In the universe, we may or may not be alone, but at least there seem to be plenty of planets. Over the last decade, extra-solar planet-finding has become a growth industry, with some 100 already identified by their effect on the motion of their central star (see Far Out Planets). The pull of the planet’s gravity makes the star wobble back and forth as the planet orbits, and the more massive the planet, the larger and more easily detected is the wobble. Consequently, the extrasolar planets that astronomers can find tend to be large, some with more than four times the mass of Jupiter.

When did the first planets form? Astronomers had predicted that planet formation would occur only in a solar system with heavy elements. Since the early universe was almost entirely hydrogen and helium, planet formation was expected only after supernova explosions had seeded the universe with the heavier elements. To test this idea, the Hubble Space Telescope looked for large, close-in planets, known as “hot Jupiters,” in the globular cluster 47 Tucanae, a densely-packed region in our galaxy containing some of the oldest stars in the universe (see image). Rather than observing a star’s wobble, the Hubble scanned many stars repeatedly, looking for a reduction in brightness due to a planet passing in front of the star (making a “transit”). The Hubble found nothing, which supported the existing theory of planet formation, but a different investigation would eventually yield surprising results.

The Hubble Space Telescope, which searched for planets in the globular cluster 47 Tucanae
(image courtesy of NASA).

In 1999, astronomers searched for extrasolar planets with the Hubble Space Telescope by looking for planetary "shadows" passing in front of stars. No large, Jupiter-like planets were detected at that time in this globular cluster of 35,000 stars. Image courtesy of Hubble Space Telescope (STScI and NASA).