Reasons to Believe

Microlensing Planets

Modern techniques have generated great interest in the search for far away, potentially habitable planets.

More than 275 extrasolar planets (planets outside of our own solar system) have been discovered using a variety of measurement techniquesincluding:
  • Astrometry, where the astronomer precisely measures the position of a star as it changes due to the gravitational pull of a planet orbiting the star.

  • Doppler spectroscopy, where the spectrum of the star shifts, due to the motion of an orbiting planet.

  • Pulsar timing, where, if the star happens to be a pulsar, the timing of the pulses will vary due to the proximity of a planet.

  • Transit method, where the parent star shows periodic dimming as a planet transits in front of the star.

  • Gravitational microlensing, where a nearby star passes precisely in front of a more distant star and magnifies the distant star’s image enough gravitationally to see its planets, or even possibly planets orbiting the nearby star.

  • Direct imaging, where the telescope has instruments that can block out the light of the parent star so the planet can be seen and measured directly.

    While the Doppler spectroscopy technique has been the most productive in finding planets, all of these detection methods have uncovered an enormous range of planetary properties. This diversity has revealed that planetary systems different from our own are common throughout the Milky Way Galaxy.

    In contrast, the relatively new microlensing technique has been recently used to obtain precise information about an exoplanet (extrasolar) system that closely mirrors some of the characteristics of our own solar system. B. S. Gaudi and colleagues published results of an extensive observational program yielding a system of planets surrounding, in this case, the nearby of two microlensed stars. The system contains a central star of a mass ~0.5 times the solar mass, circled by two planets of masses ~0.71 and ~0.27 times the mass of Jupiter, at ~2.3 and ~4.6 astronomical units (distance of Earth to the Sun) from the central star. This system resembles a scaled version of our solar system. If these numbers are doubled to make its star equal to our Sun, then the inner “Jupiter” planet is 1.4 times the mass of our Jupiter at 0.88 of its distance, and the outer “Saturn” planet is 1.8 times the mass of our Saturn at 0.97 of its distance. Based on their results, the researchers argue that such solar system analogs may be common.

    The microlensing technique is similar to what astronomers have used in the past involving gravitational bending of light by nearby galaxies to image more distant galaxies. In the case of stars, a nearby star’s gravity bends the light of a more distant star as it slowly passes in front of the distant star. Since the alignment must be very precise, such events are unusual and of short duration, perhaps only a few weeks. Therefore, astronomers must be ready for an event that they cannot predict ahead of time. Nevertheless, the authors indicate that their program has the potential of detecting nearly 700 microlensing events per year, many of which are expected to yield new exoplanets.

    We find it remarkable that this new tool can be so effective in the discovery and measurement of new solar systems. Only a few years ago did researchers begin using gravitational lensing to examine distant galaxies. RTB looks forward to the new insights to be gained by this and future techniques. However, while the discovery of new planets and planetary systems like our own is exciting, we should remember that success in finding habitable planets is hidden in the details. Where we have been able to examine the details, as in the case of Mars, success still eludes us.

Subjects: Extrasolar Planets

Dr. David Rogstad

Dr. Dave Rogstad received his PhD in physics from Caltech and worked over 30 years for NASA’s Jet Propulsion Laboratory. Though now retired, Dave continues to serve as an RTB board member and participates regularly in several RTB podcasts.