Astronomers using the Gemini South
telescope in Chile, operated by NSF’s NOIRLab, have observed the first
compelling evidence of a dying sun-like star engulfing an exoplanet. The
"smoking gun" of this event was seen in a long and low-energy outburst
from the star—the telltale signature of a planet skimming along a star’s
surface. This never-before-seen process may herald the ultimate fate of Earth
when our own sun nears the end of its life in about five billion years. Credit:
International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani
By
studying countless stars at various stages of their evolution, astronomers have
been able to piece together an understanding of the life cycle of stars and how
they interact with their surrounding planetary systems as they age. This
research confirms that when a sun-like star nears the end of its life, it
expands anywhere from 100 to 1,000 times its original size, eventually
engulfing the system's inner planets. Such events are estimated to occur only a
few times each year across the entire Milky Way. Though past observations have
confirmed the aftermath of planetary engulfments, astronomers have never caught
one in the act, until now.
With the power of the Gemini South
Adaptive Optics Imager (GSAOI) on Gemini South, one half of the International
Gemini Observatory, operated by NSF's NOIRLab, astronomers have observed the
first direct evidence of a dying star expanding to engulf one of its planets. Evidence for this event was found in a telltale
"long and low-energy" outburst from a star in the Milky Way about
13,000 light-years from Earth. This event, the devouring of a planet by an
engorged star, likely presages the ultimate fate of Mercury, Venus, and Earth
when our sun begins its death throes in about five billion years.
"These observations provide a new perspective on finding and studying the billions of stars in our Milky Way that have already consumed their planets," says Ryan Lau, NOIRLab astronomer and co-author on this study, which is published in the journal Nature.
Astronomers using the Gemini South telescope in
Chile, operated by NSF’s NOIRLab, have observed the first compelling evidence
of a dying sun-like star engulfing an exoplanet. The "smoking gun" of
this event was seen in a long and low-energy outburst from the star—the
telltale signature of a planet skimming along a star’s surface. This
never-before-seen process may herald the ultimate fate of Earth when our own
sun nears the end of its life in about five billion years. Credit: International
Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani/N. Bartmann
For most of its life, a sun-like
star fuses hydrogen into helium in its hot, dense core, which allows the star
to push back against the crushing weight of its outer layers. When hydrogen in
the core runs out, the star begins fusing helium into carbon, and hydrogen
fusion migrates to the star's outer layers, causing them to expand, and
changing the sun-like star into a red giant.
Such a transformation, however, is
bad news for any inner-system planets. When the star's surface eventually
expands to engulf one of its planets, their interaction would trigger a
spectacular outburst of energy and material. This process would also put the
brakes on the planet's orbital velocity, causing it to plunge into the star.
The first hints of this event were
uncovered by optical images from the Zwicky Transient Facility. Archival
infrared coverage from NASA's Near-Earth Object Wide-field Infrared Survey
Explorer (NEOWISE), which is able to peer into dusty environments in search of
outbursts and other transient events, then confirmed the engulfment event,
named ZTF SLRN-2020. "Our team's custom reanalysis of all-sky infrared
maps from NEOWISE exemplifies the vast discovery potential of archival survey data
sets," said NOIRLab astronomer Aaron Meisner, another co-author on the
paper.
Credit: International Gemini
Observatory/NOIRLab/NSF/AURA/P. Marenfeld
Distinguishing
a planetary-engulfment outburst from other types of outbursts, such as
solar-flare-type events and coronal-mass ejections, is difficult and requires high-resolution
observations to pinpoint the location of an outburst and long-term measurements
of its brightness without contamination from nearby stars.
Gemini South provided these essential
data thanks to its adaptive-optics capabilities.
"Gemini South continues to expand
our understanding of the Universe and these new observations support
predictions for the future of our own planet," said NSF Gemini Observatory
program director Martin Still. "This discovery is a wonderful example of
the feats we can accomplish when we combine world-class telescope operations
and cutting-edge scientific collaboration."
"With these revolutionary new
optical and infrared surveys, we are now witnessing such events happen in real
time in our own Milky Way—a testament to our almost certain future as a
planet," said Kishalay De, an astronomer at the Massachusetts Institute of
Technology and lead author on the paper.
The outburst from the engulfment lasted
approximately 100 days and the characteristics of its lightcurve, as well as
the ejected material, gave astronomers insight into the mass of the star and
that of its engulfed planet. The ejected material consisted of about 33 Earth
masses of hydrogen and about 0.33 Earth masses of dust. "That's more star-
and planet-forming material being recycled, or burped out, into the
interstellar medium thanks to the star eating the planet," said Lau. From
this analysis, the team estimated that the progenitor star is about 0.8−1.5
times the mass of our sun and the engulfed planet was 1−10 times the mass of
Jupiter.
Now that the signatures of a planetary
engulfment have been identified for the first time, astronomers have improved
metrics they can use to search for similar events happening elsewhere in the
cosmos. This will be especially important when Vera C. Rubin Observatory comes
on line in 2025. For instance, the observed effects of chemical pollution on
the remnant star when seen elsewhere can hint that an engulfment has taken
place. The interpretation of this event also provides evidence for a missing
link in our understanding of the evolution and final fates of planetary systems, including our own.
"I think there's something pretty remarkable about these results that speaks to the transience of our existence," says Lau. "After the billions of years that span the lifetime of our solar system, our own end stages will likely conclude in a final flash that lasts only a few months."
by Association of Universities for Research in Astronomy
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