“In 2003 we derived the properties of a planet that was orbiting a white dwarf star and a neutron star binary near the core of the ancient globular star cluster M4, located 5,600 light-years away in the summer constellation Scorpius using NASA’s Hubble Space Telescope, precisely measuring the mass of the oldest known planet in our Milky Way galaxy,” astrophysicist Harvey Richer told The Daily Galaxy. At an estimated age of 13 billion years, the planet is more than twice as old as Earth’s 4.5 billion years.
Formed Barely i Billion Years After the Big Bang
The ancient Jupiter-sized planet formed around a sun-like star.is about as old as a planet can be. It formed around a young, sun-like star barely 1 billion years after our universe’s birth in the Big Bang. The ancient planet has had a remarkable history because it resides in an unlikely, rough neighborhood. It orbits a peculiar pair of burned-out stars in the crowded core of a cluster of more than 100,000 stars
“There was absolutely no discussion on habitability of the planet, in fact it is almost certain that it isn’t habitable,” Richer explained to The Daily Galaxy. “The ancient M4 cluster it resides in is poor in metals needed for life. The planet is more like Jupiter than the Earth, with no solid surface, and it might have survived a supernova explosion (the neutron star in the system was likely the end product of this explosion).”
However, Richer added: “in low metallicity systems, globular clusters specifically, there is no reason for the higher metallicity globulars not to have planets formed, IF planet formation is “efficient’. The preponderance of planets found since then, including around low metallicity stars in the field of the galaxy, suggest planet formation is efficient.”
Globular Cluster Opportunity?
Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics (CfA) has observed that “a globular cluster might be the first place in which intelligent life is identified in our galaxy,” Globular star clusters are extraordinary in almost every way. Globular clusters, which are found in the halo of a galaxy, contain considerably more stars than the less dense galactic, or open clusters, which are found in the disk. They’re densely packed, holding a million stars in a ball only about 100 light-years across on average. They’re old, dating back almost to the birth of the Milky Way. And according to new research, they also could be extraordinarily good places to look for space-faring civilizations.
“Once planets form, they can survive for long periods of time, even longer than the current age of the universe,” explains DiStefano.
Each Orbit Takes a Century
The Hubble findings closed a decade of speculation and debate as to the true nature of this ancient world, which takes a century to complete each orbit. The planet is 2.5 times the mass of Jupiter. Its very existence provides tantalizing evidence that the first planets were formed rapidly, within a billion years of the Big Bang, leading astronomers to conclude that planets may be very abundant in the universe.
Planet Formation Took Place Early in the Universe
“Our Hubble measurement offers tantalizing evidence that planet formation processes are quite robust and efficient at making use of a small amount of heavier elements. This implies that planet formation happened very early in the universe,” says Steinn Sigurdsson of Pennsylvania State University, State College.
“There is no reason for the higher metallicity globulars not to have planets formed, IF planet formation is ‘efficient’ – and, the preponderance of planets found since then, including around low metallicity stars in the field of the galaxy, suggest planet formation is efficient,” Sigurdsson told The Daily Galaxy.
“The mass estimate for the M4 planet has come down a bit from the original Science paper,” Sigurdsson pointed out. “The mass quoted was a “upper mass” – because the argument to that point had been that the orbital alignment and uncertainties in the masses of the neutron star and white dwarf meant it could be a low mass star, and we established it could not be.”
Planets Abundant in Globular Clusters
“This is tremendously encouraging that planets are probably abundant in globular star clusters,” says Harvey Richer. He bases this conclusion on the fact that a planet was uncovered in such an unlikely place, orbiting two captured stars –a helium white dwarf and a rapidly spinning neutron star — near the crowded core of a globular cluster. In such a place, fragile planetary systems tend to be ripped apart due to gravitational interactions with neighboring stars.
The Hubble findings close a decade of speculation and debate about the identity of this ancient world. Until Hubble’s measurement, astronomers had debated the identity of this object. Was it a planet or a brown dwarf? Hubble’s analysis shows that the object is 2.5 times the mass of Jupiter, confirming that it is a planet. Its very existence provides tantalizing evidence that the first planets formed rapidly, within a billion years of the Big Bang, leading astronomers to conclude that planets may be very abundant in our galaxy.
Illustration Credit: Hubble/NASA, ESA and G. Bacon (STScI)