Image: SIM concept
An artist's interpretation shows how the Space Interferometry Mission might look in space.
updated 3/17/2004 4:39:03 PM ET 2004-03-17T21:39:03

Some of the smallest measurements on Earth are helping scientists prepare for a space mission that, they hope, will find Earthlike planets revolving around alien stars.

The key to those measurements is a device known as the Microarcsecond Metrology Testbed, a collection of lasers, lenses and mirrors aimed at proving the ability to determine stellar distances accurate to within one-tenth the width of a hydrogen atom — or about a millionth the thickness of a human hair.

Despite being firmly attached to terra firma, the testbed is a technological linchpin for a space observatory that will make some of the most accurate astrophysical observations ever known.

"To see an Earth-mass planet, we have to be able to make measurements that are 1,000 times better than what we presently have," explained Michael Shao, leader of the Space Interferometry Mission science team at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "And essentially we're just about there."

Using the silvery, submarine-shaped microarcsecond testbed, JPL engineers were able to make measurements of light from a simulated star that were accurate down to a single microarcsecond, Shao added.

A microarcsecond is a tiny fraction of a sky measurement called an arcsecond used by astronomers to determine the size of objects in space. There are 60 arcseconds in an arcminute, and 60 arcminutes in one degree of sky. The moon, for example, is about half a degree (30 arcminutes or 1,800 arcseconds) wide. If you're not lost yet, one microarcsecond is just one-millionth of a single arcsecond.

So with its one-microarcsecond resolution, the JPL testbed would be sensitive enough to detect the movement of a wiggling pinky finger attached to an astronaut who was standing on the moon. Better still, once a similar setup is lofted into space aboard the Space Interferometry Mission, or SIM, it could detect the presence of an Earth-mass planets by observing the "wobble" of its parent star.

Getting past jumbo-sized Jupiters
Most of the more than 100 extrasolar plants discovered to date have been gas giants the size of Jupiter or larger. Astronomers believe they may have also found at least a few Earth-size planets around other stars, but Shao and his science team wants to find more.

"We're basically hoping to find several terrestrial-type planets with SIM," he said. In addition to his role as leader of the SIM science team, Shao is principal investigator of an extrasolar planet survey using the space-based instrument. "And we'll be looking at a couple of hundred nearby stars," he added.

By looking at such a large number of stars, astronomers hope to learn more about the morphology of planetary systems and whether the nine-planet configuration of our own solar system is a run-of-the-mill arrangement across the universe.

As its name suggests, the Space Interferometry Mission is an effort to conduct interferometry in space. The mission plan calls the launch of two telescopes spaced 33 feet (10 meters) apart to combine the light they detect from identical targets. The combined light allows astronomers to learn the dimensions of a distant object better than with a traditional telescope.

The mission is scheduled for launch sometime between 2009 and 2010, but in order to get there JPL researchers had to know whether they could even design an instrument sensitive enough for their needs. The discovery of many massive extrasolar planets found to date has been possible because astronomers can see the gravitational influence they have on their parent stars. But smaller planets, such as those about the size of Earth, are more difficult to spot because their gravitational influence is much smaller — hence the microarcsecond testbed.

"We wanted to be able to demonstrate our ability to make measurements at the microarcsecond level before we put something up in orbit," Shao said, adding that one of the main challenges was stripping away the machine's ability foul up its own accuracy. "Essentially, it's like peeling an onion, because there are many sources of systematic errors that can lead to measurement error."

Even a tiny amount of moving air can interfere with results inside the microarcsecond testbed chamber, so the area is kept in vacuum during each experiment. Adjustable mirrors, lasers and a camera then detect variations in simulated starlight to determine how much the star is moving, or "wobbling."

While it took more than six years to perfect the testbed, its success has allowed NASA to move forward with other design efforts within the SIM mission. In the meantime, JPL engineers are working on a second level of accuracy, now that one microarcsecond has been achieved. They are working to give the instrument a wide-angle accuracy of four microarcseconds, which would be useful for non-planet hunters hoping to use SIM solely for astrophysical research.

Not just a planet-finder
While there are three specific planet-finding studies planned for SIM, the mission also includes a number of astrophysical surveys as well. Among them are studies of the Milky Way, the motions of galaxies and binary black holes.

SIM scientist Todd Henry, an associate astronomy professor at Georgia State University, said the mission should allow him to make measure the masses of a cross-section of stars with remarkable accuracy.

"We're going to be looking from the most massive stars all the way down to brown dwarfs," Henry told "To do that, we're going to need the full precision shown in the testbed."

The abilities showcased by the SIM testbed would allow the final SIM spacecraft to determine stellar masses with unprecedented accuracy, with the margin for error set at 1 percent or less. That would let Henry's research team collect mass information for about 200 stars and create a yardstick showing the relationship between stellar mass and luminosity that could later be applied to the universe as a whole.

Henry does concede that the most interesting part of SIM's mission is its mandate to find extrasolar planets.

"I personally would like to see if any of these binary star systems could have planets," Henry said, adding that, in his mind, the best place to look for Earth-sized planet will be around the smaller stars, where perturbations from extrasolar planet gravity will be more pronounced. "I'm looking at the little red stars, that's where they'll be."

© 2013 All rights reserved. More from

Interactive: The search for extrasolar planets


Discussion comments


Most active discussions

  1. votes comments
  2. votes comments
  3. votes comments
  4. votes comments