PSR B1620-26 / "Methusalah" System

Bill Keel, Ray White III, Chris Conselice,
Cerro Tololo Inter-American Observatory


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The lack of easily detectable,
close-orbiting "hot Jupiters" in the
core of 47 Tucanae, the Milky Way's
second largest and brightest globular,
led some astronomers in 1999 to
suggest that planets might be very
scarce among the universe's more
ancient, metal-deficient stars (more
at Astronomy Picture of the Day
and Bill Keel's Globular Cluster
47 Tucanae).

Akira Fujii, STScI, NASA




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The B1620-26 system lies
around 5,600 ly away in
globular cluster M4, boxed
in green, directly west of
red supergiant star Antares
in Constellation Scorpius.

Discovered by Philippe Loys de Chéseaux (1718-51) in 1746, M4 (NGC 6121) is one of the Milky Way's smallest but closest known globular star clusters. Unfortunately, it is obscured by dark interstellar clouds from the perspective of the Solar System. Located only around 5,600 ly (1,720 parsecs) away in the northwestern part (16:23:35.4-26:31:31.9, J2000) of Constellation Scorpius (lined photo), the Scorpion, it is also one of the most open, or loose, globulars with some 100,000 stars spread over more than 50 ly. As it contains mostly stars around 13 (12.7 +/- 0.35) billion years old (Sigurdsson et al, 2003), most of its higher-mass stars (i.e., Sol-sized and larger) have already evolved out of the main sequence to become white dwarfs, neutron stars, or black holes. (See a photo of some of M4's white dwarfs). Moreover, since most of M4's stars were formed within a billion years of the Big Bang, they are only around five percent as enriched in elements heavier than hydrogen and helium as Sol (Sigurdsson et al, 2003).

NOAO, AURA, NSF




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The unusual binary system
containing PSR B1620-26 is
located in the boxed, central
region of globular cluster M4,
which has more than 100,000
stars despite its relatively
small size (more).

In large globular clusters, a million stars may be crammed into a space that measures only a few dozen ly across. By comparison, an equivalent volume in the Solar neighborhood would contain only several hundred stars. As a result, stars in globular cores are much more likely to suffer gravitational interactions, where lower-mass stars and stellar remnants tend to be ejected from the cluster by energy transfer from higher mass stars (or binaries) which then fall in closer together towards the cluster center. Given the relatively slow motion of stars in globulars and enough time, some of their tightly packed stars, brown dwarfs, and stellar remnants (white dwarfs, neutron stars, and black holes) may eventually collide to result in stellar mergers and ejections. Direct stellar collisions with stellar remnants tend to obliterate main sequence dwarfs, but main sequence stars and brown dwarfs sometimes merge into a single, more massive star that is larger, hotter, and bluer than either pre-merger object that are know as a "blue straggler." While possibly common, collisions between binary systems that result in at least one stellar ejections are hard to uncover. It has been estimated that half of the stars in the central regions of some globular clusters may have undergone one or more collisions over the past 13 billion years (Michael M. Shara, Scientific American, November 2002).

Ann Feild, STScI, NASA


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The planet may have
first formed around
a Sol-type star before
colliding with a
pulsar binary system
which caused the
pulsar's original
stellar companion to
be flung away (more).

Empirical evidence for stellar collisions is found in the crowded cores of globulars such as 47 Tucanae, that are full of unusual compact binary star systems (where a normal star is paired with a white dwarf, neutron star, or a black hole) in unusually large numbers rarely found outside of globular cores. Such cores have been found to include exotic objects such as milli-second pulsars, which are neutron stars that are rotating extremely rapidly (between 100 to nearly 1,000 times a second) as if they have the rotational energy of formerly two objects that recently merged (more from Astronomy Picture of the Day and the Chandra X-Ray Observatory). Within M4, astronomers discovered the first known, millisecond pulsar (PSR 1821-24) in 1987. The pulsar is a neutron star that rotates and pulsates once every 3.0 milliseconds (over 300 times per second), or 10 times faster than the Crab Pulsar.

Pulsar B1620-26

Despite an estimated age of around 13 billion years, PSR B1620-26 or PSR J1623-2631 (16:23:38.22-23:31:53.8 J2000; and 16:23:38.24-26:31:53.9, ICRS 2000) is a neutron star that is spinning just under 100 times per second and emitting regular radio pulses like a lighthouse beam. It is believed to be an old neutron star that has been spun up to a millisecond period by accretion from a stellar companion in a highly circular orbit (e= 0.025) during an X-ray binary phase (Sigurdsson et al, 2003). The binary's low eccentricity of orbit may be related to the age of the millisecond pulsar and its surface magnetic field, and astronomers believe that the pulsar's current, white dwarf companion may once have lost mass to the pulsar which caused it to "spin up" (Lynne et al, 1988; and McKenna and Lynne, 1988).

STScI, NASA





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Like the much younger Crab Pulsar,
the environment of a rapidly spinning
neutron star such as PSR B1620-26
is filled with high-energy radiation
(more at STScI).

