The discovery of a planet nearly three times older than any previously known, reported at a NASA briefing on Thursday, means that planets may be far more common than expected. It could also force a reversal of the prevailing view on how planets and solar systems are born.
The antique planet, dubbed 鈥淢ethuselah鈥� after a biblical patriarch who lived to the grand old age of 969, is located in a globular star cluster called M4. The cluster is composed of stars that formed barely a billion years after the big bang, or nearly 13 billion years ago. The Earth and the rest of the solar system, by contrast, are only 4.5 billion years old, and all of the more than 100 planets discovered in recent years around nearby stars are of comparable age.
It is the first star to be found at the centre of a globular cluster, a dense and chaotic environment. Not only is it far older than expected, it is also the first planet found orbiting both stars of a binary system.
Advertisement
Precise observations with the Hubble Space Telescope revealed that an object orbiting both a white dwarf star and a pulsar in M4 is a planet with about 2.5 times the mass of Jupiter. It was thought to be a brown dwarf or another star. The observations were carried out by a team led by Steinn Sigurdsson of Pennsylvania State University.
Chain reaction
Prevailing theory says that planets form when dust particles in a disk of material surrounding a new star begin to stick together in clumps. These gradually build up into larger and larger chunks called planetesimals, in a kind of gravitational chain reaction.
When enough of these have collided to form Earth-sized bodies, some of these cores attract enough gases to build gas giants like those in the outer solar system. That process requires the disk to contain the quantities of heavy elements found in later-generation stars and dust 鈥� so- called 鈥渉igh-metallicity鈥� stars. But such stars took many billions of years to develop, far longer than the age of this newly-discovered elder planet.
鈥淭he traditional way of making planets just isn鈥檛 going to work in this case,鈥� says Alan Boss, a planet-formation theorist with the Carnegie Institution in Washington, DC. Boss, who was at the NASA briefing but was not directly involved in the discovery, called the work 鈥渁 stunning revelation.鈥�
Prevailing theory
Hubble鈥檚 find is the latest challenge to the prevailing theory of planet formation. Recently a team using observations from an infrared sky survey found that planets appear to form more rapidly than the theory requires (see New 杏吧原创 print edition, 31 May, p.23).
Another theory may offer an explanation. It holds that tidally-induced instabilities 鈥� caused by waves that naturally emerge in the dust disk 鈥� speed up the nucleation process greatly. But the theory cannot yet explain why so many planets are thought to exist, so more work needs to be done.
The new finding means not only that planets may be more abundant than expected throughout space, but that life may have started far earlier 鈥� and had more of a chance to spread 鈥� than had been thought possible. 鈥淭his means that 13 billion years ago, life could have arisen and then died out鈥� on this distant planet, Boss speculates.
Journal reference: Science (vol 301, p193)