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News Release 31 of 66

July 10, 2003 02:00 PM (EDT)

News Release Number: STScI-2003-19

Oldest Known Planet Identified

A NASA Science Update Release

July 10, 2003: NASA's Hubble Space Telescope precisely measured the mass of the oldest known planet in our Milky Way galaxy. 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. The new 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.

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Q & A: Understanding the Discovery

  1. 1. How do astronomers know the planet is so old?

  2. It was uncovered in what astronomers thought was an unlikely place, the crowded center of a globular star cluster called M4, which lies 5,600 light-years away in the summer constellation Sagittarius. Most globular clusters formed early in the universe, roughly 13 billion years ago. They have long stopped making stars and contain only old stars.

  3. 2. Why is this discovery important?

  4. The discovery suggests that the universe was quite efficient at making planets very early in its history, despite the fact that the "construction material" for planets was rare. The heavier elements needed to make at least the cores of planets weren't abundant until the first "star factories" began forging oxygen, silicon, nitrogen, etc., in their nuclear-fusion furnaces. This discovery implies that our galaxy is abundant in planets. And, where there are planets there could be life.

  5. 3. Does this raise any new mysteries?

  6. The planet's presence may challenge astronomers to revisit theories and models for how planets form. It was thought that lots of heavier elements and lots of time were needed to make a Jupiter-sized planet. But somehow the universe figured out how to do this very early in its history, when stars were just beginning to appear.

  7. 4. What does the planet look like?

  8. The planet is too dim and far away for Hubble to photograph. It probably looks like Jupiter, with belts and zones of clouds. It is so big it must be predominantly made of helium and hydrogen. The planet might have moons and rings.

  9. 5. Could anyone live there?

  10. If this planet were inhabited it would be home for conceivably one of the oldest civilizations in our galaxy. Its life forms would have been far evolved by the time the very first primitive cells were assembling in Earth's primeval oceans. However, this planet is a gas giant like Jupiter, and so has no solid surface for life as we know it to arise. But if the planet has rocky moons, like the satellites of Jupiter, they might be abodes for life. Unfortunately the planet's star is burned out, so any indigenous life that is dependent on stellar radiation would be frozen by now.

  11. 6. Why didn’t earlier Hubble searches find planets in globular clusters?

  12. A landmark Hubble observation in the late 1990s looked for transiting planets (planets passing in front of their stars) in the globular cluster 47 Tucanae. This type of observation was only sensitive to giant, close-orbiting planets, dubbed "hot Jupiters." The fact they weren't found doesn't rule out the existence of planets in globular clusters. But, intriguingly, it suggests that this type of close-orbiting planet doesn't form in globulars.

  13. 7. What type of star does the planet orbit?

  14. When the planet was born, it orbited a star like our Sun. The star was later captured by a crushed, spinning stellar core called a neutron star. It shoots out lighthouse beams of radio energy and is also called a pulsar.

  15. 8. How long did it take to find the planet?

  16. The pulsar betrayed the planet's presence in observations made in the late 1980s. The pulsar wobbles slightly, as measured by its clock-precision radio pulses. This showed the pulsar was being gravitationally tugged by two orbiting objects. The white dwarf was identified in the 1990s, but the planet's presence was debated among astronomers for over a decade. Finally Hubble telescope observations were used to calculate the planet's mass, which is only 2.5 times the mass of Jupiter. This finding rules out that the object is a brown dwarf star, which is considerably more massive.

  17. 9. How was Hubble used to measure the planet’s mass?

  18. Astronomers had to rely on some cosmic detective work to measure the planet's mass. Hubble observations were used to study a white dwarf star that the planet orbits. The two bodies are about the same distance that Uranus is from our Sun. Knowing the color and temperature of the dwarf allowed astronomers to deduce the dwarf's age and mass. This allowed them to calculate the tilt of its orbit around the pulsar. This then allowed the planet's mass to be calculated, assuming its orbit is similarly tilted.

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Credit: NASA, Brad Hansen (UCLA), Harvey Richer (UBC), Steinn Sigurdsson (Penn State), Ingrid Stairs (UBC), and Stephen Thorsett (UCSC).