WASP-33b

WASP-33b

Discovery
Discovered by	WASP
Discovery date	2010
Detection method	Transit
Orbital characteristics
Semi-major axis	0.02555±0.00017 AU
Orbital period (sidereal)	1.21987089±0.00000015 days
Inclination	87.67±1.81°
Semi-amplitude	< 0.59 km/s
Physical characteristics
Mean radius	1.497±0.095 RJ
Mass	< 4.1 MJ
Albedo	0.25+0.09 −0.10
Temperature	2710±50 K

WASP-33b is an extrasolar planet orbiting the star HD 15082. It is the first planet discovered to orbit a Delta Scuti variable star. With a semimajor axis of 0.026 AU and a mass which is likely greater than Jupiter mass, it belongs to the hot Jupiter class of planets.

Discovery

In 2010, the SuperWASP project announced the discovery of an extrasolar planet orbiting the star HD 15082. The discovery was made by detecting the transit of the planet as it passes in front of its star, an event which occurs every 1.22 days. As the planet crosses the star's disc, it causes the rotational broadening signature in the star's spectrum to change, enabling the determination of the sky-projected angle between the star's equator and the orbital plane of the planet to be determined. (This differs from the Rossiter–McLaughlin effect which is observed for radial velocity measurements.) For HD 15082 b, this angle is about 250 degrees, indicating that it is in a retrograde orbit. Limits from radial velocity measurements imply it has less than 4.1 times the mass of Jupiter. The exoplanet orbits so close to its star that its surface temperature is about 3,200 °C (5,790 °F). The transit was later recovered in Hipparcos data.

Atmosphere

June 2015 NASA reported the exoplanet has a stratosphere, and the atmosphere contains titanium oxide which creates the stratosphere. Titanium oxide is one of only a few compounds that is a strong absorber of visible and ultraviolet radiation, which heats the atmosphere, and able to exist in a gas state in a hot atmosphere.

Atmosphere of WASP-33b was detected by monitoring light as the planet passed behind its star (top)—higher temperatures result in the low stratosphere due to molecules absorbing radiation from the star (right)—lower temperatures at higher altitudes would result if there were no stratosphere (left)

Non-Keplerian features of motion for HD 15082 b

In view of the high rotational speed of its parent star, the orbital motion of HD 15082 b may be affected in a measurable way by the huge oblateness of the star and effects of general relativity.

First, the distorted shape of the star makes its gravitational field deviate from the usual Newtonian inverse-square law. The same is true for the Sun, and part of the precession of the orbit of Mercury is due to this effect. However, it is estimated to be $9\times 10^{9}$ greater for HD 15082b.

Other effects will also be greater for HD 15082b. In particular, precession due to general relativistic frame-dragging should be $3\times 10^{5}$ greater for HD 15082b than for Mercury, where it is so far too small to have been observed. It has been argued that the oblateness of HD 15082 could be measured at a percent accuracy from a 10-year analysis of the time variations of the planet's transits. Effects due to the planet's oblateness are smaller by at least one order of magnitude, and they depend on the unknown angle between the planet's equator and the orbital plane, perhaps making them undetectable. The effects of frame-dragging are slightly too small to be measured by such an experiment.