The PHASES exoplanet search
The development of multiple tools for studying new planetary systems around other stars is important because each method provides us with different information to answer questions about the sizes of exoplanets, the diversity of environments in which they are found, and how they form. As we learn more, we get closer to answering questions about where else life might exist, how common life might be, how the Earth came to be, and its role in the universe. PHASES offers a unique method for finding Jupiter-like planets around other stars.
The majority of star systems contain more than one star; the Sun is one of the exceptions, rather than the norm. However, we still don't know whether planets are as abundant in these binary systems as they are around single stars. It's not clear how planets could form when a second star is present, especially if the stars are close together. Would the second star disrupt the planet-forming dust and gases before the planets are built? Are the planets' orbits dynamically stable, or will the stars rapidly eject planets?
Two versatile configurations exist in which planets in binary systems can have stable orbits over long periods of time.1 The popular movie Star Wars depicts one configuration: the Sky-walkers' home planet Tatooine orbits two stars that are close together. These can be called circumbinary planets. In the other configuration, the planet orbits just one star, with the second star orbiting distantly. The now-disproven Nemesis hypothesis proposed that the Sun had such a distant stellar companion, periodically perturbing distant comets, causing periods of excessive bombardment on Earth and mass extinctions in the geologic record.2 Though the hypothesis has been disproven, the name remains to describe such configurations as Nemesis systems. The PHASES program is an effort to find such systems.
The search uses a method different from most extrasolar planet searches: astrometry. When a planet orbits a star, Newton's Third Law of Motion—for every action there is an equal and opposite reaction—predicts that the star will move in response. The two bodies each orbit the center of mass of the planetary system. Because the star is much more massive than the planet, this center of mass is located much closer to the star, and the star's motion is smaller, but not zero. While we cannot yet see the planets themselves, we can measure these small motions of the stars. The very successful radial velocity (RV) method measures motions in a direction along the line connecting the Earth and the target star. Astrometry measures the side-to-side motions of the stars in the two perpendicular directions that mark the positions of stars on the sky.
From the nearest stars, the Sun would appear to move by an amount smaller than a tenth of the resolution of the Hubble Space Telescope (HST). PHASES uses multiple small telescopes combined into a system called an interferometer, which can operate with a resolving power greater than 10 times that of the HST.
Astrometry requires reference stars to calibrate measurements of a target's positions. Ground-based astrometry is made difficult by the atmosphere, as turbulent winds and temperature variations introduce apparent changes in stars' separations. However, this effect is small when the stars are close together in the sky. One good way to find references is to look for pairs of stars orbiting each other. Thus, this is an excellent method for searching out planets in Nemesis systems.
The Palomar Testbed Interferometer can measure the separations of pairs of stars to extremely high precisions.3 It is a unique facility with the ability to use two cameras simultaneously: one that rapidly monitors changes in the atmosphere for calibration, and another that measures star positions with high accuracy. This double measurement configuration (called phase-referencing,4 thus the PHASES acronym) is necessary to measure the separations well enough to detect planets.
PHASES studies pairs of stars, looking for wobbles superimposed on the motion of the stars as they orbit each other. These perturbations would be caused by objects orbiting either star (the Nemesis setup), but not both simultaneously (the circumbinary case).
The PHASES measurements have successfully identified wobbles superimposed on the orbits of several systems. 5,6 Analysis has revealed that the faint companions in those systems are not small enough to be planets, but instead are fainter, lower-mass stars. This is an important auxiliary science result, as the orbital configurations tell us about the environments in which multiple stars form.
Planets as small as twice Jupiter's mass can already be ruled out by the PHASES observations of several systems. As the study continues and the technique is improved, better limits can be placed on planetary companions. If PHASES finds that many giant planets exist in the target systems, this result will show that planet formation can occur regardless of disruptions by the presence of extra stars. If not, we will have learned that the pool of stars potentially hosting planets is much smaller.
Astrometry is more sensitive to planets with long orbital periods, like those found in Earth's solar system, than other search techniques like the RV method. Future astrometric planet searches, such as the Space Interferometry Mission (SIM),7 will be used to study systems similar to our own. The PHASES program is developing many of the tools and procedures that will be used for SIM.