最后补充一类系外行星:脉冲星附近的系外行星,鉴于脉冲星实际上在恒星总数中所占比例很小,加上这类恒星的残骸附近存在行星的概率很小,实际上这类行星并不具有代表性,但是由于其是最早得到确认的系外行星,加之宇宙中其总数仍然不少,因此还是有必有介绍一二:
Pulsar planets are discovered through pulsar timing measurements, to detect anomalies in the pulsation period. Any bodies orbiting the pulsar will cause regular changes in its pulsation. Since pulsars normally rotate at near-constant speed, any changes can easily be detected with the help of precise timing measurements. The discovery of pulsar planets was unexpected; pulsars or neutron stars have previously gone supernova, and it was thought that any planets orbiting such stars would have been destroyed in the explosion.
In 1991, Andrew G. Lyne announced the first ever pulsar planet discovered around PSR 1829-10.[1] However, this was later retracted,[2] just before the first real pulsar planets were announced.
In 1992, Aleksander Wolszczan and Dale Frail announced the discovery of a multi-planet planetary system around the millisecond pulsar PSR 1257+12.[3] These were the first two extrasolar planets confirmed to be discovered, and thus the first multi-planet extrasolar planetary system discovered, and the first pulsar planets discovered. There was doubt concerning the discovery because of the retraction of the previous pulsar planet, and questions about how pulsars could have planets. However, the planets proved to be real.[4] Two additional planets of lower mass were later discovered by the same technique.
In 2000, the millisecond pulsar PSR B1620-26 was found to have a circumbinary planet (PSR B1620-26 b) that orbits both it and its companion white dwarf, WD B1620-26. This was announced as the oldest planet ever discovered, at 12.6 billion years old.[5] It is currently believed to have originally been the planet of WD B1620-26 before becoming a circumbinary planet, and therefore, while discovered through the pulsar timing method, it did not form the way that PSR B1257+12's planets are thought to have.
In 2006, the magnetar 4U 0142+61, located 13,000 light years from Earth, was found to have a circumstellar disk. The discovery was made by a team led by Deepto Chakrabarty of MIT using the Spitzer Space Telescope.[6] The disk is thought to have formed from metal-rich debris left over from the supernova that formed the pulsar roughly 100,000 years ago and is similar to those seen around Sun-like stars, suggesting it may be capable of forming planets in a similar fashion. Pulsar planets would be unlikely to harbour life as we know it, because the high levels of ionizing radiation emitted by the pulsar and the corresponding paucity of visible light.
In 2011, a planet that is theorized to be the remaining core of a star that orbited a pulsar was announced. It orbits millisecond pulsar PSR J1719-1438, and represents a path to planetary status by evaporation of a star.[7][8] The planet is estimated to have a density of at least 23 times that of water, a diameter of 55,000 km, a mass near that of Jupiter's, and a 2hr10min orbital period at 600,000 km. It is thought to be the diamond crystal core remaining from the evaporated white dwarf, with an estimated 1031 carat weight.[9][10]
There are three types of pulsar planets known so far. The PSR B1257+12 planets were formed out of the debris of a destroyed companion star that used to orbit the pulsar.[11] In PSR J1719-1438, the planet most likely is the companion, or what's left of it after being almost entirely blasted away by the extreme irradiation from the nearby pulsar. PSR B1620-26 b is most likely a captured planet.
以下为得到确认的系外脉冲星的行星:
PSR B1620-26 b,PSR B1257+12 A,PSR B1257+12 B以及PSR B1257+12 C,还有一颗系外脉冲星行星的候选者PSR J1719-1438 b,等待进一步确认,估计质量大约是一个木星质量,半长轴只有0.004个AU,呵呵,估计是个超级热木星吧。。。 |