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. The ancient planet has had a remarkable history because it has wound up in an unlikely, rough neighborhood. It orbits a peculiar pair of burned-out stars in the crowded core of a globular star cluster.
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.
The planet now lies in the core of the ancient globular star cluster M4, located 5,600 light-years away in the summer constellation Scorpius. 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 therefore 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.
"This is tremendously encouraging that planets are probably abundant in globular star clusters," says Harvey Richer of the University of British Columbia. 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, where 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.
Sigurdsson, Richer, and their co-investigators 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 astronomers estimated the white dwarf's mass. This in turn was compared to the amount of wobble in the pulsar's signal, allowing the astronomers 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 has had a rough road over the last 13 billion years. When it was born, it probably orbited its youthful yellow sun at approximately the same distance Jupiter is from our Sun. The planet survived blistering ultraviolet radiation, supernova radiation, and shockwaves, which must have ravaged the young globular cluster in a furious firestorm of star birth in its early days. Around the time multi-celled life appeared on Earth, the planet and star were plunging into the core of M4. In this densely crowded region, the planet and its sun passed close to an ancient pulsar, formed in a supernova when the cluster was young, that had its own stellar companion. In a slow-motion gravitational dance, the sun and planet were captured by the pulsar, whose original companion was ejected into space and lost. The pulsar, sun, and planet were themselves flung by gravitational recoil into the less-dense outer regions of the cluster. Eventually, as the star aged it ballooned to a red giant and spilled matter onto the pulsar. The momentum carried with this matter caused the neutron star to "spin-up" and re-awaken as a millisecond pulsar. Meanwhile, the planet continued on its leisurely orbit at a distance of about 2 billion miles from the pair (approximately the same distance Uranus is from our Sun).
It is likely that the planet is a gas giant, without a solid surface like the Earth. Because it was formed so early in the life of the universe, it probably doesn't have abundant quantities of elements such as carbon and oxygen. For these reasons, it is very improbable the planet would host life. Even if life arose on, for example, a solid moon orbiting the planet, it is unlikely to have survived the intense X-ray blast that would have accompanied the spin-up of the pulsar. Regrettably, it is unlikely that any civilization witnessed and recorded the dramatic history of this planet, which began at nearly the beginning of time itself.
The full team involved in this discovery is composed of Brad Hansen (UCLA), Harvey Richer (UBC), Steinn Sigurdsson (Penn State), Ingrid Stairs (UBC), and Stephen Thorsett (UCSC).
Background Information: The Life and Times of the Oldest Known Planet
Long before our Sun and Earth existed, or even the Milky Way galaxy, as we know it today, a planet formed around a sun-like star in one of the earliest homesteaders of our corner of the universe, a globular star cluster.
This planet, a few times more massive than Jupiter has survived the harsh conditions of a globular cluster, a gravitational collision with a binary system, and the death of its progenitor star.
The planet resembled Jupiter in several ways: its mass was only a few times that of Jupiter and its orbit was similar, somewhere between 250 and 750 million miles from its sun. The star and planet orbited untouched for almost 10 billion years as they fell into the dense heart of the cluster, where stars are so crowded together they are a fraction of a light-year apart. Like strolling into a crowded marketplace, this star system would not be independent for long without "bumping" into something.
As it passed by a binary system containing an old neutron star and a white dwarf, gravitational forces pulled the two systems together into a web of tangled orbits. Soon, the small-mass white dwarf was booted out of its original position and thrown into space by the more massive progenitor star. Meanwhile, the planet was thrown into a circumbinary orbit, a large orbit around both its original star and the neutron star.
The new system of the planet, its sun, and the neutron star recoiled from the ejected white dwarf, in much the same way a cannon jumps backwards when it fires a cannon ball. This gravitational recoil sent the new binary system out of the globular cluster's core into a less dense region of the cluster, reducing its chance for another such stellar interaction.
