NASA's TESS delivers new insights into an ultrahot world

Date:
June 30, 2020
Source:
NASA/Goddard Space Flight Center
Summary:
KELT-9 b is one of the hottest planets known. New measurements
from NASA's Transiting Exoplanet Survey Satellite (TESS) have
enabled astronomers to greatly improve their understanding of this
bizarre world.



FULL STORY ========================================================================== Measurements from NASA's Transiting Exoplanet Survey Satellite (TESS)
have enabled astronomers to greatly improve their understanding of the
bizarre environment of KELT-9 b, one of the hottest planets known.


==========================================================================
"The weirdness factor is high with KELT-9 b," said John Ahlers, an
astronomer at Universities Space Research Association in Columbia,
Maryland, and NASA's Goddard Space Flight Center in Greenbelt,
Maryland. "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." The new findings
appear in a paper led by Ahlers published on June 5 in The Astronomical Journal.

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

Between July 18 and Sept. 11, 2019, as part of the mission's yearlong
campaign to observe the northern sky, TESS observed 27 transits of KELT-9
b, 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 about 1.8 times bigger than Jupiter, with
2.9 times its mass. Tidal forces have locked its rotation so the same
side always faces its star. The planet swings around its star in just
36 hours on an orbit that carries it almost directly above both of the
star's poles.



========================================================================== KELT-9 b receives 44,000 times more energy from its star than Earth
does from the Sun. This makes the planet's dayside temperature around
7,800 degrees Fahrenheit (4,300 C), hotter than the surfaces of some
stars. This intense heating also causes the planet's atmosphere to stream
away into space.

Its host star is an oddity, too. It's about twice the size of the Sun
and averages about 56 percent 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 1,500 F (800 C).

With each orbit, KELT-9 b 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-9 b experiences
two summers and two winters every year, with each season about nine hours.

"It's really intriguing to think about how the star's temperature
gradient impacts the planet," said Goddard's Knicole Colo'n, a co-author
of the paper.

"The varying levels of energy received from its star likely produce
an extremely dynamic atmosphere." KELT-9 b'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-9 b are by far the most spectacular," said Jason Barnes,
a professor of physics at the University of Idaho and a co-author of
the paper. "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." TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts,
and managed by NASA's Goddard Space Flight Center.

Additional partners include Northrop Grumman, based in Falls Church,
Virginia; NASA's Ames Research Center in California's Silicon Valley; the Harvard- Smithsonian Center for Astrophysics in Cambridge, Massachusetts;
MIT's Lincoln Laboratory; and the Space Telescope Science Institute
in Baltimore. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.


========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Original written by Francis Reddy. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
*
YouTube_video:_NASA's_TESS_Delivers_New_Insights_Into_an_Ultrahot_World ========================================================================== Journal Reference:
1. John P. Ahlers, Marshall C. Johnson, Keivan G. Stassun, Knicole D.

Colo'n, Jason W. Barnes, Daniel J. Stevens, Thomas Beatty,
B. Scott Gaudi, Karen A. Collins, Joseph E. Rodriguez, George
Ricker, Roland Vanderspek, David Latham, Sara Seager, Joshua
Winn, Jon M. Jenkins, Douglas A. Caldwell, Robert F. Goeke, Hugh
P. Osborn, Martin Paegert, Pam Rowden, Peter Tenenbaum. KELT-9
b's Asymmetric TESS Transit Caused by Rapid Stellar Rotation and
Spin-Orbit Misalignment. The Astronomical Journal, 2020; 160 (1):
4 DOI: 10.3847/1538-3881/ab8fa3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200630125144.htm

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