A team of 41 astronomers from Australia, Japan, New Zealand, and the United States took advantage of a naturally occurring telescope to image a small planet orbiting a star somewhere between us and the Galactic Bulge (the dense concentration of stars that exists at the core of our Milky Way galaxy).1 The natural telescope consisted of a large star functioning as a gravitational lens. According to general relativity, a sufficiently massive body can bend the path of a beam of light that passes close enough to it. Therefore, if such a body lies between us and another object located directly behind it, it can magnify for us the image of the more distant object (see diagram). The more massive the lense object is, the more it will magnify.
Gravitational lenses that astronomers are fortunate enough to find exhibit widely varying magnifying properties. In this particular case the magnification exceeded twenty times.
The team’s results demonstrate that relative to the star, MACHO 98-BLG-35, that provided the gravitational lens phenomenon, the planet orbiting it is between 0.004 and 0.02 percent of the mass of the star. For star masses that could possibly give rise to such a spectacular magnification, the planet mass orbiting it would fall between 3 and 35 times the mass of the Earth (or, 0.17 and 2.0 Neptune masses).
While not so small as an Earth-sized planet, the discovered object does rank as the smallest extrasolar planet yet found. The next smallest is about ten times more massive.
Can this gravitational lens technique ever find planets as small as Earth? Yes, but probably not more than a handful and never with any certainty as to the mass of the planet, its distance from its star, or any of the features of its orbit. That job must be left to the space interferometer project discussed in the accompanying article.
The apologetic significance of the planet discovered orbiting MACHO 98-BLG-35 is that the data demonstrates the planetary system is lacking in large gas-giant planets like Jupiter and Saturn. Since such gas-giants, located where they are in our solar system and manifesting stable near-circular orbits, are essential ingredients for the support of life on Earth,2 we know that the MACHO 98-BLG-35 system is not a candidate for a possibly life-supporting planet. Indeed, not one of the 42 extrasolar planetary systems discovered so far offers any possibility of harboring a planet with the capacity to support life.
The hope expressed by many non-theists before all this research got started was that planetary systems like ours would prove to be common. This hope appears to be dashed. Our solar system so far remains alone in exhibiting the extraordinary properties necessary for the maintenance of a planet with the capacity to support life.