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First Light Captured from Extrasolar Planets

March 23, 2005

JPL — NASA’s Spitzer Space Telescope has for the first time captured the light from two known planets orbiting stars other than our Sun. The findings mark the beginning of a new age of planetary science, in which “extrasolar” planets can be directly measured and compared.

“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 Dr. Drake Deming of NASA’s Goddard Space Flight Center, Greenbelt, Md., lead author of a new study on one of the planets.

“It’s fantastic,” said Dr. David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a separate study on a different planet. “We’ve been hunting for this light for almost 10 years, ever since extrasolar planets were first discovered.” The Deming paper appears today in Nature’s online publication; the Charbonneau paper will be published in an upcoming issue of the Astrophysical Journal.

So far, all confirmed extrasolar planets, including the two recently observed by Spitzer, have been discovered indirectly, mainly by the “wobble” technique and more recently, the “transit” technique. In the first method, a planet is detected by the gravitational tug it exerts on its parent star, which makes the star wobble. In the second, a planet’s presence is inferred when it passes in front of its star, causing the star to dim, or blink. Both strategies use visible-light telescopes and indirectly reveal the mass and size of planets, respectively.

In the new studies, Spitzer has directly observed the warm infrared glows of two previously detected “hot Jupiter” planets, designated HD 209458b and TrES-1. Hot Jupiters are extrasolar gas giants that zip closely around their parent stars. From their toasty orbits, they soak up ample starlight and shine brightly in infrared wavelengths.

To distinguish this planet glow from that of the fiery hot stars, the astronomers used a simple trick. First, they used Spitzer to collect the total infrared light from both the stars and planets. Then, when the planets dipped behind the stars as part of their regular orbit, the astronomers measured the infrared light coming from just the stars.

This pinpointed exactly how much infrared light belonged to the planets. “In visible light, the glare of the star completely overwhelms the glimmer of light reflected by the planet,” said Charbonneau. “In infrared, the star-planet contrast is more favorable because the planet emits its own light.”

The Spitzer data told the astronomers that both planets are at least a steaming 1,000 Kelvin (727 degrees Celsius, 1340 Fahrenheit). These measurements confirm that hot Jupiters are indeed hot. Upcoming Spitzer observations using a range of infrared wavelengths are expected to provide more information about the planets’ winds and atmospheric compositions.

The findings also reawaken a mystery that some astronomers had laid to rest. Planet HD 209458b is unusually puffy, or large for its mass, which some scientists thought was the result of an unseen planet’s gravitational pull.

If this theory had been correct, HD 209458b would have a non-circular orbit. Spitzer discovered that the planet does in fact follow a circular path. “We’re back to square one,” said Dr. Sara Seager, Carnegie Institution of Washington, Washington, co-author of the Deming paper. “For us theorists, that’s fun.”

Spitzer is ideally suited for studying extrasolar planets known to transit, or cross, stars the size of our Sun out to distances of 500 light-years. Of the seven known transiting planets, only the two mentioned here meet those criteria.

As more are discovered, Spitzer will be able to collect their light — a bonus for the observatory, considering it was not originally designed to see extrasolar planets. NASA’s future Terrestrial Planet Finder coronagraph, set to launch in 2016, will be able to directly image extrasolar planets as small as Earth.

Shortly after its discovery in 1999, HD 209458b became the first planet detected via the transit method. That result came from two teams, one led by Charbonneau. TrES-1 was found via the transit method in 2004 as part of the NASA-funded Trans-Atlantic Exoplanet Survey, a ground-based telescope program established in part by Charbonneau.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center, at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

Videos and Animations

A Planet in a Different Light

This artist’s animation shows first what a fiery hot star and its close-knit planetary companion might look like close up in visible light, then switches to infrared views. In visible light, a star shines brilliantly, overwhelming the little light that is reflected by its planet. In infrared, a star is less blinding, and its planet perks up with a fiery glow.

Astronomers using NASA’s Spitzer Space Telescope took advantage of this fact to directly capture the infrared light of two previously detected planets orbiting stars outside our solar system. Their findings revealed the temperatures and orbits of the planets. Upcoming Spitzer observations using a variety of infrared wavelengths may provide more information about the planets’ winds and atmospheric compositions.

In this animation, the colors represent real differences between the visible and infrared views of the system. The initial visible view shows what our eyes would see if we could witness the system close up. The hot star is yellow, because like our Sun, it is brightest in yellow wavelengths. The warm planet, on the other hand, is brightest in infrared light, which we can’t see. Instead, we would see the glimmer of star light that the planet reflects.

In the second half of the animation, the colors reflect what our eyes might see if we could retune them to the invisible, infrared portion of the light spectrum. The hot star is less bright in infrared light than in visible and appears fainter. The warm planet peaks in infrared light, so is shown brighter. Their hues represent relative differences in temperature. Because the star is hotter than the planet, and because hotter objects give off more blue light than red, the star is depicted in blue, and the planet, red.

