The “teapot” of Sagittarius steams low across the southwest on late summer evenings. The spout of the teapot lines up in front of the center of the galaxy, which is about 27,000 light-years away.

Astronomers have discovered about 50 exoplanets in that region — planets orbiting stars other than the Sun. And most of them were discovered using an unusual technique, known as microlensing.

It takes advantage of the fact that the gravity of a massive object, such as a star or planet, can act as a lens. It both bends and magnifies the light of objects behind it. So if a star and planet pass in front of another star, the background star gets two “bumps” in its light — a big one from the foreground star, and a smaller one from its planet.

From the size and length of the bump, astronomers can learn about the planet’s mass and its orbit. But they don’t get a second look. Most of the planets discovered through microlensing are thousands of light-years away, which is too far to study with other techniques.

Two microlensing projects have discovered most of the planets in this region. One project, called OGLE, uses a telescope in Chile. The other, MOA, is in New Zealand. They look toward the galactic center because there are huge numbers of stars in that direction. That gives them a better chance of seeing the random alignments that produce microlensing events — alignments that can reveal distant planets.

More about Sagittarius tomorrow.

Script by Damond Benningfield