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PSR B1620-26c

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: PSR B1620-26c

     Extrasolar planet          Lists of extrasolar planets

          Artist's impression of PSR B1620-26c looking at its suns.

                             Parent star
   Star                   PSR B1620-26 A / B
   Constellation          Scorpius ( M4)
   Right ascension    (α) 16^h 23^m 38^s
   Declination        (δ) −26° 31′ 53″
   Spectral type          Pulsar / DB
                          Orbital elements
   Semimajor axis     (a) 23 AU
   Eccentricity       (e) low
   Orbital period     (P) ~100 y
   Inclination        (i) 55°
   Longitude of
   periastron         (ω)  ?°
   Time of periastron (τ)  ? JD
                      Physical characteristics
   Mass               (m) 2.5 ± 1 M[J]
   Radius             (r)  ? R[J]
   Density            (ρ)  ? kg/ m^3
   Temperature        (T)  ? K
                        Discovery information
   Discovery date         1996
   Discoverer(s)          Backer et al.
   Detection method       Pulsar timing
   Discovery status       Confirmed
                         Other designations
   PSR J1623-2631c

   PSR B1620-26c is an extrasolar planet orbiting the pulsar PSR B1620-26
   in the Messier 4, about 12,400 light-years away in the constellation of
   Scorpius. The planet is the oldest known extrasolar planet. It is
   believed to have been formed about 12.7 billion years ago.

The PSR B1620-26 system

   PSR B1620-26c orbits a binary pair of stars. One, the pulsar, is a
   neutron star. The second is a white dwarf with a mass of 0.34 solar
   mass. These stars orbit each other at a distance of 1 AU about once
   every half year. As the third object found in the system, the planet's
   official designation is PSR B1620-26c. The planet has a mass of 2.5
   times that of Jupiter, is in an orbit similar to that of Uranus around
   the Sun, but slightly larger; PSR B1620-26c orbits at a distance of 23
   AU (3400 million km). Each orbit of the planet takes about 100 years.

   The triple system is just outside the core of the globular cluster M4.
   The age of the cluster has been estimated to be about 12.7 billion
   years, and because all stars in a cluster form at about the same time,
   and planets form together with their host stars, it is likely that PSR
   B1620-26c is also about 12.7 billion years old. This is much older than
   any other known planet, and nearly three times as old as Earth. The
   nickname "Methuselah" was given as a reference to the long-lived
   Biblical Methuselah. However, this name is not generally used in the
   astronomical literature, and has not been accepted by the International
   Astronomical Union, which governs the naming of astronomical objects.

Discovery of PSR B1620-26c

   The location of the pulsar and planet.
   Enlarge
   The location of the pulsar and planet.

   Like nearly all extrasolar planets discovered to date, PSR B1620-26c
   was originally detected through the Doppler shifts its orbit induces on
   radiation from the star it orbits (in this case, changes in the
   apparent pulsation period of the pulsar). In the early 1990s, a group
   of astronomers led by Donald Backer, studying what they thought was a
   binary pulsar, determined that a third object was needed to explain the
   observed Doppler shifts. Within a few years, the gravitational effects
   of the planet on the orbit of the pulsar and white dwarf had been
   measured, giving an estimate of the mass of the third object that was
   too small for it to be a star. The conclusion that the third object was
   a planet was announced by Stephen Thorsett and his collaborators in
   1993.

   The study of the planetary orbit allowed the mass of the white dwarf
   star to be estimated as well, and theories of the formation of the
   planet suggested that the white dwarf should be young and hot. On July
   10, 2003, the detection of the white dwarf and confirmation of its
   predicted properties were announced by a team led by Steinn Sigurdsson,
   using observations from the Hubble Space Telescope. It was at a NASA
   press briefing that the name Methuselah was introduced, capturing press
   attention around the world.

Evolutionary history

   The origin of this pulsar planet is still uncertain, but it probably
   did not form where we see it today. Because of the decreased
   gravitational force when the core of star collapses to a neutron star
   and ejects most of its mass in a supernova explosion, it is unlikely
   that a planet could remain in orbit after such an event. It is more
   likely that the planet formed in orbit around the star that has now
   evolved into the white dwarf, and that the star and planet were only
   later captured into orbit around the neutron star.
   The evolution of the PSR B1620-26 system.
   Enlarge
   The evolution of the PSR B1620-26 system.

   Stellar encounters are not very common in the disk of the Milky Way,
   where our Sun lives, but in the dense core of globular clusters they
   occur frequently. At some point during the 10 billion years, the
   neutron star is thought to have encountered and captured the host star
   of the planet into a tight orbit, probably losing a previous companion
   star in the process. About half a billion years ago, the newly captured
   star began to expand into a red giant (see stellar evolution).

   Typical pulsar periods for young pulsars are of the order one second,
   and they decrease with time; the very short periods exhibited by
   so-called millisecond pulsars are due to the transfer of material from
   a binary companion. The pulse period of PSR B1620-26 is a few
   milliseconds, providing strong evidence for matter transfer. It is
   believed that as the pulsar's red giant companion expanded, it filled
   and then exceeded its Roche lobe, so that its surface layers started
   being transferred onto the neutron star.

   The infalling matter produced complex and spectacular effects. The
   infalling matter 'spun up' the neutron star, due to the transfer of
   angular momentum, and for a few hundred million years, the stars formed
   a low-mass X-ray binary, as the infalling matter was heated to
   temperatures high enough to glow in X-rays.

   Mass transfer came to an end when the surface layers of the mass-losing
   star were depleted, and the core slowly shrunk to a white dwarf. Now
   the stars peacefully orbit around each other. The long-term prospects
   for PSR B1620-26 c are poor, though. The triple system, which is much
   more massive than a typical isolated star in M4, is slowly drifting
   down into the core of the cluster, where the density of stars is very
   high. In a billion years or so, the triple will probably have another
   close encounter with a nearby star. The most common outcome of such
   encounters is that the lightest companion is ejected from the multiple
   star system. If this happens, PSR B1620-26c will most likely be ejected
   completely from M4, and will spend the rest of its existence wandering
   alone in interstellar space as an interstellar planet.

   Retrieved from " http://en.wikipedia.org/wiki/PSR_B1620-26c"
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