How a Planet Can Mess Up a Star's Looks
Recently, beautiful photos of auroras have been in the news.
These colorful light shows were generated by solar storms, and provide a vivid
demonstration of activity on the Sun affecting the Earth. The pummeling
experienced by our home planet is an example of our one-way relationship with
the Sun: it can have a noticeable effect on the Earth, but the Earth has a
negligible effect on the Sun. Further afield in the galaxy, this isn't always
the case. In a few other systems planets can
have a big effect on their star, changing their looks in surprising ways.
A spectacular picture of auroras by photographer Mike Taylor taken over Unity Pond in Waldo County, Maine on September 12, 2014. Credit: Mike Taylor photography. |
As explained in the latest press
release from NASA's Chandra X-ray
Observatory, an exoplanet called WASP-18b appears to be causing the star it
orbits to act much older than it actually is. WASP-18b is an example of a hot
Jupiter, with a mass about ten times that of Jupiter and an orbit that is less
than 24 hours long. The host star, WASP-18, is estimated to have an age that
lies between about 500 million and 2 billion years, relatively young by
astronomical standards.
Younger stars tend to be more active stars, with stronger
magnetic fields, larger flares, and more intense X-ray emission than their
older counterparts. Magnetic activity, flaring, and X-ray emission are linked
to the stellar rotation, which generally declines with age. However, when
astronomers took a long look with Chandra at WASP-18, they didn’t detect any
X-rays. Using established relations between the magnetic activity and X-ray
emission of stars and their age indicates that WASP-18 is about 100 times less
active than it should be at its age.
The researchers argue that tidal forces from the
gravitational pull of the massive planet – similar to those the Moon has on
Earth’s tides but on a much larger scale – may have disrupted the magnetic
field of the star. The strength of the magnetic field depends on the amount of
convection in the star. Convection is the process where hot gas stirs the
interior of the star.
The planet’s gravity may cause motions of gas in the
interior of the star that weaken the convection, causing the magnetic field to
weaken and activity to decline. This causes the appearance of premature aging
in the star. WASP-18 is thought to have a shallow convection zone, making it
unusually susceptible to tidal effects.
Shown in the main part of this graphic is an artist's impression of the star WASP-18 and, in the foreground, its hot Jupiter WASP-18b. The insets show the star in the optical image and its non-detection in X-rays with Chandra. Credit: X-ray: NASA/CXC/SAO/I.Pillitteri et al; Optical: DSS; Illustration: NASA/CXC/M.Weiss |
What about other hot Jupiters that are relatively massive
and close to their star? In some cases - where they orbit a different type of
star to WASP-18 - the effect of hot Jupiters can be flipped and they can make a
star appear younger than it really
is. In the cases of HD 189733
and CoRoT-2a
the presence of the planet appears to have increased
the amount of activity in the star. In these cases the stars have much deeper
convection zones than WASP-18 and tidal effects have little influence on
convection and hence on the star's dynamo. Instead, the planets may be speeding
up their star's rotation, leading to a more powerful dynamo and more activity
than expected for the star's age. In these cases having a companion makes the
star act younger than it really is. That makes sense for people and, in a few
cases, for stars.
An artist's impression of the star CoRoT-2a and its hot Jupiter exoplanet, CoRoT-2b. Credit: NASA/CXC/M.Weiss |
I've discussed the effects that extreme hot Jupiters can
have on their host star. In such systems, what effect does the star have on its
planet? In the cases of HD 189733 and CoRoT-2a, strong X-rays and ultraviolet
radiation from the active star are evaporating the atmospheres of the planet.
For HD 189733, astronomers estimate the planet is losing 100 to 600
million kilograms per second, and for CoRoT-2a astronomers estimate the
planet is losing about 5 billion
kilograms per second. For WASP-18, with much weaker X-ray emission and
ultraviolet radiation, there is much less evaporation of the nearby planet's
upper atmosphere than there would be if the star was more active. In effect,
the planet is protecting itself. Its gravity causes the nearby star to be less
active, and that causes the planet to be struck with less damaging radiation.
HD 189733b and CoRoT-2b, on the other hand, are behaving in a self-destructive
manner.
Talk of planet destruction isn’t necessary for our present-day
solar system, where the planets are much further away from the Sun than hot
Jupiters are. However, that won’t always be the case. A few billion years in the
future, the Sun will dramatically expand in size when it becomes a red giant. Our
oceans will boil away, never to return and what’s left of the Earth may end up
spiraling in towards the Sun. We don't know the exact fate of our home planet, but it is clear that our aurora-viewing days are numbered.
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