TESS offers new insights into an ultra-hot world

Measurements from NASA’s Transit Exoplanet Study Satellite (TESS) have enabled astronomers to greatly improve their understanding of the strange environment of KELT-9b, one of the hottest known planets.

“The rarity factor is high with KELT-9b,” said John Ahlers, an astronomer at the Universities Space Research Association in Columbia, Maryland, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It is a giant planet in a very close, almost polar, orbit around a rapidly rotating star, and these characteristics complicate our ability to understand the star and its effects on the planet.”

The new findings appear in an article led by Ahlers published June 5 in The astronomical diary.

Located about 670 light-years away in the Cygnus constellation, KELT-9 b was discovered in 2017 because the planet passed one part of each orbit in front of its star, an event called transit. Transits regularly dim light from the star by a small but detectable amount. KELT-9 b transits were first observed in the KELT Traffic Survey, a project that collected observations from two robotic telescopes located in Arizona and South Africa.

Between July 18 and September 11, 2019, as part of the mission’s yearlong campaign to observe the northern sky, TESS observed 27 KELT-9 b transits, taking measurements every two minutes. These observations allowed the team to model the system’s unusual star and its impact on the planet.

KELT-9 b is a gas giant world approximately 1.8 times larger than Jupiter, with 2.9 times its mass. The tidal forces have blocked its rotation, so the same side always faces its star. The planet rotates around its star in just 36 hours in an orbit that takes it almost directly over the star’s two poles.

KELT-9 b receives 44,000 times more energy from its star than Earth from the Sun. This makes the planet’s daytime temperature around 7,800 degrees Fahrenheit (4,300 C), hotter than the surface of some stars. This intense warming also causes the planet’s atmosphere to flow into space.

Its host star is also a rarity. It is about twice the size of the Sun and averages 56 percent more heat. But it spins 38 times faster than the Sun, completing a full rotation in just 16 hours. Its quick twist distorts the shape of the star, flattening it at the poles and widening its midsection. This causes the star’s poles to heat up and light up as its equatorial region cools and dims, a phenomenon called gravity darkening. The result is a difference in temperature at the star’s surface of nearly 1,500 F (800 C).

With each orbit, KELT-9 b experiences twice the full range of stellar temperatures, producing what amounts to a peculiar seasonal sequence. The planet experiences “summer” when it balances on each hot pole and “winter” when it passes over the coldest midsection of the star. So KELT-9 b experiences two summers and two winters each year, with each season approximately nine hours.

“It is really intriguing to think about how the star’s temperature gradient impacts the planet,” said Knicole Colón de Goddard, co-author of the article. “Variable energy levels received from your star are likely to produce an extremely dynamic atmosphere.”

KELT-9 b’s polar orbit around its flattened star produces clearly asymmetric 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 fainter equator. This asymmetry provides clues to changes in temperature and brightness across the star’s surface, and allowed the team to reconstruct the star’s rounded shape, how it is oriented in space, its range of surface temperatures, and other factors. that affect the planet.

“Of the planetary systems we have studied using gravity darkening, the effects on KELT-9b are by far the most spectacular,” said Jason Barnes, professor of physics at the University of Idaho and co-author of the article. . “This work goes a long way in unifying the darkening of gravity with other techniques that measure planetary alignment, which in the end we hope to uncover secrets about the formation and evolutionary history of planets around high-mass stars.”