queloz (1995) discovered the first extrasolar planet orbiting a solar-type star, named 51 Peg. These landmark discoveries have opened an entirely new exciting research area called exoplanetary astronomy. At the time of this writing, 708 candidates of exoplanets have been detected by radial velocity or astrometric measurements (Extrasolar Planets Encyclopedia; Schneider et al 2011).

The presence of planets around pulsars was unexpected and has raised many stimulating questions. One of the basic questions has to do with the formation mechanism of these pulsar planetary systems (Podsiadlowski 1993; Phinney & Hansen 1993). Pulsars are highly magnetized, rotating neutron stars (the remnants of supernova explosions) emitting a periodic beam of electromagnetic radiation. In particular, millisecond pulsars are believed to be very old (~109yr) and to originate from dead pulsars1 which have been spun up by accretion of matter from a companion star in a binary stellar system (Bhattacharya & van den Heuvel 1991). According to one of the proposed formation scenarios (Podsiadlowski 1993; Phinney & Hansen 1993), it is believed that pulsar planets are second generation planets, i.e. they are formed from a disk of material around the pulsar (neutron star), instead of being the remains of an original planetary system (first generation planets) around the pulsar's parent star that survived the supernova explosion.

Recently Bailes et al. (2011) discovered a Jupiter-mass object orbiting the 5.7 ms pulsar PSR J1719-1438, having an orbital period of 2.177 hours. Assuming that the companion of PSR J1719-1438 fits inside its Roche lobe, Bailes et al. (2011) derived a minimum value ρcmin= 23 g/cm3 for the average density of the companion object. This density is far in excess of the average density (< 2 g/cm3) for gaseous giant planets. This Jupiter-mass object has thus been interpreted by Bailes et al. (2011) as an ultralow-mass carbon white dwarf, and it was assumed that this system has evolved from an ultracompact low mass X-ray binary (see also van Haaften et al. 2012a; 2012b).

A different and unconventional interpretation for the nature and the formation mechanism of the PSR J1719-1438 planetary system is discussed in a paper by Jorge E. Horvath (RAA, 2012, 12, 813) on this issue. The starting point of Horvath's argument is the minimum value of 23 g/cm3 (Bailes et al. 2011) for the average density of the companion of PSR J1719-1438. Horvath goes a step further and assumes that the companion of PSR J1719-1438 is an exotic object composed of strangeness-bearing matter at supranuclear densities (ρ> 2.8×1014g/cm3).

As a first possibility, Horvath considers a planet-mass lump of strange quark matter (SQM) or, in other words, a strange planet.

The possible existence of a class of astrophysical compact objects, which are completely (or almost completely) made of deconfined u,d,s quark matter (SQM), has received wide attention in the last three decades (Alcock et al. 1986; Haensel et al. 1986; Glendenning et al. 1995; Li et al. 1999; Dey et al. 19981999; Bhattacharyya et al. 2001; Jaikumar et al. 2006), particularly in the case of objects with masses of about the mass of the Sun: these are the so called strange stars. This possibility is a direct consequence of the so called strange matter hypothesis (Bodmer 1971; Terazawa 1979; Witten 1984). According to this hypothesis, SQM is the true ground state of matter. In other words, the energy per baryon of SQM (at the baryon density where the pressure is equal to zero) is supposed to be less than the lowest energy per baryon found in atomic nuclei, which is about 930.4 MeV for the most tightly bound nuclei (62Ni, 58Fe, 56Fe) existing in nature.

If the strange matter hypothesis is true, then a nucleus with A nucleons could in principle lower its energy by converting to a strangelet (a droplet of SQM). However, this process requires the simultaneous weak decay of about a number A of u and d quarks in the nucleus to strange quarks. The probability for such a process is proportional to GF2A, with GF being the Fermi constant. Thus, for a large enough baryon number (A>Amin~5), this probability is extremely low, and the mean life time for an atomic nucleus to decay to a strangelet is much higher than the age of the Universe. In addition, finite size effects (surface, Coulomb and shell effects) place a lower limit (Amin~10-103, depending on values of the model parameters) on the baryon number of a stable strangelet even if SQM is stable in bulk (Farhi & Jaffe 1984; Madsen 1999). On the other hand, a step by step production (i.e. at different times) of s quarks will produce hyperons in the nucleus, that is to say, a system (hypernucleus) with a higher energy per baryon with respect to its original nucleus. Thus according to the strange matter hypothesis, the ordinary state of matter, in which quarks are confined within hadrons, is a metastable state with a mean lifetime much higher than the age of the Universe.

There is a striking qualitative difference between the mass-radius (MR) relation of strange stars compared to that of neutron stars. For strange stars with ``small'' mass (i.e. M not to close to Mmax, the maximum mass of the stellar sequence), M is proportional to R3. By contrast, neutron stars have radii that decrease with increasing mass. This difference in the MR relation is a consequence of the differences in the underlying interactions between the stellar constituents for the two types of compact stars. In fact, ``low'' mass strange stars are bound by the strong interaction, contrary to the case of neutron stars, which are bound by gravity2. As is well known, there is a minimum mass for a neutron star (Mmin~0.1 Msun). In the case of SQM objects, there is essentially no minimum mass, and one could have self-bound objects with planet-mass or asteroid-mass down to small lumps where the baryon number becomes so low that finite size effects destabilize them (Madsen 1999).

Horvath, later on, analyzes other possible exotic types of astrophysical objects made of alternative forms of strange hadronic matter. In particular, he considers H-matter and Qα-matter bosonic stars, i.e. astrophysical compact objects whose constituents are charge neutral spin-0 unconventional multi-quark systems with baryon number B=2, in the case of the H particle (Jaffe 1977), or B=6, in the case of the Qα particle (Michel 1988). He also emphasizes the considerable difference in size (radius) between these bosonic stars and the so called strange dwarfs (Glendenning et al. [1995]) or the strangelet dwarfs (Alford et al. [2011]).

In summary, in his paper, Horvath has presented various arguments to support the possibility that the Jupiter-mass companion of PSR J1719-1438 is a compact exotic object. His suggestion definitely departs from the standard accepted view of the planetary science community. Nevertheless, it is a very appealing possibility and, if corroborated by future observational data, it could provide strong additional support for the existence of exotic (in the sense discussed in this note) forms of dense matter in astrophysical compact objects with masses ranging from the mass of our Sun down to the mass of a planet.

Footnote

1
i.e. pulsars that have ceased to emit periodic electromagnetic radiation as their spin period reaches a few seconds.
2
As an idealized example, remember that pure neutron matter is not bound by nuclear forces.

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