Campus & Community

Light detected from alien planets

4 min read

Historic moment for astronomy

Light from two worlds far from our solar system has been detected for the first time. The planets that emit it are too hot to be inhabited, at least by intelligent beings, but an Earthly satellite has gotten the best views yet of planets orbiting other stars like the sun.
“It’s an awesome experience to realize we are seeing the direct glow of distant worlds,” says David Charbonneau of the Harvard-Smithsonian Center for Astrophysics. “When I first saw the data, I was ecstatic.”

The “data” is not light blinking coded messages into space; rather it consists of infrared radiation that you would need special goggles to see. Still, it’s a historic moment for astronomy. “Such light tells us things about faraway planets that we could never learn before, such as their temperatures and what gases are in their atmospheres.”

The otherworldly shines were detected by instruments aboard the National Aeronautics and Space Administration’s Spitzer Space Telescope. Both planets are hot balls of gas hundreds of times more massive than Earth. Because they orbit much closer to their stars than our planet does to the sun, their surfaces are heated to about 1,500 degrees Fahrenheit. “They would be rough places to call home,” Charbonneau comments.

A better view

One planet is called HD 209458b, the other TrES-1. “We don’t get to pick their names,” Charbonneau noted when asked about their seeming awkwardness. HD 209458b’s name is taken from the star it orbits; TrES-1 comes from the survey that discovered it. HD 209458b was first found by an international team of skywatchers led by David Latham, a Harvard-Smithsonian astronomer. This team found it, not by its light, but by the wobble induced on the star by the gravitational tug of the planet, which can be measured from Earth. Most of the more than 130 planets outside our solar system have been found this way.

Latham got Charbonneau interested in HD 209458b, and the planet became the focus of Carbonneau’s Ph.D. thesis. Realizing that a lot more can be learned about the variety of planets in the Milky Way, Charbonneau, now 30 and an assistant professor of astronomy at Harvard, has made a career out of such alien worlds.

When NASA launched the Spitzer telescope in August 2003, Charbonneau and his colleagues all over Earth saw it as a way to get a better view of these worlds. Cameras and other instruments aboard the satellite see the universe in infrared “light,” radiation from the planets that shines more brightly than the weak visible light they give off.

A team from NASA’s Goddard Space Flight Center in Greenbelt, Md., started watching HD 209458 through one instrument on Spitzer. Charbonneau and his crew chose to study TrES-1. Charbonneau describes the relationship between the two competing teams as “very friendly.”

TrES-1 is roughly 3,000 trillion miles away, in the summer constellation Lira. Light from it traveled for 500 years before reaching the Spitzer camera “That sounds like a long way,” admits Charbonneau, “but TrES-1 is virtually our neighbor when you consider the vast distances of the universe.”

On Halloween day 2004, Charbonneau, colleagues Lori Allen, Tom Megeath, and others first saw the light. To make it sweeter, it came through an infrared camera designed by their Harvard-Smithsonian colleagues.

When astronomers detect a planet indirectly, by the way it wobbles its mother star, they can’t tell much about it. If aliens looked at the sun this way, they could discover that Jupiter is about 300 times more massive than Earth. But they wouldn’t be aware of Jupiter’s colorfully banded clouds or Earth’s watery surface. They would miss the fact that Jupiter is a big ball of hot gas, while Earth is a cool rock coated with living things. So a new window has opened through which Earthlings can get a better look at some of our Milky Way neighbors.