Tiny Planet SIM

"We are witnessing the birth of a new observational science: the discovery and characterization of extrasolar planetary systems." –G. Marcy, UC Berkeley

Even though astronomers have discovered more than 100 planets around stars other than the Sun in recent years, the "holy grail" of the search — an Earth-sized planet capable of supporting life — remains elusive. The main problem is that an Earthlike planet would be much smaller than any of the gas giants detected so far.

SIM reference design
SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL

Planets orbiting other stars are too dim to be observed directly, but scientists infer their presence by the tiny gravitational "wobble" they induce in their parent stars. Observed from tens of light years away (one light-year is 5.88 trillion miles), this movement becomes very tiny indeed. The smaller the planet, the less the star parent wobbles.

To detect the stellar wobble caused by a planet as small as Earth, scientists need an instrument of almost unbelievable sensitivity. Let’s say there’s an astronaut standing on the moon, wiggling her pinky. You’d need an instrument sensitive enough to measure that movement from Earth, a quarter million miles away.

In order to do that, the instrument needs to be a "ruler" accurate to within just one-tenth the width of a hydrogen atom. That’s about 1 millionth of the width of the thickest human hair.

Is such precision possible? After a six-year struggle, engineers at the Jet Propulsion Laboratory recently proved that the answer is yes.

Such sub-atomic measurements were conducted for the first time ever within a vacuum-sealed chamber called the Microarcsecond Metrology Testbed.

By doing this, the engineers proved they can measure the movements of stars with an astonishing degree of accuracy never before achieved in human history.

The testbed, which resembles a shiny silver submarine, is jammed with mirrors, lasers, lenses and other optical components. Because even small air movements can interfere with the measurements, all air is pumped out of the chamber before each experiment is run. Laser beams, moving mirrors and a camera are used to help detect movements of an artificial star, which simulates the light that would be emitted by a real star.

The MAM interferometer includes all the functionality of the flight system in a reduced-scale experiment, enabling MAM to pathfind the ground performance testing methodology for the flight instrument. The MAM optics, metrology system, and artificial star are placed in a vibration-isolated, thermally stabilized vacuum chamber. This eliminates index of refraction fluctuations in air to achieve the goal of 50-picometer optical path measurement accuracy.
The instrument that engineers have demonstrated in the laboratory will become the heart of a revolutionary new space telescope known as the Space Interferometry Mission.

Interferometry Animation
Animation: Click on the image above for a description of Interferometry.

"Six-and-a-half years ago, this technology was unproven and unsubstantiated," said Brett Watterson, the mission’s deputy project manager. "It was just a remote possibility that we could do it. It was through ingenuity, insight, leadership and sheer perseverance that the team was able to overcome these difficult technological challenges."

SIM will be an interferometer, which means it will combine interacting light waves from its three component telescopes. This interaction, called interference, makes the individual telescopes, which are separated from each other on the spacecraft, act as though they were a single, larger telescope with greater light-gathering ability.

SIM, in solar orbit, won’t actually see Earth-sized planets. Instead, it will use astrometry, measuring the angle between two stars (as viewed from the spacecraft, which will form the third point of a triangle). By repeatedly measuring the angle between a target star and each of several more distant background stars, SIM will be able to determine whether the target star wobbles periodically because of gravity from orbiting planets, including planets as small as Earth.

NASA recently gave the go-ahead for the second stage of development for the mission, which will not only be able to search for Earth-like planets around other stars, but will also measure cosmic distances several hundred times more accurately than currently possible. SIM’s primary mission will be to measure distances to stars with 100 times greater precision than now is possible. This will improve estimates of the size of the universe and help astronomers determine the true brightness of stars, and thus learn more about their chemical composition and evolution. Scheduled to launch in 2009, it will scan the heavens for five years and provide astronomers with the first truly accurate road map of our Milky Way galaxy.

Cassini Jupiter
Giant Red Spot in background, one of Jupiter’s moon in foreground with eclipse shadow cast on gas giant. The scale of gas giants greatly exceeds what a rocky inner planet might offer optically for detection. Credit: NASA/JPL Cassini

"This is a historical time that we’re intimately involved with," Watterson said. "Unlike any other culture in history, we have the technological means, the budget, and the will to determine the occurrence of Earthlike planets orbiting other stars. Everyone on the team is aware of their role in this pivotal stage in the search for life elsewhere in the universe."

What’s Next

Scientists hope to launch six new space-borne missions over the next few years to search for terrestrial planets.

They include France’s small-scale COROT, NASA’s more-ambitious Kepler mission, the European Space Agency’s (ESA) Eddington and NASA’s Space Interferometry Mission (SIM).

The French COROT mission, approved and due for launch in late 2004, will study asteroseismology, or oscillations within stars, and likely will be the first orbiting telescope to search for extrasolar planets.It will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission, says Pierre Barge, an astronomer at the Laboratory of Astrophysics in Marseille and leader of COROT’s exoplanets group. COROT – for Convection, Rotation and Planetary Transits – is a mission of CNES, the French National Center for Space Studies, in partnership with ESA, Italy, Belgium and Germany. When searching for extrasolar planets, COROT’s 27-centimeter (10.6-inch) telescope will use a method called photometry, in which sensitive light detectors look for a slight drop in a star’s brightness as a small planet "transits" the star (crosses the face of the star as viewed from COROT).

The Kepler mission is scheduled for launch into solar orbit in October 2006. Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet. To highlight the difficulty of detecting an Earth-sized planet orbiting a distant star, Borucki, Kepler’s principal investigator, points out it would take 10,000 Earths to cover the Sun’s disk. One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars. Because most of the gas giant planets found so far orbit much closer to their stars than Jupiter does to the Sun, Borucki believes that during the four- to six-year mission, Kepler will find a large proportion of planets quite close to stars. If that proves true, he says, "We expect to find thousands of planets."

In 2008 or later, the European Space Agency hopes to launch the Eddington mission (named for the late British astronomer Arthur Eddington). Eddington primarily would study stars’ interior structures and the processes that govern how stars evolve, but it would spend three years scanning 500,000 stars for planets, including terrestrial planets in habitable zones. Like Kepler and COROT, Eddington would use a camera with a large optical telescope, in this case a wide-field 1.2-meter (47-inch) one, to detect planetary transits using photometry.

Also due for launch in 2009 is the almost $1 billion NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope. Planet hunting will be "a minor part of its job, " says Penny. Like Hubble, NGST will be a general-purpose telescope with an emphasis on cosmology. But it will investigate stars with dusty disks – the early stage of planet formation – and may also be able to study Jupiter-size planets.

The Space Interferometry Mission is managed by JPL as part of NASA’s Origins program.

Related Web Pages

Star chart
Kepler mission
Space Interferometry Mission
Extrasolar Planets Encyclopedia
Info on the PLANET collaboration
Extrasolar Encyclopedia site in France