HD 15082

Lore & Background

HD 15082 is an Am star, making its stellar classification challenging. The hydrogen lines and effective temperature resemble spectral type A8, the calcium II K line resembles A5, and the metallic lines are more similar to F4, leading to the classification kA5hA8mF4. The star is about 100 million years old and spins with a projected rotational velocity of 86 km/s. It has 1.55 times the mass of the Sun and 1.51 times the Sun's radius. The intrinsic variability of HD 15082 was discovered in 2011 by Enrique Herrero et al. As a Delta Scuti variable, it exhibits many pulsation modes, with 8 measured high frequency p-modes. Another proposed non-radial mode, possibly induced by tidal interactions with the planet, would make this star also a Gamma Doradus variable. The star has the GCVS variable star designation V807 Andromedae. In 2010, the SuperWASP project announced the discovery of an exoplanet, designated WASP-33b, orbiting the star. The discovery was made by detecting the transit of the planet as it passes in front of its star every 1.22 days. It had first been identified as a planetary candidate in 2006.

Reader's Guide

HD 15082 holds significance as the first Delta Scuti variable star confirmed to host a planet, bridging the study of stellar pulsations and exoplanetary systems. Its discovery by the SuperWASP project in 2010, following its identification as a candidate in 2006, highlighted the potential for finding planets around pulsating stars. The star's complex spectral classification as an Am star (kA5hA8mF4) underscores the difficulty in typing such objects. The detection of eight high-frequency p-modes typical of Delta Scuti variables, along with a proposed non-radial mode possibly induced by tidal interactions with the hot Jupiter WASP-33b, suggests a dynamic interplay between the star and its planet. This system provides a unique laboratory for studying how a close-in giant planet may influence stellar oscillations, and it remains a key object for understanding the coexistence of planetary transits and stellar variability.

Did You Know?

• HD 15082 is the first Delta Scuti variable known to host a planet.
• The planet WASP-33b orbits every 1.22 days and was first identified as a candidate in 2006.
• The star's spectral type is written kA5hA8mF4 due to differing line strengths.

Discovery and Classification as a Hot Jupiter

In 2010, the SuperWASP survey team announced the detection of an extrasolar planet orbiting HD 15082. The signal was identified through the transit method, tracking the periodic dimming of starlight as the planet crossed the stellar disk on a 1.22-day cycle. The finding carried special significance because HD 15082 is a Delta Scuti variable star, making WASP-33b the first transiting exoplanet ever identified around a star of that pulsation class. Orbital parameters place the planet squarely in the hot Jupiter regime: its semimajor axis is a mere 0.026 AU, about 3.9 million kilometers, and radial-velocity constraints indicate a mass greater than Jupiter's but below 4.1 Jupiter masses. The extreme closeness to the host star drives the planetary surface temperature to roughly 3,200 °C. Years after the original announcement, researchers recovered the transit signature in archival Hipparcos photometry, providing an independent confirmation of the SuperWASP detection.

Retrograde Orbit and Orbital Misalignment

A 2012 study leveraging the Rossiter–McLaughlin effect uncovered one of WASP-33b's most remarkable traits: its orbit is strongly misaligned with the equatorial plane of HD 15082. The measured angle of −107.7 ± 1.6° means the planet travels in a retrograde direction, circling opposite to the star's spin. Such a configuration challenges straightforward disk-formation scenarios, which typically yield aligned orbits, and points toward a more complex evolutionary history involving migration or dynamical interactions. The same analysis revealed that the periastron node of the orbit is precessing, completing a full cycle in approximately 709 years, with an uncertainty of +33/−34 years. This slow nodal drift is consistent with the gravitational pull of the star's oblate figure, a connection that subsequent research would exploit to probe the internal structure of HD 15082 with extraordinary precision.

Atmospheric Chemistry and the Stratosphere

In June 2015, NASA researchers reported that WASP-33b hosts a stratosphere sustained by titanium monoxide, a compound that absorbs visible and ultraviolet radiation strongly while remaining gaseous at the planet's extreme temperatures. The initial detection was confirmed via high-resolution spectroscopy with the High Dispersion Spectrograph on Subaru's 8.2-meter telescope. The narrative then grew complicated: by 2020, higher-quality data under different observational settings yielded only an upper limit of one part-per-billion for titanium monoxide volume mixing ratio, failing to reproduce the original signal. Later work reconfirmed the molecule's presence at concentrations below HARPS-N's detection threshold. Neutral iron and silicon were also identified. Most strikingly, emission spectra revealed hydroxyl radicals on the dayside—the first such detection on any planet outside the Solar System—produced when intense heat dissociates water molecules.

Non-Keplerian Dynamics and Gravitational Probes

Because HD 15082 rotates rapidly, its shape deviates noticeably from a sphere, and this oblateness distorts the gravitational field felt by WASP-33b. The resulting departure from a simple Newtonian inverse-square force is estimated to be roughly nine billion times larger than the analogous effect on Mercury's orbit. General-relativistic frame-dragging, far too tiny to measure around the Sun, should be about 300,000 times stronger in this system, though it remains just beyond current observational reach. By 2021, researchers had measured the nodal precession of WASP-33b caused by the star's oblateness, extracting a gravitational quadrupole moment of 6.73 ± 0.22 × 10⁻⁵. Theoretical work shows a decade-long transit-timing analysis could pin down the star's oblateness to percent-level accuracy, while the expected non-Keplerian precession, roughly 500 times smaller, has yet to be detected.