A group of British researchers has made the first direct observation of a world beyond our solar system by capturing light from a so-called extrasolar planet and devising a method to measure the body's size and composition.

The milestone comes just four years after the first indirect evidence of a planet beyond our solar system was announced.

The astronomers' method involves measuring reflected light from the star around which the huge planet orbits.

The planet is estimated to have a diameter roughly 1.6 to 1.8 times Jupiter's and is about eight times as massive. First discovered by indirect observations in 1997, the newly measured object orbits a star called Tau Bootis.

The planet is said to be blue-green in color.

Using the wobble method inherently favors the discovery of very massive planets orbiting close to their stars, but it does not reveal the planets' size. Knowing density and size, combined with studies of reflected light, will help researchers figure out what distant worlds are made of.

"If we can study the colors in enough detail, it'll tell us what trace gases other than hydrogen are present in the atmospheres of these giant planets," said the new study's lead researcher Andrew Collier Cameron of the University of St. Andrews in Scotland. "Knowing the chemical composition will tell us a lot about the evolution."

Cameron said the observations are not certain: There is a one-in-20 chance that they could be incorrect. But if further observations, possibly as soon as next spring, prove the method out, then the floodgates will open on our knowledge of extrasolar worlds. Knowing a planet's mass and size means researchers can figure its gravity at the surface, which helps paint a picture of the incredibly hot atmosphere of these worlds that orbit so close to their stars, Cameron said.

"This allows you to start producing climate models for these roasted planets, which can predict what gases we should be able to see," he said. "If we can push that a little bit deeper and start to measure their spectra, we'll be able to test these models directly. It's going to broaden the horizons of planetary-atmosphere research considerably."

Along with other evidence, the new study confirms that giant gaseous planets can exist very close to their stars -- an arrangement very unlike our solar system.

"And that's a surprise," Cameron told space.com, "since gas-giant planets are supposed to form only in the relatively cool environment of the outer reaches of a planetary system, where the giant planets in our own system are found. The most plausible explanation of how these giant planets get into such close orbits is that their orbits "decay" because of drag in the gaseous protoplanetary disc in which they first condensed. This would make our own system -- where the giants have stayed put -- something of an oddity."

A planet orbiting a distant star reflects light back into space. (We see the other planets in our solar system because they reflect light from our star -- the sun.) But the planet in this study is 20 times closer to its giant star as Earth is to the sun -- too close for its reflected light to be distinguished from as far away as Earth, or even from the orbiting Hubble telescope.

So the researchers used a common technique among astronomers -- observing the light's Doppler shift. The shift in light's wavelength is akin to the sound changes one hears as an ambulance approaches, passes and then recedes. The waves are compressed as the vehicle approaches, and they are elongated as it races away.

Because it is so large and close to Tau Bootis, the planet observed by Cameron and his colleagues' moves swiftly. The wavelength of its light changes when the body is moving rapidly toward us in its orbit compared to when it is moving just as rapidly away from us. Capitalizing on this Doppler shift, the researchers picked the planet's light signal out, separating it from the light emitted directly from the star.

"Unfortunately, the detection of reflected light is extremely difficult and so reliability is not high," write University of Arizona scientists Adam Burrows and Roger Angel in a separate analysis for Nature. The pair note that previous observations by other researchers did not come to similar conclusions.

Still Burrows and Angel, who were not involved in the research, expressed their enthusiasm for the new technique: "Our understanding of planetary science will be enormously increased by direct measurements of the physical properties of extrasolar planets."

Andrew Collier Cameron explains the accompanying image:

"As the planet orbits around the star, its spectral signature is expected to appear as a dark feature snaking back and forth, following the dotted line. Time increases from bottom to top."

"The signature is strongest when the planet is on the far side of the star with its illuminated face turned towards us, halfway up the diagram. In the right-hand panel, a synthetic planet signature has been injected into the data, and the resulting dark streak is clearly seen crossing from right to left halfway up the diagram. The slope of the streak tells us that the orbit is inclined at 60 degrees to the line of sight.

"The left-hand panel shows the actual data without the injected planet signature. In this case, the dotted line shows the path of the planet as inferred from the signal analysis. With the orbit tilted at 30 degrees to the line of sight, the signature is much harder to see in the actual data because the planet is correspondingly fainter."