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Lone star has two planets

A NEW GIANT PLANET has been located in a star system about 250 light-years from Earth. The planet, perhaps twice the mass of Jupiter, could help researchers learn more about how extrasolar planets are formed.

An extrasolar (‘beyond the Solar System’) planet is one that belongs to a star system other than our own.

The star system harbouring the new planet contains only one star, as do the other three systems with extrasolar planets analysed by San Francisco State University astronomer Stephen Kane, an assistant professor of physics and astronomy, and his colleagues. It is a surprising finding, given the high rate of multiple-star systems in our solar neighbourhood.

“There is a great interest in these stars that are known to host planets,” Kane explained, since astronomers suspect that planet formation in a multi-star system would be very different from planet formation in a single-star system like our own.

False-colour image of star HD 4230

This false-colour image shows the star HD 4230, located about 250 light-years from Earth. Measurements indicate the presence of two, unseen, planets orbiting it.

A multiple-star system “might have not one but two” flattened clouds, called discs, where planets form, he said. “Or it could be that having an extra star would be disruptive, and its gravity could cause any protoplanets to pull apart.”

Relatively few extrasolar planets have been found in multiple-star systems, “but we know that they are there,” Kane said.

A wobbling star

In the four systems studied by the researchers, using optical imaging data collected at the Gemini North observatory in Hawaii, there were some intriguing signs that perhaps a second star – or something else – was present.

In each system, the extrasolar planets were discovered by the radial velocity technique, which measures variations in the speed at which a star moves away and toward Earth, “wobbled” by the gravitational pull of a nearby cosmic body. Depending on the radial velocity signature, astronomers can calculate whether the wobble is coming from a planet or star.

In the star systems studied by Kane and his colleagues, there was a part of the radial velocity data that couldn’t be explained entirely by the pull of an orbiting planet. And at the same time, the planets that had already been discovered in these systems followed eccentric orbits, swinging away from their stars in a less circular and more elliptical fashion, “more like that of a comet,” Kane said.

An unexplained velocity

With these two clues, the researchers wondered if the radial velocity and eccentric orbits might be explained by the presence of another star in the system. But when they took a closer look at the systems, they were able to rule out the possibility that another star was perturbing the system.

Depiction of planetary orbits around HD 168443

The two solid circles marked b and c, depict the orbits of planets circling HD 168443, one of the stars studied by Stephen Kane and his colleagues. The dashed circles represent the size of the orbits of Mercury, Venus Earth and Mars in our Solar System.

“I thought we were likely to find stellar companions, and when all four didn’t have a binary star, that did surprise me,” Kane said.

But in the case of one star, HD 4230, the unexplained radial velocity appears to be coming from the pull of a previously undiscovered giant planet, the researchers report. They confirmed the planet’s presence with additional radial velocity data collected at Hawaii’s Keck observatory.

Given that the researchers did not find any stellar companions, Kane says it is very likely that the leftover radial velocity is instead a signal that there are additional planets to be found in all four systems. The researchers feel this is especially true for the system called HD 168443, where their ability to detect a companion star was very strong.

Adapted from information issued by San Francisco State University.

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