NASA’s Hubble Space Telescope precisely measured the mass of the oldest known planet in our Milky Way galaxy near the core of the ancient globular star cluster M4, located 5,600 light-years away in the summer constellation Scorpius. At an estimated age of 13 billion years, the planet is more than twice as old as Earth’s 4.5 billion years. It’s 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.
Long before our Sun and Earth ever existed, a Jupiter-sized planet formed around a sun-like star. Now, 13 billion years later, NASA’s Hubble Space Telescope has precisely measured the mass of this farthest and oldest known planet.
“A globular cluster might be the first place in which intelligent life is identified in our galaxy,” says Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics (CfA). 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.
So if habitable planets can form in globular clusters and survive for billions of years, what are the consequences for life should it evolve? Life would have ample time to become increasingly complex, and even potentially develop intelligence.
Such a civilization would enjoy a very different environment than our own.
The nearest star to our solar system is four light-years, or 24 trillion miles, away. In contrast, the nearest star within a globular cluster could be about 20 times closer – just one trillion miles away. This would make interstellar communication and exploration significantly easier.
” We call it the ‘globular cluster opportunity,'” says DiStefano. “Sending a broadcast between the stars wouldn’t take any longer than a letter from the U.S. to Europe in the 18th century. Interstellar travel would take less time too. The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster,” she adds.
The closest globular cluster to Earth is still several thousand light-years away, making it difficult to find planets, particularly in a cluster’s crowded core. But it could be possible to detect transiting planets on the outskirts of globular clusters. Astronomers might even spot free-floating planets through gravitational lensing, in which the planet’s gravity magnifies light from a background star.
A more intriguing idea might be to target globular clusters with SETI search methods, looking for radio or laser broadcasts. The concept has a long history: In 1974 astronomer Frank Drake used the Arecibo radio telescope to broadcast the first deliberate message from Earth to outer space. It was directed at the globular cluster Messier 13 (M13) shown at the top of the page.
The new Hubble findings close 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.
Globular clusters are deficient in heavier elements because they formed so early in the universe that heavier elements had not been cooked up in abundance in the nuclear furnaces of stars. Some astronomers have argued that globular clusters cannot contain planets. This conclusion was bolstered in 1999 when Hubble failed to find close-orbiting “hot Jupiter”-type planets around the stars of the globular cluster 47 Tucanae. Now, it seems that astronomers were just looking in the wrong place, and that gas-giant worlds orbiting at greater distances from their stars could be common in globular clusters.
“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.
“This is tremendously encouraging that planets are probably abundant in globular star clusters,” says Harvey Richer of the University of British Columbia (UBC), Vancouver, Canada. . 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 story of this planet’s discovery began in 1988, when the pulsar, called PSR B1620-26, was discovered in M4. It is a neutron star spinning just under 100 times per second and emitting regular radio pulses like a lighthouse beam. The white dwarf was quickly found through its effect on the clock-like pulsar, as the two stars orbited each other twice per year. Sometime later, astronomers noticed further irregularities in the pulsar that implied that a third object was orbiting the others. This new object was suspected to be a planet, but it could also be a brown dwarf or a low-mass star. Debate over its true identity continued through the 1990s.
Astronomers settled the debate by at last measuring the planet’s actual mass through some ingenious celestial detective work. They had exquisite Hubble data from the mid-1990s, taken to study white dwarfs in M4. Sifting through these observations, they were able to detect the white dwarf orbiting the pulsar and measure its color and temperature.
Using evolutionary models computed by Brad Hansen of the University of California, Los Angeles, the team estimated the white dwarf’s mass. This in turn was compared to the amount of wobble in the pulsar’s signal, allowing the team to calculate the tilt of the white dwarf’s orbit as seen from Earth. When combined with the radio studies of the wobbling pulsar, this critical piece of evidence told them the tilt of the planet’s orbit, too, and so the precise mass could at last be known. With a mass of only 2.5 Jupiters, the object is too small to be a star or brown dwarf, and must instead be a planet. The planet is likely a gas giant without a solid surface like the Earth.
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: NASA, ESA and G. Bacon (STScI)