At its new position in the cluster, the planet slowly traced out a wide orbit around the neutron star and its progenitor star at a distance of approximately 2 billion miles, which is similar to Uranus's orbit around our Sun. From this vantage point the planet witnessed the death of its progenitor star over the course of the next billion years. The sun-like star aged into a red giant and poured matter onto the neutron star. The neutron star's acquisition of mass caused it to rotate faster and faster on its axis, eventually spinning up into a pulsar. Now the neutron star makes almost 100 rotations per second on its axis (that's 10 times faster than a humming bird flaps its wings!) Once all the excess gas left the star, it became a small, bright, helium-core white dwarf. All the while, the planet continued on its sweeping orbit. This is the state, established less than one billion years ago, in which astronomers discovered the planet.
So, how could researchers tell that this planet had survived such dynamic cosmic forces, or existed at all? Using Hubble data, scientists used the white dwarf's color and temperature to determine its age and mass, which they compared to the wobble of the neutron star. In addition, radio studies of the pulsar revealed irregularities in its signal that could not be caused solely by its white dwarf companion star. Putting this information together, researchers obtained a tilt for the white dwarf's orbit, after which they could infer the tilt of the third orbiting body. From there, astronomers were able to determine the mass of the third body, which is too small to be a brown dwarf or a low-mass star; thus, the planet revealed itself through its subtle tug on the system. "We probably would never have found this planet if it had just stayed with its original star," remarked Steinn Sigurdsson of Pennsylvania State University. "Its history put it in the right place; the interactions helped us see it."
Furthermore, the planet's orbit and place in the globular cluster give us clues to its past. For the proposed scenario to be plausible, the white dwarf must have lost its gaseous envelope after it and the planet joined the neutron star; therefore, the white dwarf should be young, bright, and low mass, which evidence suggests is the case. In addition, the planet's presence in a wide near-circular orbit reveals that the mass transfer from the progenitor star, now the white dwarf, to the neutron star, spinning up into a pulsar, did occur after the planet was in an orbit around the pair.
The wide orbit also makes the planet more vulnerable to the gravitational forces of nearby stars, in which case the planet's continuing presence suggests the system has been in the lower-density portion of the cluster since its current configuration was established. Because such a system would return to the cluster's core on a time scale of a billion years and we know that the system has not yet returned, we can establish the time scale for the current configuration, the tumultuous series of events leading to the present, and an age for the planet.
This planet's tale also gives astronomers an idea of where planets may reside and how many could exist. The planet was born before many heavier elements existed in space, such as oxygen, carbon and silicon. It's birth in such an element-poor globular cluster like M4 may imply that planets are more common in such environments than once thought. "This is a big hint that there are more out there," said Sigurdsson. "There are 100 pulsars like [the one this planet orbits] out there, this one was just extremely well researched." Having theorized the planet's existence 10 years ago, Sigurdsson says the discovery of this planet means that we must "overcome theoretical prejudices" and "suggests we should make more of an effort to look for [such planets]." Coming in at 13 billion years old, this planet also makes a case for planet formation occurring earlier and more abundantly than previously thought.
Background Information: Chronology of the Discovery of a Planet Orbiting the Pulsar B1620-26 and White Dwarf Companion
A British team finds pulsar, PSR B1620-26, in the core of M4, a globular cluster about 5,600 light-years away. It is suspected to be part of a binary system, with the companion a white dwarf star. But the team has to wait half the white dwarf's orbit time (200 days) to confirm it.1
The team publishes the discovery paper. Since it is one of the first of its kind to be discovered, the pulsar sparks many follow-up papers describing how the neutron star became a pulsar and explaining the white dwarf's origins.
Using pulsar timing, three groups find an anomaly in the pulsar's signal indicating it is being gravitationally pulled by one or more unseen objects.
At a conference, Don Backer of UC Berkeley presents a paper contending that the anomaly discovered using pulsar timing is a third object in the system. At the same conference, Steinn Sigurdsson proposes that "it might be possible to see planets "stolen" from their parent stars by pulsars."
