PSR B1620-26 b
PSR B1620-26 b (also PSR B1620-26 c in popular scientific articles) is a pulsar planet orbiting the pulsar PSR B1620-26 in the globular cluster Messier 4 . This is located about 12,400 light years away in the constellation Scorpio. The exoplanet is also the oldest existing known exoplanet, as it is believed to be around 12.7 billion years old.
The PSR-B1620-26 system
PSR B1620-26 b orbits a binary star system . One star, the pulsar, is a neutron star . The second is a white dwarf with a mass of about 0.34 solar masses. Both stars orbit each other at a distance of about 1 astronomical unit, with an orbital period of about half a year. PSR B1620-26 b has about 2.5 times the mass of Jupiter and moves around the two central bodies at a distance of almost 23 astronomical units. It is thus a circumbinary planet . It takes about 100 years to complete one cycle. Due to the high mass, it is certain that it is a gas planet.
The triple system is located just outside the center of M4. The age of the star cluster has been determined to be around 12.7 billion years. Since all stars in such a cluster form at the same time and the planets form together with their stars, it is likely that the exoplanet is also roughly this age. This is much older than any other known planet, and nearly three times as old as Earth. Because of this, he is unofficially known as Methuselah .
Discovery of PSR B1620-26 b
As with almost all other extrasolar planets discovered so far, PSR B1620-26 b was also discovered through Doppler shifts that the planet triggers at the central star. In the early 1990s, a group of astronomers led by Donald C. Backer studied the observed Doppler shifts. To explain this it had to be assumed that a third body must be present in this system. Within a few years the gravitational influence of the planet on the two central bodies was measured, from which the mass of the planet could be calculated. The calculated mass was too small to suspect another star, which is why Stephen Thorsett and his colleagues announced the discovery of the exoplanet in 1993.
By closely observing the orbits , the mass of the white dwarf could also be estimated, which led to theories that assumed that the white dwarf is relatively young and hot. On July 10, 2003, Hubble images showed that the predicted properties of the white dwarf were correct. The name "Methuselah" appeared publicly for the first time in NASA's press release.
Evolutionary history
The planet's origin is still uncertain, but it is likely that it did not originate where it is today. Because the gravitational force between the star and the planet decreases when the star's core collapses into a neutron star and ejects most of its mass into space, it is unlikely that a planet will be on a stable orbit after such an event. It is more likely that the planet evolved in orbit around today's white dwarf and was subsequently captured by the neutron star.
Close encounters of stars don't happen very often in the disk of the Milky Way . This often happens in the dense core regions of globular clusters . At some point in the last 10 billion years, the neutron star likely approached this system, capturing today's white dwarf. It may also have lost a previous companion star in the process. Half a billion years ago, the newly captured star swelled into a red giant.
Typical pulsar periods of young pulsars are on the order of seconds. The extremely short-period pulsars in the millisecond range are created by material transfer from a companion star. The pulsation period of PSR B1620-26 is a few milliseconds, which is a strong indication of material transfer from a companion. It is assumed that by the time the companion star expanded into a red giant , it crossed the Roche limit, and an exchange of material occurred.
Since torque was also transmitted to the incoming matter at the same time , this matter heated up so much that it radiated in the X-ray range .
The exchange of materials was over when the surface layers of the mass-giving star were exhausted and the core slowly shrank into a white dwarf. Since then, both objects have been orbiting each other.
Looking into the future for a very long time, the system will move into the very star-rich central region of M4. It is therefore very likely that there will be another close encounter with a star, with a high probability that the lightest companion of the stars will be thrown out of the system. Should this scenario occur, PSR B1620-26 b will most likely be completely ejected from M4, and will migrate as an interstellar planet through interstellar space.
Web links
Individual evidence
- ↑ Oldest known planet
- ↑ Article on PSR B1620-26 b on planeten.ch ( Memento from June 16, 2013 in the Internet Archive )
- ↑ Discussion at Raumfahrer.net