Big Advance in Study of Extrasolar Planets

The Astro-comb, an astounding advance in taking the spectra of stars, soon may allow Earthlings to know whether a small, distant planet is an ocean world, a rocky planet like our own or something completely different.

Ron Walsworth, senior physicist at the Harvard-Smithsonian Center for Astronomy, Cambridge, Mass., told of the technology he and colleagues invented, during a lecture Wednesday night in the University of Utah’s Aline Wilmot Skaggs Biology Building. The free public talk, part of the U.’s Frontiers of Science lecture series, was sponsored by the College of Science and the College of Mines and Earth Science.

[Ron Walsworth lecturing at the University of Utah Wednesday night. Photo by Cory Bauman]

The energetic Walsworth charmed the hundreds in the audience, as he moved around, gesturing to show concepts like stretching of star spectra or the orbits of planets.

Those who don’t remember their high-school chemistry are reminded that light can be broken into spectra. In 1672 Sir Isaac Newton created a spectrum when he sent sunlight through a prism, dividing the light into its component colors ranging from red to violet. He could not see others, infrared and ultraviolet, because they are beyond the range that can be detected by the human retina, but they were there too.

Those who weren’t passing notes or dozing in Mr. Norton’s chemistry class also may recall that a spectrum shows black lines that correspond to elements or molecules within the material emitting the light. When analyzed, these absorption lines tell scientists what substances are in a sample being burned or in a star giving off light.

In elementary physics we learned about Doppler shifts. A classic example happens when a train passes: the whistle is high-pitched when it’s approaching, but shifts to a lower range as it recedes, because the sound waves are squished together ahead of the train and lengthened when they trail behind. The shorter the wavelength the higher the pitch. The same with light. Light waves from a star that happens to be going toward Earth are compressed to a higher frequency, that is, shifted to the blue, while the light from a star moving away is red-shifted.

The terms don’t mean the light looks different between coming and going stars of the same type. Blue-shifted means shifted toward the blue, while red-shifted means toward the red end of the spectrum. The shifts can be extremely small.

But how can we tell if light is shifted at all, if it really doesn’t look different? That’s where those black lines come in. Starlight carries absorption lines that indicate the material of which the star is made. If these characteristic patterns are shifted toward the blue, we know the star is approaching; if toward the red, receding.

As a planet orbits, its gravity tugs on the central stars. The larger and closer the planet, the greater the effect; the smaller and more distant, the less. Astronomers studying Doppler shifts in starlight can tell when a planet is tugging the star toward Earth and when it’s on the other side, pulling it away.

These motions are tiny and hard to find. So far, Earth-based observatories can only detect relatively massive exoplanets, with an emphasis on those that are closest to their stars, because their gravity has the most powerful effect on the stars.

Many of the 400-plus planets found so far outside our solar system are “hot Jupiters,” called that because they are huge gas giants orbiting close to the stars. The bias in their favor isn’t necessarily because they’re more common than other planets but because the detectors haven’t been sensitive enough to detect the gentle attraction of smaller distant planets.

Small planets like ours, at the right distance from the stars, seem most likely to host life, at least as we know it. If an exoplanet is the right size and composition, orbiting in the “Goldilocks zone” where it’s neither too hot nor too cold for liquid water, maybe it has life.

The Kepler probe, which NASA launched a little more than a year ago, has been staring at 100,000 stars in a patch of space in the Cygnus and Lyra regions of the constellations. Its sensors are so acute they know when a star’s light dims by a minute fraction. If a Jupiter-size planet happens to pass in front of its star, it blocks about the same amount of light that Jupiter blocks from our sun, 1 percent, during that eclipse period.

Walsworth said Kepler’s CCD array is so sensitive that it could detect the passage of an Earth-size planet in front of its star by the dip of only 0.01 percent of the starlight, the amount our planet blocks.

When a planet clears the star, the usual amount of light strikes the instruments. If an eclipsing planet has a one-year orbit, like Earth, the drop in light output would happen once a year. Confirming that an Earth-size planet orbits in the Goldilocks zone around a sun-like star could take two or three years, depending on the number of orbits the researchers require before announcing their discovery.

Kepler can determine a planet’s size and its distance from the star; that is, whether it’s Earth-size or not, and whether it is in the Goldilocks zone. But it can’t tell anything about the planet’s mass.

Planets have wildly varying masses. Saturn’s density is so low that it is less dense than water. Earth is like rock and metal in its density because that’s what it is mostly.

Enter Walsworth and his team’s Astro-comb. The device is named for its ability to comb out interesting information from starlight spectra.

Using laser pulses firing at a millionth of a billionth of a second, precision timing and special filtering, the Astro-comb will calibrate starlight, breaking down a star’s spectrum into thousands of detailed segments. As many as 100,000 absorption lines will show up. This offers greater resolution in the details of the star’s chemistry and movement.

Like the spectra of infrared and ultraviolet that Newton couldn’t see, the thousands of absorption lines are in starlight. They’ve only been undetectable until now.

The lines will be “creating this fingerprint pattern,” said Walsworth. The fingerprints will give an enormous amount of information about the star’s composition. Amazingly, the detail will be so fine that small Doppler shifts caused by the pull of little Earth-like planets should show up as the planet is in front of the star, then behind it.

The force of the gravitational tug, measured by the amount of shifting, will help determine the planet’s mass.

“The goal is to observe the Earth-like planets being found by Kepler,” he said.

In 2008, in a demonstration project, the team installed the Astro-comb in a telescope at the Whipple Observatory, owned by the Smithsonian south of Tucson. In 2011 the team intends to install it in the 4.2-meter (nearly 14-foot diameter) William Herschel Telescope in the Canary Islands, where it can operate to examine planets found by Kepler.

With Kepler showing the size of a planet and its period (any by extension, its distance from the star), the Astro-comb will determine its mass. We will know whether the planet is a water world or a rocky planet like Earth.

In the next five years, Walsworth predicted, Kepler will have discovered several Earth-size planets. Eventually, the planned James Webb Space Telescope should be able to detect spectra from the light of distant planets, which might indicate the presence of life.

Oxygen is famously reactive, not remaining in a free state for long after it is released by plants. It remains at around 20 percent of our atmosphere only because it’s constantly resupplied by vegetation. If the Webb telescope finds free oxygen in an exoplanet’s atmosphere, Walsworth said, “hum, hum, hum, that probably means there’s something going on there — something growing …. Mushrooms in the basement.”

He added, “We don’t know yet but we’ll soon know, perhaps in the next decade.”

The discoveries that are coming, Walsworth said, will help humankind “in knowing our place in the universe.”

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