In just one year, four papers are published discussing possible explanations for the anomaly in the system. Theories for the object's celestial designation range from a black hole to a Saturn-like planet. Debates run high as this list of third-body candidates grows. At this time Sigurdsson presents his paper on the anomaly in which he rejects several different models to predict the exchange interaction scenario. Shortly afterward, scientists rule out the possibility of a black hole. This implies that it is a half solar mass star or a Jupiter-sized planet. Even more papers are now published, most concerning the orbits of the system's objects. Within the current scope of knowledge about the system, the white dwarf's orbit should be perfectly circular, but turns out to be slightly elliptical, like Earth's orbit, which is a great surprise. In addition, the eccentricity of the third object, predicted to be quite large, is instead rather small. Because of such ongoing uncertainties, many new explanations for the system arise, including reformed theories about the evolution of the system after it settled into its current configuration.
Multiple parties begin to look for a proposed third star in the system. One team claims to have found it, but it is discovered instead to be a nearby star that is not actually in the system. Following that close call, new theories about the possibilities of a faint cold star or a white dwarf arise.
After more than 10 years of perplexity over this system, a paper is published analyzing a decade's worth of data concerning this system. The system is subsequently observed in radio wavelengths, but scientists still possess no complete answer because there is no data about the system's tilt. An unknown inclination gives researchers a wide range of possibilities. Later, however, individual probabilities are calculated for different solutions. Researchers find there to be a low chance that the third body is a star; rather, they find a higher probability of the object being a low-mass object, like a Jupiter-mass planet or brown dwarf.
A student's theory paper suggests a mechanism for the planet being in the system. This is a well-known effect to explain the squished orbit of the white dwarf, but researchers had yet to apply it to this situation.
Hubble data sets, one taken starting in 1995 and another recent one, are compared to distinguish the movement of the white dwarf within the cluster. After a long wait, scientists are now able to determine the white dwarf's true mass, inclination, time of formation, and the age of the system. This new data boosts enormously the probability of the object being a planet. Determination of the planet's inclination is now possible, because all three components are not coplanar. From this calculation, the team is able to find the mass of the planet.
Space Science Update: Presenter Biographies
Dr. Harvey Richer was born in Montreal, Quebec, Canada. He studied at McGill University in Montreal and obtained his doctorate in physics and astronomy from the University of Rochester. His doctoral program supervisor, Dr. Stewart Sharpless, was one of the first astronomers to recognize that we live in a spiral galaxy. Sharpless also mapped out the distribution of ionized gas clouds by using a tiny wide-field camera. He was fond of simple experiments that had far-reaching consequences.
Richer has been at the University of British Columbia in Vancouver since the early 1970s. For the past three years, he has been the Gemini Scientist for Canada. Last year he was awarded a Canada Council Killam Fellowship that allows him to work full time on his research. His research is largely focused on stellar astronomy and on what resolved systems of stars can tell us about dark matter, the age of the universe, the dynamical evolution of stellar systems, and the formation of galaxies. To investigate these diverse subjects, he observes a wide range of objects, including nearby stars, open and globular star clusters, and the resolved components of our neighboring galaxies. To accomplish his research goals, he uses a variety of telescopes, particularly the twin Gemini Telescopes, the Canada-France- Hawaii Telescope and the Hubble Space Telescope.
The current result on the Hubble Space Telescope emerged from discussions at a meeting in Santa Barbara in January 2003 where the first 3 authors were present at a meeting on globular star clusters. However, it is actually the culmination of research started more than 20 years ago. In the early 1980s, Richer used photographic plates and the Canada-France-Hawaii Telescope to identify the few brightest white dwarfs in Messier 4. This work continued from ground-based telescopes over the years and in other globular clusters until 1995, when Messier 4 was first observed with the Hubble Space Telescope by Richer and his collaborators. This provided, for the first time, a large sample of faint white dwarfs in an ancient star cluster. Second epoch images were secured in 2000 (by another group) and in 2001 (by Richer and his collaborators), which allowed for a clean separation between cluster members and non- members via their motion.
