The Lost World?

The Lost World?

The star HD 192263

Discovering extrasolar planets is a difficult business. Too faint to see with even the most powerful telescopes, the planets must be found by the subtle gravitational effects they exert on their host stars. A planet tugs back and forth on its star as it orbits around, making the starlight spectrum shift from red (receding light) to blue (approaching light). Called "radial velocity" or the "Doppler effect," this phenomenon has enabled astronomers to locate over 100 extrasolar planets to date.

However, Gregory Henry, an astronomer at Tennessee State University in Nashville, recently conducted a study that calls one of these planets into question. Henry and his colleagues looked at HD 192263, a star located 65 light-years (390 trillion miles) from Earth in the constellation Aquila (the Eagle). Three years ago, astronomers reported finding a planet orbiting this star. At least three-fourths the mass of Jupiter and only .15 AU away from its star, the planet was determined to complete one full orbit every 24 days.

But instead of a planet, Henry says the radial velocity measurements are due to "starspots" – dark, cooler areas on a star generated by twists and kinks in the star’s magnetic field.

While the gravitational tugging of an orbiting planet results in red and blue-shifted light, stars also produce a Doppler effect due to their own rotation. As the star rotates on its axis, the part of the star moving away from us results in red-shifted light. Likewise, the part of the star moving toward us results in blue-shifted light. If the surface area of the star radiates light equally, the spectrum would always have the same blue-versus-red shift. But starspots alter the amount of light radiating from a star’s surface.

Gregory Henry
Gregory Henry, astronomer.

As a star rotates, starspots on the surface are carried in and out of our view. A starspot on the approaching side of the star results in less blue-shifted light, and the net spectrum of the star appears slightly red-shifted. As the starspot rotates toward the receding side of the star, there’s less red-shifted light and the net spectrum becomes slightly blue-shifted. In this way, starspots can create a light spectrum that shifts back and forth much like the spectrum of a star with an orbiting planet.

Just as extrasolar planets cannot be seen, Henry and his team could not actually see starspots on the star’s surface. But the light curve of HD 192263 looks very similar to those of other active young stars, and such stars are believed to have many large starspots. Henry and his team also detected small, systematic differences in the maxima and minima of the radial velocity variations from cycle to cycle -another indication of starspots, which have a transitory nature and can move around or disappear entirely.

When HD 192263’s planet was discovered in 1999, the scientists had asked Henry’s team to validate their find. At the time, Henry and his team were unable to comply, but between April 2001 to July 2002 they managed to conduct photometric observations of the star. Analyzing data from an array of robotic telescopes in Arizona, Henry’s team found that the star’s brightness varied with a 24-day period. They interpreted this period to represent one full stellar rotation, with starspots creating the variations in brightness. In other words, the starspots created a pattern of light that repeated as they were rotated back to the same point every 24 days.

Because the star’s rate of rotation overlapped with the rate of Doppler variations – approximately 24 days – the scientists concluded that the variations were due to starspots and not an orbiting planet. Their findings were published recently on the Astrophysical Journal’s web site. Henry’s colleagues are Robert Donohue and Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics.

The discovery of the planet orbiting HD 192263 had been simultaneously announced by a team led by Steven Vogt of the University of California in Santa Cruz, and by a team led by Nuno Santos in Geneva, Switzerland. Based on the amount of star activity, the Geneva team determined the rotation rate of the star to be only 9 days. (The Henry report suggests this discrepancy might be due to sampling bias: the star may have been more active at the time of the Geneva team’s observations).

An example of starspots. A computer rendering shows the star AB Doradus mottled by starspots that reveal its twisting rotation.
Credit: University of St. Andrews

The Geneva team had originally concluded that starspots were not the cause of the radial velocity variations. Not only was the estimated stellar rotation period of 9 days much shorter than the radial velocity period of 24 days, but there were no light variations indicating starspots from observations made in 1997. The team also checked for line-bisector variations correlated with their radial velocity measurements, but did not find anything significant.

The Geneva planet hunters do not agree with the recent conclusion that starspots are generating the radial velocity measurements of HD 192263. They still believe that a planet closely orbiting the star causes these variations.

"I don’t think the results obtained by Henry, et al., can be seen as conclusive," says Santos. "I think they simply don’t have enough data to be able to discard that the planet is there."

Even if the radial velocity measurements of this star are due to starspots instead of a planet, that doesn’t affect the status of most other extrasolar planet discoveries. Most of the known extrasolar planets orbit much older stars that do not possess strong magnetic fields, and therefore have few, if any, large starspots.

"This is not really a very important point in the global context of exoplanet searches," says Santos.

What’s Next

Santos and his team are currently conducting several tests to verify the existence of the planet around HD 192263, including photometry, radial-velocities, and line-bisector analyses. They hope to publish a new study on their results in the near future.

Related Web Pages

Aquila Constellation
Sun Spot Cycle
Geneva Group
California Group
Extrasolar Planet Catalog