The University of Toronto actively participates in the search for extra-solar planets. Professor Howard Yee of U of T’s department of astronomy and astrophysics leads the EXPLORE Project, an international effort that is looking for planets around other stars by a very direct approach.

The team examines the light curves of stars. (A light curve is a measure of the brightness of a star over time.) When a planet orbiting another star passes in front of that star, some of its light is blocked until the planet moves on. Such episodes often take several hours [see sidebar]. But since planets are much smaller than stars, the light from a typical star is only dimmed by about one percent during the eclipse of a large Jupiter-like planet.

Yee’s team specifically looks for these minute changes, because any larger variations may be the effect from larger objects that cannot be planets. EXPLORE (which stands for EXtra-solar PLanet Occultation REsearch) surveys star fields along the galactic plane, or the disc of our galaxy. The team is currently planning a new survey that will use the Magellan telescopes in Chile to broaden their search to star clusters. Said Yee, “Extra-solar planets is the next big thing. It will be extremely prominent in the near future.”

Five ways to find a planet

Doppler shift

Just as a police siren lowers in pitch as it passes by, the light from a star changes wavelength when it moves towards or away from us. Periodic changes in the wavelength of light coming from a star betray the presence of a planet. Most extra-solar planets have been discovered by this method.

Transits

A planet passing in front of its parent star causes a temporary dip in the star’s brightness. Transits are rare because the planet’s orbit must be edge-on from our point of view, so relatively few planets have been discovered this way.

Microlensing

Einstein’s relativity theory predicts that light rays will bend when they pass a massive object. This means that when a massive object, such as a planet, passes in front of a distant background star, it can act as a lens, focusing the light of the background star and making it appear temporarily brighter. Few planets have been discovered this way because it depends on a rare alignment of the background star, the planet, and the Earth.

Astrometric detection

Stars pull on planets to keep them in orbit, but the planets also pull back, creating tiny, recurring shifts in a star’s position that can give away the planet’s presence. Not many planets have been discovered this way because the shifts are usually too small to observe.

Direct imaging

Actually taking a picture of a planet. Planets have no internal light source and shine only by reflected starlight, making them tough to spot. The first pictures of extra-solar planets should arrive within two years.

—David Shiga