Dr. Steinn Sigurdsson received his B.Sc. in mathematical physics with honors from the University of Sussex in 1986. He earned his M.Sc. in physics and his Ph.D. in theoretical astrophysics from the California Institute of Technology in 1988 and 1991 respectively. Sigurdsson was a research associate at the University of California's Lick Observatory in Santa Cruz from 1991 to 1994. He was a PPARC and Marie Curie Research Fellow from 1994 to 1998 at the Institute of Astronomy and a member of King's College in Cambridge, England. He is currently an assistant professor in the Astronomy Department at Pennsylvania State University.
Steinn's Ph.D. thesis was on formation of pulsars in globular clusters, working with Sterl Phinney at Caltech. The discovery by Alex Wolszczan at about that time of the first known planets orbiting PSR 1257+12 prompted him to consider how we could use the precision measurements possible with pulsar observations to constrain planet formation around solar-like stars. One way to do this is through exchanges of planets in regions of high density. Steinn works on the dynamics of compact objects and dense stellar systems, including planet dynamics, and relativistic binaries. Steinn is a member of the Center for Gravitational Wave Physics and the Penn State Astrobiology Research Center.
Dr. Alan P. Boss is a staff member at the Carnegie Institution's Department of Terrestrial Magnetism (DTM). Boss received his doctorate in physics from the University of California at Santa Barbara in 1979. He spent two years as a postdoctoral fellow at NASA's Ames Research Center before joining the staff of DTM in 1981. Boss's research focuses on using three- dimensional hydrodynamics codes to model the formation of stars and planetary systems. He has been helping NASA plan its search for extrasolar planets ever since 1988, and continues to be active in helping to guide NASA's efforts. Boss chairs the International Astronomical Union's Working Group on Extrasolar Planets, the group charged with maintaining the IAU's official list of extrasolar planets.
The formation of giant planets like Jupiter has long been regarded as the foremost unsolved problem in the origin of our solar system. In June 1997 Boss published a paper in Science that resurrected a long-forgotten means for making giant planets, through a gravitational instability of the protoplanetary disk, showing that it could make giant planets around even the youngest stars. The alternative mechanism requires about several million years to operate. A paper describing the different predictions of the two competing theories of giant planet formation was published in the May 14, 1998 issue of Nature. Boss's mechanism for Solar System formation implies that planetary systems similar to our own, with habitable planets, may be much more prevalent than would be the case for the competing formation mechanism.
Dr. Anne Kinney is director of the astronomy and physics division in the Office of Space Science at NASA Headquarters in Washington, DC. Previously, she was the director of the Origins Program at NASA Headquarters. As such, she oversaw the ongoing process of the Hubble Space Telescope, as well as the progress on upcoming missions such as SIRTF (Space Infrared Telescope Facility), the fourth of the Great Observatories; SOFIA (Stratospheric Observatory for Infrared Astronomy), a 2.7-meter telescope which flies above the atmosphere in a 747 airplane; ST-3 (Space Technology 3), a technology demonstrator for formation flying and space interferometry; SIM (Space Interferometry Mission); and ultimately, TPF (Terrestrial Planet Finder), a mission to find and characterize Earth-like planets.
Kinney is originally from Wisconsin, where she earned her B.Sc. in 1975. She then studied in Denmark for several years at the Niels Bohr Institute. In 1984, she received her doctorate in physics from New York University.
Kinney is an expert in extragalactic astronomy and has worked on characterizing the optical and ultraviolet spectra of quasars, blazars, active galaxies, and normal galaxies. She has studied signatures of accretion disks in active galaxies and demonstrated that the disks lie at random angles relative to their host galaxies. She was instrument scientist on one of the original instruments to fly on the Hubble Space Telescope, the Faint Object Spectrograph. She worked in education and public outreach with the Hubble Space Telescope and was involved in creating the program Amazing Space (http://amazing-space.stsci.edu), an educational web site for children learning the basic principles of science, math, and astronomy.
Kinney served on the Council of the American Astronomical Society and has been a visiting scholar at the Institute of Astronomy in Cambridge, UK.
