Planetary bodies observed for first time in 'habitable zone' of dead
star

Date:
February 11, 2022
Source:
University College London
Summary:
A ring of planetary debris studded with moon-sized structures has
been observed orbiting close to a white dwarf star, hinting at a
nearby planet in the 'habitable zone' where water and life could
exist, according to a new study. White dwarfs are glowing embers
of stars that have burned through all their hydrogen fuel. Nearly
all stars, including the Sun, will eventually become white dwarfs,
but very little is known about their planetary systems.



FULL STORY ==========================================================================
A ring of planetary debris studded with moon-sized structures has been
observed orbiting close to a white dwarf star, hinting at a nearby planet
in the "habitable zone" where water and life could exist, according to
a new study led by UCL researchers.


========================================================================== White dwarfs are glowing embers of stars that have burned through
all their hydrogen fuel. Nearly all stars, including the Sun, will
eventually become white dwarfs, but very little is known about their
planetary systems.

In the study, published in Monthly Notices of the Royal Astronomical
Society, an international team of researchers measured light from a
white dwarf in the Milky Way known as WD1054-226, using data from ground-
and space-based telescopes.

To their surprise, they found pronounced dips in light corresponding
to 65 evenly spaced clouds of planetary debris orbiting the star every
25 hours. The researchers concluded that the precise regularity of the transiting structures -- dimming the star's light every 23 minutes --
suggests they are kept in such a precise arrangement by a nearby planet.

Lead author Professor Jay Farihi (UCL Physics & Astronomy) said: "This
is the first time astronomers have detected any kind of planetary body
in the habitable zone of a white dwarf.

"The moon-sized structures we have observed are irregular and dusty (e.g.

comet-like) rather than solid, spherical bodies. Their absolute
regularity, one passing in front of the star every 23 minutes, is a
mystery we cannot currently explain.



==========================================================================
"An exciting possibility is that these bodies are kept in such an
evenly-spaced orbital pattern because of the gravitational influence of
a nearby planet.

Without this influence, friction and collisions would cause the structures
to disperse, losing the precise regularity that is observed. A precedent
for this 'shepherding' is the way the gravitational pull of moons around Neptune and Saturn help to create stable ring structures orbiting these planets.

"The possibility of a planet in the habitable zone is exciting and also unexpected; we were not looking for this. However, it is important to
keep in mind that more evidence is necessary to confirm the presence of
a planet. We cannot observe the planet directly so confirmation may come
by comparing computer models with further observations of the star and
orbiting debris." It is expected that this orbit around the white dwarf
was swept clear during the giant star phase of its life, and thus any
planet that can potentially host water and thus life would be a recent development. The area would be habitable for at least two billion years, including at least one billion years into the future.

More than 95% of all stars will eventually become white dwarfs. The
exceptions are the largest stars that explode and become either black
holes or neutron stars.

Professor Farihi added: "Since our Sun will become a white dwarf in a
few billion years, our study provides a glimpse into the future of our
own solar system." When stars begin running out of hydrogen, they expand
and cool, becoming red giants. The Sun will enter this phase in four to
five billion years, swallowing Mercury, Venus, and possibly Earth. Once
the outer material has gently blown away and hydrogen is exhausted, the
hot core of the star remains, slowly cooling over billions of years --
this is the star's white dwarf phase.



========================================================================== Planets orbiting white dwarfs are challenging for astronomers to detect
because the stars are much fainter than main-sequence stars (like the
Sun). So far, astronomers have only found tentative evidence of a gas
giant (like Jupiter) orbiting a white dwarf.

For the new study, researchers observed WD1054-226, a white dwarf 117
light years away, recording changes in its light over 18 nights using
the ULTRACAM high-speed camera fixed on to the ESO 3.5m New Technology Telescope (NTT) at the La Silla Observatory in Chile. In order to better interpret the changes in light, the researchers also looked at data from
the NASA Transiting Exoplanet Survey Satellite (TESS), which allowed
the researchers to confirm the planetary structures had a 25-hour orbit.

They found that the light from WD1054-226 was always somewhat obscured by enormous clouds of orbiting material passing in front of it, suggesting
a ring of planetary debris orbiting the star.

The habitable zone, sometimes called the Goldilocks zone, is the area
where the temperature would theoretically allow liquid water to exist on
the surface of a planet. Compared to a star like the Sun, the habitable
zone of a white dwarf will be smaller and closer to the star as white
dwarfs give off less light and thus heat.

The structures observed in the study orbit in an area that would have
been enveloped by the star while it was a red giant, so are likely to
have formed or arrived relatively recently, rather than survived from
the birth of the star and its planetary system.

The study received funding from the UK's Science and Technology Facilities Council (STFC) and involved a team of researchers from six countries,
including Boston University, the University of Warwick, Lund University,
the University of Cambridge, the University of St Andrews, Wesleyan
University, the University of La Laguna, Naresuan University, the
University of Sheffield, and the Instituto de Astrofi'sica de Canarias.

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


========================================================================== Related Multimedia:
* An_artist's_impression_of_the_white_dwarf_star_WD1054-226 ========================================================================== Journal Reference:
1. J Farihi, J J Hermes, T R Marsh, A J Mustill, M C Wyatt, J A Guidry,
T G
Wilson, S Redfield, P Izquierdo, O Toloza, B T Ga"nsicke, A
Aungwerojwit, C Kaewmanee, V S Dhillon, A Swan. Relentless and
complex transits from a planetesimal debris disc. Monthly Notices
of the Royal Astronomical Society, 2022; 511 (2): 1647 DOI:
10.1093/mnras/stab3475 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220211080603.htm

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