WASP-18: NASA's Chandra X-ray Observatory Finds Planet That Makes Star Act Deceptively Old

WASP-18
Credit: X-ray: NASA/CXC/SAO/I.Pillitteri et al; Optical: DSS; Illustration: NASA/CXC/M.Weiss

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A new study using data from NASA's Chandra X-ray Observatory has shown that a planet is making the star that it orbits act much older than it actually is, as explained in our latest press release. The artist's illustration featured in the main part of this graphic depicts the star, WASP-18, and its planet, WASP-18b. 

WASP-18b is a "hot Jupiter," a giant exoplanet that orbits very close to its star, located about 330 light years from Earth. Specifically, the mass of WASP-18b is estimated to be about ten times that of Jupiter, yet it orbits its star about once every 23 hours. By comparison, it takes Jupiter about 12 years to complete one trip around the Sun from its great distance.

The new Chandra data of the WASP-18 system show that this huge planet is so close to its star that it may be causing a dampening of the star's magnetic field. As stars age, their X-ray and magnetic activity decreases. Astronomers determined that WASP-18 is only between 500 million and 2 billion years old, a relatively young age for a star. Given this age, astronomers expect that WASP-18 would be giving off copious amounts of X-rays.

Surprisingly, the long Chandra observations reveal no X-rays being emitted from WASP-18, as seen in the lower inset box. The same field-of-view in the upper inset box shows that in optical light WASP-18 is a bright source. Using established relations between the magnetic activity and X-ray emission of stars and their age, the researchers concluded that WASP-18 is about 100 times less active than it should be at its age.

The low amount of magnetic activity from WASP-18 is shown in the artist's illustration by the lack of sunspots and strong flares on the surface of the star. The weak X-ray emission from the star has relatively little effect on the outer atmosphere of the nearby planet, giving it a symmetrical appearance. By contrast, much stronger X-rays from the star CoRoT-2a are eroding the atmosphere of its nearby planet, producing a tail-like appearance.

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 be responsible for disrupting the magnetic field of the star. The strength of the magnetic field in a star depends on the amount of convection, the process by which hot gas moves around the stellar interior. The planet's gravity may cause motions of gas inside the star that weaken the convection. Because WASP-18 has a narrower convection zone than most stars, it is more vulnerable to the impact of tidal forces that tug at it.

The effect of tidal forces from the planet may also explain an unusually high amount of lithium found in earlier, optical studies of WASP-18. Lithium is usually abundant in younger stars, but over time convection carries lithium to the hot inner regions of a star, where it is destroyed by nuclear reactions. If there is less convection, the lithium does not circulate into the interior of the star as much, allowing more of it to survive.

These results were published in the July 2014 issue of Astronomy and Astrophysics and are available online. The first author is Ignazio Pillitteri of the Istituto Nazionale di Astrofisica (INAF)-Osservatorio Astronomico di Palermo in Italy. The co-authors are Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; Salvatore Sciortino also from INAF-Osservatorio Astronomico di Palermo in Italy and Victoria Antoci from Aarhus University in Denmark.

Fast Facts for WASP-18:

Scale: Inset image is about 5.3 arcmin across (about 0.5 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 01h 37m 25.00s | Dec -45º 40' 40.30"
Constellation: Phoenix
Observation Date: 26 Feb 2013
Observation Time: 23 hours 43 min.
Obs. ID: 14566
Instrument: ACIS
References: Pillitteri, I et al, 2014, A&A, 567, 128; arXiv:1406.2620
Color Code: Inset: X-ray (Pink), Optical (Red, Green, Blue)
Distance Estimate: About 326 light years

Source: NASA’s Chandra X-ray Observatory

CoRoT-2a: Star Blasts Planet With X-rays

CoRoT-2a

Credit: X-ray: NASA/CXC/Univ of Hamburg/S.Schröter et al;
Optical: NASA/NSF/IPAC-Caltech/UMass/2MASS, UNC/CTIO/PROMPT;
Illustration: NASA/CXC/M.Weiss

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Press Room: CoRoT-2a - View on the Sky (WWT)

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X-ray and Optical Images of CoRoT-2A

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This graphic contains an image and illustration of a nearby star, named CoRoT-2a, which has a planet in close orbit around it. The separation between the star and planet is only about 3 percent of the distance between the Earth and the Sun, causing some exotic effects not seen in our solar system.

The planet-hosting star is located in the center of the image. Data from NASA's Chandra X-ray Observatory are shown in purple, along with optical and infrared data from the Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT) and the Two Micron All Sky Survey (2MASS). CoRoT-2a is surrounded by a purple glow showing that it is an X-ray source.

This star is pummeling its companion planet -- not visible in this image -- with a barrage of X-rays a hundred thousand times more intense than the Earth receives from the Sun. Data from Chandra suggest that high-energy radiation from CoRoT-2a is evaporating about 5 million tons of matter from the nearby planet every second, giving insight into the difficult survival path for some planets. The artist's representation shows the material, in blue, being stripped off the planet.

The Chandra observations provide evidence that CoRoT-2a is a very active star, with bright X-ray emission produced by powerful, turbulent magnetic fields. This magnetic activity is represented by the prominences and eruptions on the surface of the star in the illustration.

Such strong activity is usually found in much younger stars and may be caused by the proximity of the planet. The planet may be speeding up the star's rotation, causing its magnetic fields to remain active longer than expected. Support for this idea comes from observations of a likely companion star to CoRoT-2a that orbits at a distance about a thousand times greater than the distance between the Earth and the Sun. This star is visible in the image as the faint, nearby star located below and to the right of CoRoT-2a. It is also shown as the bright background star in the illustration. This star is not detected in X-rays, perhaps because it does not have a close-in planet like CoRoT-2b to cause it to stay active.

The planet, CoRoT-2b, was discovered by the French Space Agency's Convection, Rotation and planetary Transits (CoRoT) satellite in 2008. It is located about 880 light years from Earth and has a mass about 3 time that of Jupiter.

Fast Facts for CoRoT-2a:

Scale: Image is 4.5 arcmin across (1.15 light years)
Category: Normal Stars & Star Clusters
Coordinates: (J2000) RA 19h 27m 07.00s | Dec +01° 23' 02.00''
Constellation: Aquila
Observation Date: 24 June 2010
Observation Time: 4 hours 10 min
Obs. ID: 10989
Color Code: X-ray (Purple); Optical (Red, Green, Blue)
Instrument: ACIS
References: S.Schröter et al. 2011 A&A 53A A3; arXiv:1106.1522
Distance Estimate: About 880 light years