Astronomers have detected a planet in a solar system 90 light-years away that seems to heat its own sun.
For over a year, Evgenya Shkolnik and Gordon Walker of the University of British Columbia and David Bohlender of the National Research Council of Canada have tracked a planet orbiting the star designated HD 179949. The planet, similar in size to Jupiter but located at a much closer distance from its sun, completed over 100 orbits of about three days each.
Appearing in the ultraviolet wavelength range, the sunspot indicates that the star's chromosphere is heated by an additional 750 degrees Fahrenheit (from an original 14,000). The researchers believe that the additional energy comes from interaction with the planet's magnetic field.
"Mainly, we expect the heating to take the form of Joule heating, i.e. resistivity of the high-energy charged particles flowing along the magnetic field lines and through microflaring on the surface of the star," Shkolnik said. "The planet's magnetic field is merely stimulating increased magnetic activity in the stellar chromosphere by increasing the local turbulent velocities."
Stars and planets exhibit magnetic fields arising from the motion of charged particles within them, such as liquid metal in a planet's core.
"We can compare the planet to our own Jupiter," Astronomy Prof. Zhi-Yun Li said. "It is very difficult to predict planetary magnetic fields. Even on Earth, they are not completely understood."
Any two magnetic fields will interact, so it is possible that the heating of the star's chromosphere is due to the planet's magnetic field.
"The star would have some magnetic field, and the magnetic field of the planet would interact with it like two magnets," Li said. "But there is a complication: It is possible that the planet's magnetic field would interact with the solar wind, particles mostly from the star's corona [the layer outside the chromosphere]."
Although this phenomenon was predicted by astronomers in 2000, this is the first observed case of a planet heating its sun.
"From a theoretical point of view, you observe interactions between binary stars: the magnetic activity seems to pick up," Li said. "That led some people to speculate that there might be similar behavior with one star replaced by a planet, and one possibility is enhanced activity in the chromosphere."
Detecting planets outside of our solar system is difficult. The first was discovered in 1991, and since then the number has reached about 110.
"Most extrasolar planets are Jupiter-sized or larger, a selection effect because we tend to find large planets close to stars," Li said. "It is very difficult to detect planets directly because they can be hidden in the glare of the star. By monitoring the light output of a star, you can tell when a planet is in front."
If the theory is correct, it may offer a new way of detecting planets closely orbiting other stars tracking their sunspots.
Magnetic field interactions may play a role in other aspects of solar-system formation.
"There is a theory that giant planets cannot form to close to a sun it's too hot so they must have moved toward the sun if they are located there," Li said. "But if at one time they were moving toward the star, what would stop the motion and keep the planet from falling into the sun? One theory is that if the planet has a magnetic field, it could exchange angular momentum with its surrounding disks."
However, more evidence may be needed to confirm the source of the sunspot.
"They see emission of light in certain wavelengths varying with a period that tracks the period of the planet," Li said. "The alternative hypothesis is that the star is rotating at the same rate. This would be a tidal lock-in, like the moon is locked into the Earth's rotation so that only one hemisphere is seen but in this case the same part of the sun always faces the planet."
An alternative explanation "is not 100 percent ruled out since there are no long-term photometric observations done of HD 179949 yet to determine a proper rotation period," Shkolnik said. "But we do have four pieces of suggestive evidence as to why the period should be between seven and 10 days and not three days."
Hence, there is a small possibility that the sunspot results from something other than magnetic interaction, and the star and planet coincidentally have synchronized orbits.
"My feeling is that it's a very interesting result," Li said. "If it can be firmed up by future observations, it will be a great achievement."
The researchers are investigating further.
"We are looking at other chromospheric/coronal activity indicators and, of course, looking at other such tight systems to search for a similar effect," Shkolnik said.
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