Enormous planet quickly orbiting a tiny, dying star

Date:
September 16, 2020
Source:
University of Wisconsin-Madison
Summary:
Thanks to a bevy of telescopes in space and on Earth -- and even
a pair of amateur astronomers in Arizona -- astronomers have
discovered a Jupiter-sized planet orbiting at breakneck speed
around a distant white dwarf star.



FULL STORY ========================================================================== Thanks to a bevy of telescopes in space and on Earth -- and even a pair
of amateur astronomers in Arizona -- a University of Wisconsin-Madison astronomer and his colleagues have discovered a Jupiter-sized planet
orbiting at breakneck speed around a distant white dwarf star.


==========================================================================
The system, about 80 light years away, violates all common conventions
about stars and planets. The white dwarf is the remnant of a sun-like
star, greatly shrunken down to roughly the size of Earth, yet it retains
half the sun's mass.

The massive planet looms over its tiny star, which it circles every 34
hours thanks to an incredibly close orbit. In contrast, Mercury takes a comparatively lethargic 90 days to orbit the sun. While there have been
hints of large planets orbiting close to white dwarfs in the past, the
new findings are the clearest evidence yet that these bizarre pairings
exist. That confirmation highlights the diverse ways stellar systems
can evolve and may give a glimpse at our own solar system's fate. Such
a white dwarf system could even provide a rare habitable arrangement
for life to arise in the light of a dying star.

"We've never seen evidence before of a planet coming in so close
to a white dwarf and surviving. It's a pleasant surprise," says lead
researcher Andrew Vanderburg, who recently joined the UW-Madison astronomy department as an assistant professor. Vanderburg completed the work while
an independent NASA Sagan Fellow at the University of Texas at Austin.

The researchers published their findings Sept. 16 in the journal Nature.

Vanderburg led a large, international collaboration of astronomers
who analyzed the data. The contributing telescopes included NASA's exoplanet-hunting telescope TESS and two large ground-based telescopes
in the Canary Islands.

Vanderburg was originally drawn to studying white dwarfs -- the remains
of sun- sized stars after they exhaust their nuclear fuel -- and their
planets by accident. While in graduate school, he was reviewing data
from TESS's predecessor, the Kepler space telescope, and noticed a white
dwarf with a cloud of debris around it.

"What we ended up finding was that this was a minor planet or asteroid
that was being ripped apart as we watched, which was really cool," says Vanderburg. The planet had been destroyed by the star's gravity after
its transition to a white dwarf caused the planet's orbit to fall in
toward the star.



==========================================================================
Ever since, Vanderburg has wondered if planets, especially large ones,
could survive the journey in toward an aging star.

By scanning data for thousands of white dwarf systems collected by TESS,
the researchers spotted a star whose brightness dimmed by half about every one-and- a-half days, a sign that something big was passing in front of
the star on a tight, lightning-fast orbit. But it was hard to interpret
the data because the glare from a nearby star was interfering with TESS's measurements. To overcome this obstacle, the astronomers supplemented
the TESS data from higher- resolution ground-based telescopes, including
three run by amateur astronomers.

"Once the glare was under control, in one night, they got much nicer and
much cleaner data than we got with a month of observations from space,"
says Vanderburg. Because white dwarfs are so much smaller than normal
stars, large planets passing in front of them block a lot of the star's
light, making detection by ground-based telescopes much simpler.

The data revealed that a planet roughly the size of Jupiter, perhaps a
little larger, was orbiting very close to its star. Vanderburg's team
believes the gas giant started off much farther from the star and moved
into its current orbit after the star evolved into a white dwarf.

The question became: how did this planet avoid being torn apart during
the upheaval? Previous models of white dwarf-planet interactions didn't
seem to line up for this particular star system.



==========================================================================
The researchers ran new simulations that provided a potential answer
to the mystery. When the star ran out of fuel, it expanded into a red
giant, engulfing any nearby planets and destabilizing the Jupiter-sized
planet that orbited farther away. That caused the planet to take on an exaggerated, oval orbit that passed very close to the now-shrunken white
dwarf but also flung the planet very far away at the orbit's apex.

Over eons, the gravitational interaction between the white dwarf and
its planet slowly dispersed energy, ultimately guiding the planet
into a tight, circular orbit that takes just one-and-a-half days to
complete. That process takes time -- billions of years. This particular
white dwarf is one of the oldest observed by the TESS telescope at
almost 6 billion years old, plenty of time to slow down its massive
planet partner.

While white dwarfs no longer conduct nuclear fusion, they still release
light and heat as they cool down. It's possible that a planet close
enough to such a dying star would find itself in the habitable zone,
the region near a star where liquid water can exist, presumed to be
required for life to arise and survive.

Now that research has confirmed these systems exist, they offer a
tantalizing opportunity for searching for other forms of life. The unique structure of white dwarf-planet systems provides an ideal opportunity
to study the chemical signatures of orbiting planets' atmospheres,
a potential way to search for signs of life from afar.

"I think the most exciting part of this work is what it means for both habitability in general -- can there be hospitable regions in these
dead solar systems -- and also our ability to find evidence of that habitability," says Vanderburg.

This work was supported in part by the National Science Foundation
(including grant AST-1824644) and by NASA (including grants RSA-1610091, RSA-1006130, 80NSSC19K1727 and 80NSSC19K0388). This work was performed
in part under contract with the California Institute of Technology/Jet Propulsion Laboratory funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute.


========================================================================== Story Source: Materials provided
by University_of_Wisconsin-Madison. Original written by Eric
Hamilton. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
*
YouTube_video:_TESS,_Spitzer_Spot_Potential_Giant_World_Circling_Tiny
Star ========================================================================== Journal Reference:
1. Vanderburg, A., Rappaport, S.A., Xu, S. et al. x. A giant planet
candidate transiting a white dwarf. Nature, 2020 DOI:
10.1038/s41586-020- 2713-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200916113547.htm

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