Smithsonian Research Reports, Spring 1994
Hunting for evidence of suspected "exoplanets" could be the major goal of Precision Optical Interferometry in Space (known as POINTS), an instrument being developed by scientists at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts in collaboration with colleagues at NASA's Jet Propulsion Laboratory (JPL) in California and Itek, a Massachusetts-based optics company.
For now, this proposed robotic spacecraft -- designed to orbit Earth -- exists only on paper and in laboratory demonstrations of its technology. However, the researchers hope to start the project toward an exoplanet odyssey early in the next century. The space agency has recently cited the search for other solar systems as one of its next major challenges.
"Planetary detection is a very exciting field, to both astronomers and the general public," says SAO physicist Robert Reasenberg, who heads the POINTS team. "In fact, it's an extension of the question 'What's out there?' that was probably asked long before recorded history."
However, as Reasenberg notes, "The real issue is not the discovery of a few scattered planets, but rather the prevalence and characteristics of planetary systems that might be counterparts of our own."
The POINTS spacecraft won't actually be able to "see" exoplanets, explains Reasenberg. The overwhelming brilliance of a star drowns out the relatively faint light reflected off a nearby planet. Rather, the instrument is designed to detect the minuscule counterbalancing motion of a star as it and its planet orbit around a common center of mass. (For the Sun and Jupiter, that common point is actually quite near the Sun's surface.) This barely detectable displacement in the star's position creates a "signature" that might indicate the presence of another body.
The star's precise position is determined by astrometric optical interferometry, a technique in which two separate telescopes gather light from a single object. The light beams are combined to form a signal that is very sensitive to position. The POINTS instrument will consist of two such interferometers, one turned on the target object, the other on a reference star, separated by about 90 degrees on the sky. Repeated measurements of the angle between the two objects will collect the tell-tale planetary signature by detecting any motion between the target and reference star.
Of course, as Reasenberg says, the reference star itself could have a companion planet, which would confound the measurements. To account for that, he says, POINTS will measure many different pairs of stars, all over the sky. During the course of the mission, which could last up to 10 years, they plan to look at more than 2000 stars at distances up to 30 parsecs (a parsec is 3.26 light years, or 19 trillion miles.)
Initially, critics were skeptical of POINTS, calling the vast amount of data the mission would generate hopelessly confusing, Reasenberg says. To address that concern, the SAO scientists created an imaginary universe, complete with randomly scattered stars, many accompanied by planets of various sizes. Astrophysicist John Chandler, another POINTS contributor, generated the "universe" by entering a range of variables, such as size and orbital periods, that could produce different planet signatures into a computer program that used a random number generator. The program created first the test planets and then "pseudodata," complete with errors and phoney signatures.
To test their planet-seeking technique, Chandler then gave the data set to POINTS colleague Robert Babcock, who analyzed it to "discover" the planets. "In fact, John didn't know what he created--and he didn't tell Bob what the variables were. Thus we conducted double-blind numerical experiments," Reasenberg says.
By comparing Babcock's discoveries with Chandler's universe, and repeating the experiment many times, the scientists determined the potential level of sensitivity for their data analysis techniques -- that is, just how small a signature they could detect.
Seen from a distance of 10 parsecs, Jupiter (which is 318 times the mass of the Earth) produces a signature of only 500 microarcseconds in the motion of the Sun. Incredibly, POINTS is sensitive enough to detect a signature of five microarcseconds. "That's the width of a human hair as seen from across the continent," Jim Phillips, another POINTS physicist at SAO, says.
Two factors contribute to the instrument's amazing precision. Because POINTS will be in orbit, the starlight it receives won't be blurred and bent by the Earth's turbulent atmosphere. But the true secret of its accuracy is the system of internal laser gauges, which serve to monitor and adjust for any deformations of the instrument during the mission.
Invented and developed by the SAO team of Reasenberg, Phillips, and physicist Charley Noecker, the gauges also may have commercial applications, including the manufacture of computer chips and large optical mirrors. A patent assigned to the Smithsonian is pending for the new gauges, which are about 1000 times more accurate than current commercial units.
Because the instrument can accurately measure distances to stars within our Galaxy, it should enable astrophysicists to extrapolate cosmic distances with much greater accuracy. Such knowledge would further our understanding of the age and size of the universe.
The fact that POINTS can address astrophysical questions like these and can look for planets bodes well for its selection as a space mission, Reasenberg says. The development work has been supported by NASA and the Smithsonian Institution.
To Reasenberg, a POINTS mission is the next logical step in humankind's long tradition of exploration. "Think of Marco Polo travelling to China, the Europeans who sailed across the Atlantic, or the first peoples who crossed the Bering Strait into North America." Exploration, he asserts, is "characteristic of great societies and a hallmark of our species."
Perhaps in the next century, humans will discover not just new continents, but new worlds.