Using this artist’s rendition as a guide, you can see why some scientists still debate what distinguishes a small brown dwarf from a large planet. From left to right, this image depicts our sun, a very cool star, a warm brown dwarf, a cooler brown dwarf and Jupiter.

Image courtesy Robert Hurt/NASA

Planet Hunting

More than 220 exoplanets -- planets outside of our solar system -- had been discovered around the time of the announcement of TrES-4. Teams of scientists, often called planet hunters, use networks of telescopes around the world to seek out undiscovered celestial bodies.

There are two techniques used by planet hunters. Most exoplanets have been discovered using the wobble method. Astronomers using the wobble method look for stars' "wobbling" due to the gravitational pull of orbiting planets. The technique also allows scientists to deduce a planet's mass.

The second technique is known as the transit method. As a planet passes between Earth and the path of light from its parent star, the visibility of that light is partially disrupted. Scientists take note of these disruptions and use them to figure out the locations of planets. The transit method allows scientists to learn much more information than they could through the wobble method. Besides finding out a planet's mass, scientists can learn information about a planet's size, chemistry and orbit. TrES scientists used the transit method to find TrES-4. (For more information about planet hunting, see How Planet Hunting Works.)

Although TrES-4 is the biggest planet ever discovered, it's not the most massive. That honor belongs to XO-3b, an orb that's 13 times more massive than Jupiter. Like TrES-4, XO-3b puzzles scientists. It has a very short, elliptical orbit -- rather than the expected circular orbit -- and completes a revolution in less than four days, meaning it's very close to its parent star. In fact, no other planet this big has been found orbiting so close to a star.

Some scientists question whether XO-3b is a planet at all. It may actually be a brown dwarf. The issue is contentious because brown dwarf classifications are a matter of debate, at least regarding what distinguishes a very large planet from a small brown dwarf. Stars are generally considered to be any body more than 80 times the mass of Jupiter, or any body able to perform hydrogen fusion. Scientists generally define brown dwarfs as bodies that are less massive than stars but more massive than planets -- or at least 13 Jupiter masses. But some say that mass is not the most important determining feature of a brown dwarf [source: Space.com]. The way a brown dwarf develops -- as part of a grouping of planets or independently -- may be the most important factor.

If XO-3B is indeed a brown dwarf, the title of most massive planet likely passes to another planet residing in the Hercules constellation. HAT-P-2, which was discovered through the transit method, weighs as much as eight Jupiters.

For more information about exoplanets, planet hunting and other related topics, please check out the links on the next page.