New observations from NASA’s Transiting Exoplanet Survey Satellite (TESS) have enabled astronomers to improve their understanding of the bizarre environment of KELT-9b, an ultrahot Jupiter exoplanet transiting the rapidly rotating early-A-type star KELT-9.
This illustration shows how the ultrahot Jupiter KELT-9b sees its host star. Image credit: NASA’s Goddard Space Flight Center / Chris Smith, USRA.
KELT-9b belongs to a family called ultrahot Jupiters, incredibly hot giant exoplanets that orbit very close to their parent stars.
Discovered in 2017, the planet is a gas giant about 1.8 times bigger than Jupiter, with 2.9 times its mass.
It orbits its host star, KELT-9, once every 36 hours on an orbit that carries it almost directly above both of the star’s poles.
At around 9,900 degrees Celsius (17,850 degrees Fahrenheit), the host star is at the dividing line between stars of type A and B.
Also known as HD 195689, the star is about 650 light-years away in the constellation of Cygnus.
It’s about twice the size of our Sun and averages about 56% hotter. But it spins 38 times faster than the Sun, completing a full rotation in just 16 hours.
Its rapid spin distorts the star’s shape, flattening it at the poles and widening its midsection. This causes the star’s poles to heat up and brighten while its equatorial region cools and dims — a phenomenon called gravity darkening. The result is a temperature difference across the star’s surface of almost 800 degrees Celsius (1,500 Fahrenheit).
KELT-9b is tidally locked to the star and its day side is perpetually bombarded by stellar radiation. It receives 44,000 times more energy from its star than Earth does from the Sun.
With a day-side temperature peaking at 4,327 degrees Celsius (7,820 degrees Fahrenheit), the planet is hotter than the surfaces of most stars.
“The weirdness factor is high with KELT-9b. It’s a giant planet in a very close, nearly polar orbit around a rapidly rotating star, and these features complicate our ability to understand the star and its effects on the planet,” said Dr. John Ahlers, an astronomer at Universities Space Research Association and NASA’s Goddard Space Flight Center.
TESS observed 27 transits of KELT-9b, taking measurements every two minutes, between July 18 and Sept. 11, 2019.
These observations allowed Dr. Ahlers and colleagues to model the system’s unusual star and its impact on the planet.
According to the team, with each orbit, KELT-9b twice experiences the full range of stellar temperatures, producing what amounts to a peculiar seasonal sequence.
The planet experiences ‘summer’ when it swings over each hot pole and ‘winter’ when it passes over the star’s cooler midsection.
So KELT-9b experiences two summers and two winters every year, with each season about 9 hours.
“It’s really intriguing to think about how the star’s temperature gradient impacts the planet,” said Dr. Knicole Colón, an astronomer at NASA’s Goddard Space Flight Center.
“The varying levels of energy received from its star likely produce an extremely dynamic atmosphere.”
KELT-9b’s polar orbit around its flattened star produces distinctly lopsided transits.
The planet begins its transit near the star’s bright poles and then blocks less and less light as it travels over the star’s dimmer equator.
This asymmetry provides clues to the temperature and brightness changes across the star’s surface, and they permitted the team to reconstruct the star’s out-of-round shape, how it’s oriented in space, its range of surface temperatures, and other factors impacting the planet.
“Of the planetary systems that we’ve studied via gravity darkening, the effects on KELT-9b are by far the most spectacular,” said University of Idaho’s Professor Jason Barnes.
“This work goes a long way toward unifying gravity darkening with other techniques that measure planetary alignment, which in the end we hope will tease out secrets about the formation and evolutionary history of planets around high-mass stars.”
The team’s paper was published in the Astronomical Journal.
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John P. Ahlers et al. 2020. KELT-9 b’s Asymmetric TESS Transit Caused by Rapid Stellar Rotation and Spin-Orbit Misalignment. AJ 160, 4; doi: 10.3847/1538-3881/ab8fa3
This article is based on text provided by the National Aeronautics and Space Administration.
