Young Planet Challenges Old Theories

The Spitzer Space Telescope has detected the youngest planet ever found, claim NASA scientists. Planets are thought to take many millions of years to form after a star is born, but the discovery of a million-year old star with a planet already in orbit around it means scientists may have to rethink planetary formation models.

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Out of the Dust, A Planet is Born
Image Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

Edward Churchwell, an astronomer with the University of Wisconsin and a principal investigator for GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire), said the new findings "knocked our socks off. We were really excited."

The star CoKu Tau 4 is surrounded by a dusty disc, which is typical of very young stars. A star is born inside a dense cloud of gas and dust. Within this cloudy envelope, a flat, dusty disc encircles the star, and planets develop from the material in this disc.

Spitzer’s infrared spectrograph (IRS) observed a clearing in the dusty disc around CoKu Tau 4. The clearing is about 10 AU in size, or 10 times the Earth-Sun distance. The theory is that a planet orbiting CoKu Tau 4 at about 10 AU scooped up much of the inner disc material, and prevents the dusty outer ring of material from falling in towards the star.

Such empty regions separating a star from its dusty disc have been seen before, but never in so young a star. Because CoKu Tau 4 is about one million years old, the possible planet would be even younger.

In the standard core accretion model of planet formation, dusty grains hit and clump together as they swirl around a star. Over time, these clumps grow bigger and bigger, eventually becoming proto-planetary lumps that acquire even more material through gravity.

Scientists believe gas giant planets like Jupiter form first in a solar system, gathering much of the dust and gas of the star’s original disc. It is thought that gas giants take about 4 million years to form through core accretion.

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Locations of RCW 49 and Taurus Star-Forming Regions
Credit: ESO

Alan Boss, an astronomer with the Carnegie Institution of Washington, has an alternative theory that could account for a gas giant forming so early in the life of CoKu Tau 4. In his disc instability model, points of instability within the dusty disc create gravity wells that form clumps. These clumps increase in density to become gas giant proto-planets in only a few thousand years.

"If that planet actually formed by disc instability, that has profound implications for the prevalence of planetary systems similar to our own," says Boss. "That means you can make gas giant planets – a major component of our own solar system – in a short time scale, in even the shortest-lived disc."

Another explanation for the cleared region around the star could be an unseen stellar companion (a binary star system), but scientists have failed to find even the faintest glimmer of a companion star. The clearing also could be caused by the formation of asteroids and comets, or by the heat and light of the star blowing the material outwards. But the inner edge of the dusty disc is very sharp, says Dan Watson, an astronomer with the University of Rochester, N.Y. who works with the IRS science team on Spitzer. Watson says that only a planet could be responsible for such a sharply defined inner margin, and planetary formation is also the only explanation that could give the degree of clearing seen around CoKu Tau 4.

"I expect Spitzer will discover many more of these important objects, each one a unique laboratory into disc evolution, and quite likely planet formation," says Anne Kinney, director of the Astronomy and Physics Division of the Office of Space Science at NASA Headquarters in Washington.

Spitzer also detected icy organic materials in the dust discs encircling other young stars. These stars are in the constellation Taurus, 420 light years from Earth. The icy particles are coated with water, methanol and carbon dioxide. These ice grains could develop into comets, which could then provide water and organic materials to any terrestrial planets that eventually may form. Scientists believe comets provided the early Earth with some of the water and organic materials that made life possible. This is the first time such icy organic materials were seen unambiguously in the dust of planet-forming discs.

SIRTF
In the early morning of August 25, 2003, NASA launched the Space Infrared Telescope Facility (SIRTF) the fourth and final element in NASA’s family of Great Observatories.
Credit: Ball Aerospace & Technologies Corp., 2003

Spitzer also looked at a star nursery called RCW 49 and spotted 300 newborn stars in one image. This star-forming region is approximately13,700 light years from Earth and can be seen in the constellation Centaurus. Two of the stars in this region are surrounded by proto-planetary dust discs. Churchwell said it is possible that all 300 stars in the nursery could harbor such discs.

"We’ve seen evidence from these observations with Spitzer that there are ices and simple organic compounds such as methanol in these discs," says Boss. "The UV (ultraviolet) flux in addition, if it’s in a disc (in a star-forming region), could perhaps convert these ices into more advanced organic compounds such as polycyclic aromatic hydrocarbons."

Polycyclic aromatic hydrocarbons (PAHs) are the building blocks for more complex organic molecules, and they appear to be widespread throughout the universe. PAHs may have played a vital role in the origin of life on Earth.

"Even amino acids – alanine and guanine – can form, as has been shown by laboratory studies, by UV irradiation of simple ices," adds Boss. "So one can imagine a system where the formation of life-bearing elements is hastened along by pre-biotic chemistry that occurs in the disc, even before things start getting going on a habitable planet."

Papers on the research will appear in the September 1 issue of the journal Astrophysical Journal Supplements.

The Spitzer Space Telescope is the fourth of NASA’s Great Observatories, which include the Hubble Space Telescope (visible light), the Chandra X-ray Observatory and the Compton Gamma Ray Observatory. The Spitzer Space Telescope detects infrared radiation, or heat, from distant, cold, and dust-obscured celestial objects.

Launched on August 25, 2003, the Spitzer Space Telescope is orbiting the sun, trailing behind and receding from the Earth. This unusual orbit allows the telescope to avoid the Earth’s heat, and it also prevents the Earth from blocking the telescope’s field of view. The telescope will move away from Earth at about a tenth of an AU per year. The Spitzer Space Telescope has a projected life span of 5 years.