First Light From Distant Planets Seen by Telescope

Brian Handwerk
for National Geographic News
March 23, 2005
NASA's Spitzer Space Telescope has detected the light from two distant planets. The scientific first opens a new era in which scientists can directly observe the extrasolar worlds that orbit distant stars.

It's also the first step toward actual photographs of planets outside our solar system.

Two separate teams made the discoveries, which were announced yesterday at a NASA press conference.

"Spitzer has provided us with a powerful new tool for learning about the temperatures, atmospheres, and orbits of planets hundreds of light-years from Earth," said Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The scientist is the lead author of a new study on one of the planets.

More Discoveries Likely

Just ten years ago extrasolar planets were nearly unknown. But indirect observation techniques have produced many recent discoveries.

Indirect techniques include detecting a planet from the "wobble" its gravity causes in the planet's host star. Another method is to observe the "darkening" of a star when an otherwise indetectable planet passes in front of it.

The new Spitzer observations are the first times extrasolar planets have been detected on their own merits, so to speak, rather than because of their effects on other celestial bodies.

"The past decade has been an exciting time for the study of planets," said David Charbonneau, an assistant professor of astronomy at the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Massachusetts.

"We've detected over 130 [extrasolar planets] and been able to infer some of their properties indirectly," said Charbonneau, the lead author of the second study. "Now, for the first time, we can isolate the light from the planet itself—that's a precious piece of information."

Launched in August 2003, the infrared Spitzer Space Telescope trails behind Earth during its orbit around the sun.

Spitzer captured infrared emissions from the two extrasolar planets, known as HD 209458b and TrES-1. The two planets shine brightly because of their status as "hot Jupiters." The gas giants are similar to Jupiter but orbit so close to their respective stars that they become superheated to some 1,000° Kelvin (727° Celsius/1,340° Fahrenheit).

Researchers collected infrared light from both the planets and their nearby stars. When an orbiting planet passed behind its star, new measurements revealed the light originating from the star alone. The difference is the exact amount of infrared light coming from each planet.

"In visible light, the glare of the star completely overwhelms the glimmer of light reflected by the planet," Charbonneau explained. "In infrared, the star-planet contrast is more favorable, because the planet emits its own light."

The Web site of the science journal Nature published the Deming paper yesterday. The Charbonneau paper will appear in an upcoming issue of the Astrophysical Journal.

Additional observations will map each planet's infrared light spectrum, which can tell scientists about the composition of the two distant worlds.

Yesterday's announcement also highlighted the mystery surrounding HD 209458b's unusual, puffy size.

"So far there is no particularly promising explanation of why it is so large," Deming said. "In this case we're left with a mystery to be solved."

Many more such quandaries will surely arise now that planets can be detected with infrared light.

"Spitzer was not designed to find planets, but it has led us to this first discovery and whets our appetite for what's coming in the next ten years," said Alan Boss, a staff research astronomer with the Carnegie Institution in Washington, D.C.

"Hot Jupiters are interesting, but in the end we really want to find warm Earths as well," he said.

Old, Cold, and Dirty

All celestial objects with temperatures above absolute zero (–273° Celsius/–460° Fahrenheit) emit infrared radiation, or heat. With an infrared orbiting telescope such as Spitzer, astronomers can detect distant or faint sources of radiation that would otherwise be distorted by Earth's atmosphere.

Spitzer is the last of NASA's four orbiting "Great Observatories." The group includes the Hubble Space Telescope, the Compton Gamma-Ray Observatory, and the Chandra X-Ray.

Researchers use this multifaceted tool kit to glimpse the origins of galaxies, stars, and planets.

Spitzer team scientists like to describe their mission as a search for "the old" (the earliest stars and galaxies), "the cold" (brown dwarfs, possibly stars that failed to ignite, and circumstellar discs, broad rings of material orbiting a star) and "the dirty" (dust-obscured processes such as star and planetary formation).

The Spitzer telescope is also a key component of the NASA Origins Program, which seeks to answer the fundamental questions: Where did we come from, and are we alone?

The planetary observations are the second major Spitzer announcement in less than a month.

On March 1, NASA revealed that the telescope had discovered a hidden group of exceptionally bright galaxies some 11 billion light-years away in deep space.

Though the galaxies were as bright as 10 trillion suns, their great distance and a cloak of cosmic dust had hidden them from all but Spitzer's finely tuned infrared apparatus.

The telescope recently recorded its ten-thousandth unique observation. The instrument has focused on everything from stars and brown dwarfs (radiation-emitting celestial objects that are smaller and dimmer than stars) to interstellar clouds of gas.

Many more observations should follow over the next few years. Spitzer is expected to enjoy a lifetime of two and a half to five years or more, while gradually drifting away from Earth's orbit toward deep space.

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