Life after death: Astronomers find a planet that shouldn't exist

Date:
June 28, 2023
Source:
University of Hawaii at Manoa
Summary:
The star would have inflated up to 1.5 times the planet's orbital
distance -- engulfing the planet in the process -- before shrinking
to its current size at only one-tenth of that distance.


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FULL STORY ==========================================================================
When our Sun reaches the end of its life, it will expand to 100 times its current size, enveloping the Earth. Many planets in other solar systems
face a similar doom as their host stars grow old. But not all hope is
lost, as astronomers from the University of Hawaii Institute for Astronomy
(UH IfA) have made the remarkable discovery of a planet's survival after
what should have been certain demise at the hands of its sun.

The Jupiter-like planet 8 UMi b, officially named Halla, orbits the red
giant star Baekdu (8 UMi) at only half the distance separating the Earth
and the Sun.

Using two Maunakea Observatories on Hawaiii Island -- W. M. Keck
Observatory and Canada-France-Hawaii Telescope (CFHT) -- a team
of astronomers led by Marc Hon, a NASA Hubble Fellow at UH IfA, has
discovered that Halla persists despite the normally perilous evolution of Baekdu. Using observations of Baekdu's stellar oscillations from NASA's Transiting Exoplanet Survey Satellite (TESS), they found that the star
is burning helium in its core, signaling that it had already expanded enormously into a red giant star once before.

The star would have inflated up to 1.5 times the planet's orbital
distance - - engulfing the planet in the process -- before shrinking to
its current size at only one-tenth of that distance.

The study is published in today's issue of the journal Nature.

"Planetary engulfment has catastrophic consequences for either the
planet or the star itself -- or both. The fact that Halla has managed
to persist in the immediate vicinity of a giant star that would have
otherwise engulfed it highlights the planet as an extraordinary survivor,"
said Hon, the lead author of the study.

Maunakea Observatories Confirm the Survivor The planet Halla was
discovered in 2015 by a team of astronomers from Korea using the radial velocity method, which measures the periodic movement of a star due to
the gravitational tug of the orbiting planet. Following the discovery
that the star must at one time have been larger than the planet's orbit,
the IfA team conducted additional observations from 2021-2022 using Keck Observatory's High-Resolution Echelle Spectrometer (HIRES) and CFHT's
ESPaDOnS instrument. These new data confirmed the planet's 93-day,
nearly circular orbit had remained stable for over a decade and that
the radial velocity changes must be due to a planet.

"Together, these observations confirmed the existence of the planet,
leaving us with the compelling question of how the planet actually
survived," said IfA astronomer Daniel Huber, second author of the
study. "The observations from multiple telescopes on Maunakea was
critical in this process." Escaping Engulfment At a distance of 0.46 astronomical units (AU, or the Earth-Sun distance) to its star, the
planet Halla resembles 'warm' or 'hot' Jupiter-like planets that are
thought to have started on larger orbits before migrating inward close to
their stars. However, in the face of a rapidly evolving host star, such
an origin becomes an extremely unlikely survival pathway for planet Halla.

Another theory for the planet's survival is that it never faced the
danger of engulfment. Similar to the famous planet Tatooine from Star
Wars, which orbits two suns, the team believes the host star Baekdu may
have originally been two stars. A merger of these two stars may have
prevented any one of them from expanding sufficiently large enough to
engulf the planet.

A third possibility is that Halla is a relative newborn -- that the
violent collision between the two stars produced a gas cloud from which
the planet formed. In other words, the planet Halla may be a recently-born 'second generation' planet.

"Most stars are in binary systems, but we don't yet fully grasp how
planets may form around them. Therefore, it's plausible that more
planets may actually exist around highly evolved stars thanks to binary interactions," explained Hon.

* RELATED_TOPICS
o Space_&_Time
# Extrasolar_Planets # Stars # Astronomy # Jupiter #
Kuiper_Belt # Eris_(Xena) # Mars # Pluto
* RELATED_TERMS
o Venus o Eris_(dwarf_planet) o Precession o Uranus o Axial_tilt
o Moon o Neptune o Definition_of_planet

========================================================================== Story Source: Materials provided by University_of_Hawaii_at_Manoa. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Artist's_impression_of_the_planet's_survival ========================================================================== Journal Reference:
1. Marc Hon, Daniel Huber, Nicholas Z. Rui, Jim Fuller, Dimitri
Veras, James
S. Kuszlewicz, Oleg Kochukhov, Amalie Stokholm, Jakob Lysgaard
Ro/rsted, Mutlu Yıldız, Zeynep C,elik Orhan, Sibel O"rtel,
Chen Jiang, Daniel R. Hey, Howard Isaacson, Jingwen Zhang, Mathieu
Vrard, Keivan G.

Stassun, Benjamin J. Shappee, Jamie Tayar, Zachary R. Claytor,
Corey Beard, Timothy R. Bedding, Casey Brinkman, Tiago L. Campante,
William J.

Chaplin, Ashley Chontos, Steven Giacalone, Rae Holcomb, Andrew
W. Howard, Jack Lubin, Mason MacDougall, Benjamin T. Montet, Joseph
M. A. Murphy, Joel Ong, Daria Pidhorodetska, Alex S. Polanski,
Malena Rice, Dennis Stello, Dakotah Tyler, Judah Van Zandt,
Lauren M. Weiss. A close-in giant planet escapes engulfment by its
star. Nature, 2023; 618 (7967): 917 DOI: 10.1038/s41586-023-06029-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/06/230628130343.htm

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