The pulsar's white dwarf companion had been quickly found through its eclipsing effect on the clock-like pulsar, as the two stars orbited each other about twice per year. Within a few years, however, astronomers noticed further irregularities in the pulsar that implied that a third object was orbiting the others (Frederic A. Rasio, 1994; and Backer et al, 1993). As it was estimated to have a much lower mass, the new companion object was suspected to be a planet, brown dwarf, or low-mass star. Debate over its true nature continued through the 1990s and confirmation an optical telescope was sought despite of its possibly undetectable dimness (Joshi and Rasio, 1997; Shearer et al, 1996; Steinn Sigurdsson, 1995; and Bailyn et al, 1994). B1620-26 is the only radio pulsar known to reside in a triple system, and the only triple system known in any globular cluster. Based on the pulsar's timing, the neutron star is estimated to have around 1.35 times Solar-masses, much greater than its lower mass, white-dwarf and planetary companions (Arzoumanian et al, 1996).

NASA Observatorium



The magnetic poles
of a pulsar are not
aligned with its
axis of rotation.



See a discussion
of neutron stars as
part of stellar
evolution and death.

White Dwarf

Based on the pulsar's timing and wobbles in its signal as well as the color and temperature of the white dwarf companion given evolutionary models, this helium white dwarf is estimated to have around 34 +/-4 percent of Sol's mass, which is "undermassive" compared with white dwarfs whose progenitor stars have not loss mass to a close binary companion (Sigurdsson et al, 2003; and Arzoumanian et al, 1996). Astronomers have estimated that its progenitor star was a yellow-orange dwarf less massive than Sol about half a billion years ago (480 +/- 120 million). The planetary companion discovered in 2003 is believed to have formed with the white dwarf's progenitor around 13 billion years ago.

Harvey Richer, STScI, NASA



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The planet orbits a binary
composed of a pulsar and a
white dwarf that was imaged
in visible light with the
Hubble Space Telescope
(more images at STScI).

Planet "Methusalah"

In the mid-1990s, astronomers determined from the pulsar's timing that it had a second, much-lower-mass companion in a low-eccentricity, 191-day orbit (Ford et al, 2000 and 1999; Thorsett et al, 1999; and Arzoumanian et al, 1996). Nicknamed "Methusalah" for its presumed age of around 13 billion years like many of the stars in the M4 globular cluster, the planet is now believed to have around 2.1 times (+/- 1) Jupiter-masses. Its appears to orbit the pulsar and white dwarf binary system at an average distance of around 23 AUs, beyond the orbital distance of Uranus in the Solar System. This orbit takes around 100 years to complete (Sigurdsson et al, 2003).

NASA

Cassini-Huygens Mission
to Saturn and Titan


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"Methusalah" may be a
large gas giant like Jupiter
(shown here with Europa).

Astronomers believe that the planet has had an unusually violent life over the past 13 billion years. After birth, it probably orbited its youthful yellow-orange sun at approximately the orbital distance of Jupiter in the Solar System. Then, the planet survived intense ultraviolet radiation, supernova radiation, and shockwaves, which would have ravaged the young globular cluster during an initial period rapid of star birth. Billions of years later, at around the time that multi-celled life appeared on Earth, the planet and star plunged into the core of M4. While passing through the densely crowded region, the planet and its star passed too close to an ancient pulsar, formed in a supernova when the cluster was young, that had its own stellar companion. In a slow-motion gravitational interaction, the planet and its host star were eventually captured by the pulsar while its original companion (possibly a high-mass white dwarf of 0.5 to 0.7 Solar-masses) was then ejected from the new star system (Sigurdsson et al, 2003). The pulsar, star, and planet were then flung by gravitational recoil into the less-dense outer regions of the cluster. Eventually, as the yellow-orange star aged, it swelled to a red giant and lost matter onto its closely orbiting, neutron star companion. The momentum carried with the accretion of this "stolen" matter caused the neutron star to "spin-up" and "re-awaken" as a millisecond pulsar. Meanwhile, the planet was flung into a farther orbit around both stellar mass objects at a distance of around two billion miles (over three billion kilometers).

Ann Feild, STScI, NASA





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The discovery of the planet
in an extremely metal-poor,
globular star cluster of the
Milky Way suggests that giant
planets can form directly from
the collapse of a gaseous
clump within a new-born star's
protoplanetary disk, without
first accreting a planetary core
through the agglomeration
of planetismals of ice and
rock before developing a
gas envelope.

It is likely that the substellar object is a gas giant rich in hydrogen and helium, without a solid surface like the Earth. Because it was formed so early in the life of the universe, it probably doesn't have abundant quantities of heavier elements such as carbon and oxygen and so would be unlikely to host Earth-type life on a rocky or icy moon. Even if life did arise on a solid moon orbiting the planet, moreover, it is unlikely to have survived the intense X-ray blast that would have accompanied the spin-up of the pulsar.