Ancient Planet Discovered

Planets started forming very soon after the Big Bang, according to a new report by astronomers.

The Big Bang that ushered in the universe occurred about 13.7 billion years ago. Stars began to form soon after, and astronomers now say they have evidence of a planet in a 13 billion-year-old globular star cluster.

"All the stars in this cluster are about the same age, so the presumption is that the planet is that old also," says Harvey Richer of the University of British Columbia in Vancouver, Canada.

All the other extrasolar planets discovered so far are in orbit around 5 billion-year-old stars (stars the same age as our sun). The existence of the more ancient planet suggests that planets have been forming for a long time, and they may be abundant in the universe.

Three unlikely companions – two burned-out stars and a planet – orbit each other near the crowded core of an ancient globular cluster of more than 100,000 stars.
Credit:NASA and H. Richer (University of British Columbia)

Not only is this the oldest known planet, it’s also the most distant. The globular cluster, known as M4, is located 5,600 light-years away, in the constellation Scorpius.

Rather than orbiting a single star, the planet orbits a binary system composed of a white dwarf with half the mass of our sun, and a pulsar, or rapidly spinning neutron star. The planet orbits at a distance of 23 AU, or 23 times the Earth-Sun distance (or roughly the distance of Neptune from our Sun). At this distance, it takes 100 years for the planet to complete an orbit.

This is the only planet that has been found in orbit around a binary star system. There have been several planets found orbiting around a single star in a binary system, but this planet orbits both stars. The planet’s orbit around a pulsar and white dwarf system is what led to its discovery.

The pulsar, named PSR B1620-26, was discovered in 1988. The pulsar spins about 100 times per second, and astronomers noticed that the radio emissions of the pulsar indicated a binary orbit. They soon determined that the other object in the binary was a white dwarf, but further irregularities in the radio emissions indicated a third object. This new object was suspected to be a planet, but it also could have been a brown dwarf or a low-mass star. Debate over the identity of this third object continued for almost 15 years.

By combining Hubble Space Telescope observations with the radio astronomy data, the astronomers determined that the third object has two to three times the mass of Jupiter. According to Steinn Sigurdsson of Pennsylvania State University, an object of this mass is definitely a planet, since stars and brown dwarfs are much more massive.

Over the course of its lifetime, the planet has seen remarkable changes. The planet formed in a Jupiter-like orbit of about 5 to10 AU from a star that was a little less massive than the sun.

This solar-like star formed in the outskirts of the globular cluster, but it eventually it worked its way down into the crowded center of the cluster, pulling its planet with it.

Within the cluster, a neutron star in a binary system collided with its companion. This companion star was thrown out of the cluster, and the solar-like star swapped in, forming a new binary with the neutron star.

An enormous amount of energy was released during this process, pushing the triple ‘neutron star – solar star – planet’ system out of the center of the globular cluster. The triple system is currently in the outskirts of the globular cluster, but it is working its way back to the center and will arrive there in another billion years.

The solar-like star eventually reached the end of its lifetime and died, becoming a red giant. The dying star transferred its gas envelope onto the neutron star, spinning it up and turning the neutron star into a recycled pulsar. The white dwarf star we see today is the burnt out core of the solar-like star that died.

Throughout all of this, the orbiting planet managed to hang on. But other planets in globular clusters probably would get stripped out of their orbits due to the gravitational interactions between stars.

"We expect for every planet you see, you’ll see many free floaters," says Sigurdsson. "You’re going to see planets that have been ripped away from their parent star, and they are just out there by themselves in interstellar space."

The Milky Way and Scorpius Constellation. Small green box upper-right shows Scorpius.
Credit: Akira Fujii

Since planets are made from conglomerations of rock and ice, some astronomers have argued that the low amount of this material (or "metallicity") in globular clusters means planets can’t form there.

The standard "accretion" model of planet formation is that rocks and dust collide and accumulate in the protoplanetary gas disk that surrounds a young star. For Jupiter mass planets, a rocky core of 5 to 10 Earth masses must form before it can start to pull in gas from the disk. But without enough metallicity to grow that core, Jupiter mass planets couldn’t form. The M4 planetary system has roughly 30 times less metallicity than the sun.

"In this case, it’s unclear if that process could work because of the very shortage of the rocky elements that you need in order to build up that core in the first place," says Alan Boss of the Carnegie Institution in Washington.

Boss says that for the gas giant in M4 to form, a different mechanism may have been at work. Boss’s model of gas giant formation says that planets can form from clumps caused by the self-gravity of gas in the protoplanetary disk. A runaway process then occurs where the clump gathers more and more gas. If the planet in M4 formed this way, it would not matter that the system has low metallicity.

"It could be that in this weird system – in this M4 globular cluster – we have evidence for at least one planet that formed by this particular mechanism," says Boss.

The reason this cluster has low metallicity is because of its age. 13 billion years ago, there weren’t many elements in the universe except hydrogen and helium. When stars die, they often explode, and this produces a fusion process that creates heavier elements, which are then scattered out into space. New stars incorporate these heavier elements as they are born. So the "first generation" stars were composed of mostly hydrogen and helium, and when they died they produced heavier elements that helped make up the second generation of stars. Our sun is a third generation star, so it has abundant metallicity.

While there was not enough metallicity in the M4 planetary system to form a gas giant core, there could have been enough to make two or three terrestrial-size planets.

"They would probably be somewhat different from the Earth, in the sense that they would be less abundant in iron and more abundant in ices – they would ice or water worlds," notes Sigurdsson.

"In this case, we have a star where we presumably could have formed not only a gas giant planet, but perhaps also a habitable planet where life could have arisen and died out long ago, well before we came along to the galactic party," says Boss.

What’s Next

"It would be nice to say that the universe is full of very old planets," says Richer.

To see if this is a possibility, Richer says scientists need to look for similar systems in globular clusters and other very metal-poor environments. He says radio astronomers have been continually monitoring the many pulsars in globular clusters.

"None of them have yet shown evidence for planets," he says, "but with time that may change because the longer the data set, the more subtle effects you can find. So they may turn up eventually."

The full team involved in this discovery are Brad Hansen of the University of California, Los Angeles, Harvey Richer, Steinn Sigurdsson, Ingrid Stairs of the University of British Columbia, and Stephen Thorsett of the University of California in Santa Cruz.

Web Links

Hubble Report: Ancient Planet
How Old is the Universe?: WMAP
A Shortage of Planets
Jupiter Mass Planets: Scientific American