The overall look of the planet is inspired by theoretical models of hot, gas giant planets. These “hot Jupiters” are similar to Jupiter in composition and mass, but are expected to look quite different at such high temperatures. The models are courtesy of Drs. Curtis Cooper and Adam Showman of the University of Arizona, Tucson.

Slow Connections (160×120): Windows Media (100 KB) | QuickTime 4.0 (68 KB)
Fast Connections (320×240): Windows Media (604 KB) | QuickTime 4.0 (1 MB) | QuickTime 6.0 (1 MB) | MPEG 1 (1.3 MB) | MPEG 4 (780 KB)
Full-Size (640×480): Windows Media (2.1 MB) | QuickTime 6.0 (4 MB) | MPEG 4 (3.1 MB)
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Distant Planet Flaunts its Light

This artist’s animation shows a close-up view of a distant giant planet passing behind its star as a regular part of its orbit. By studying “secondary eclipses” like this in infrared light, astronomers can capture and study the direct light of known extrasolar planets. Though they cannot actually distinguish the planet from its star, they can detect changes in the system’s total light.

Why a secondary eclipse? When a planet transits, or passes in front of, its star, it partially blocks the light of the star. When the planet swings around behind the star, the star completely blocks its light. This drop in total light can be measured to determine the amount of light coming from just the planet.

Why infrared? In visible light, the glare of a star overwhelms its planetary companion and the little light the planet reflects. In infrared, a star shines less brightly, and its planet gives off its own internal light, or heat radiation, making the planet easier to detect.

By observing these secondary eclipses at different infrared wavelengths, astronomers can obtain the planet’s temperature, and, in the future, they may be able to pick out chemicals sprinkled throughout a planet’s atmosphere. The technique also reveals whether a planet’s orbit is elongated or circular.

This strategy will not work for all known extrasolar planets. It is ideally suited to study those Jupiter-sized planets previously discovered to cross, or transit, between us and the Sun-like stars they orbit, out to distances of 500 light-years. NASA’s Spitzer Space Telescope was the first to successfully employ this technique.

In this animation, the colors reflect what our eyes might see if we could retune them to the invisible, infrared portion of the light spectrum. The hot star is a bit fainter than it would appear in visible light, and the planet glows more intensely. Their hues represent relative differences in temperature. Because the star is hotter than the planet, and because hotter objects give off more blue light than red, the star is depicted in blue, and the planet, red.

Slow Connections (160×120): Windows Media (88 KB) | QuickTime 4.0 (64 KB)
Fast Connections (320×240): Windows Media (504 KB) | QuickTime 4.0 (900 KB) | QuickTime 6.0 (984 KB) | MPEG 1 (1.1 MB) | MPEG 4 (720 KB)
Full-Size (640×480): Windows Media (1.7 MB) | QuickTime 6.0 (3.7 MB) | MPEG 4 (2.8 MB)
Broadcast Quality

How to Measure a Planetary Eclipse

This artist’s animation shows a close-up view of a distant giant planet passing behind its star as a regular part of its orbit. By studying “secondary eclipses” like this in infrared light, astronomers can capture and study the direct light of known extrasolar planets. Though they cannot actually distinguish the planet from its star, they can detect changes in the system’s total light.

Why a secondary eclipse? When a planet transits, or passes in front of, its star, it partially blocks the light of the star. When the planet swings around behind the star, the star completely blocks its light. This drop in total light can be measured to determine the amount of light coming from just the planet, as demonstrated in the graph overlay.

Why infrared? In visible light, the glare of a star overwhelms its planetary companion and the little light the planet reflects. In infrared, a star shines less brightly, and its planet gives off its own internal light, or heat radiation, making the planet easier to detect.

By observing these secondary eclipses at different infrared wavelengths, astronomers can obtain the planet’s temperature, and, in the future, they may be able to pick out chemicals sprinkled throughout a planet’s atmosphere. The technique also reveals whether a planet’s orbit is elongated or circular.

This strategy will not work for all known extrasolar planets. It is ideally suited to study those Jupiter-sized planets previously discovered to cross, or transit, between us and the Sun-like stars they orbit, out to distances of 500 light-years. NASA’s Spitzer Space Telescope was the first to successfully employ this technique.

In this animation, the colors reflect what our eyes might see if we could retune them to the invisible, infrared portion of the light spectrum. The hot star is a bit fainter than it would appear in visible light, and the planet glows more intensely. Their hues represent relative differences in temperature. Because the star is hotter than the planet, and because hotter objects give off more blue light than red, the star is depicted in blue, and the planet, red.

Slow Connections (160×120): Windows Media (88 KB) | QuickTime 4.0 (64 KB)
Fast Connections (320×240): Windows Media (508 KB) | QuickTime 4.0 (896 KB) | QuickTime 6.0 (992 KB) | MPEG 1 (1.1 MB) | MPEG 4 (<720 KB)
Full-Size (640×480): Windows Media (1.8 MB) | QuickTime 6.0 (3.8 MB) | MPEG 4 (2.9 MB)
Broadcast Quality

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On the Net:

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Spitzer Space Telescope


First Light Captured from Extrasolar Planets First Light Captured from Extrasolar Planets


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