Flaring Red Dwarf Star (Artist's Illustration)
Cool dwarf stars are hot targets for exoplanet hunting right now. The discoveries of planets in the habitable zones of the TRAPPIST-1 and LHS 1140 systems, for example, suggest that Earth-sized worlds might circle billions of red dwarf stars, the most common type of star in our galaxy. But, like our own sun, many of these stars erupt with intense flares. Are red dwarfs really as friendly to life as they appear, or do these flares make the surfaces of any orbiting planets inhospitable?
To address this question, a team of scientists has combed 10 years of
ultraviolet observations by the Galaxy Evolution Explorer (GALEX)
spacecraft looking for rapid increases in the brightnesses of stars due
to flares. Flares emit radiation across a wide swath of wavelengths,
with a significant fraction of their total energy released in the
ultraviolet bands where GALEX observed. At the same time, the red dwarfs
from which the flares arise are relatively dim in the ultraviolet. This
contrast, combined with the time resolution of the GALEX detectors,
allowed the team to measure events with less total energy than many
previously detected flares. This is important because, although
individually less energetic and therefore less hostile to life, smaller
flares might be much more frequent and add up over time to produce an
inhospitable environment.
“What if planets are constantly bathed by these smaller, but still
significant, flares?” asked Scott Fleming of the Space Telescope Science
Institute (STScI) in Baltimore, Maryland. “There could be a cumulative
effect.”
To detect and accurately measure these flares, the team had to slice
the GALEX data into very high time resolution. From images with exposure
times of nearly half an hour, the team was able to reveal stellar
variations lasting just seconds.
First author Chase Million of Million Concepts in State College,
Pennsylvania, led a project called gPhoton that reprocessed more than
100 terabytes of GALEX data held at the Mikulski Archive for Space
Telescopes (MAST), located at STScI. The team then used custom software
developed by Million and Clara Brasseur (STScI) to search several
hundred red dwarf stars and detected dozens of flares.
“We have found dwarf star flares in the whole range that we expected
GALEX to be sensitive to, from itty bitty baby flares that last a few
seconds, to monster flares that make a star hundreds of times brighter
for a few minutes,” said Million.
The flares GALEX detected are similar in strength to flares produced
by our own sun. However, because a planet would have to orbit much
closer to a cool, red dwarf star to maintain a temperature friendly to
life as we know it, such planets would be subjected to more of a flare’s
energy than Earth.
Large flares can strip away a planet’s atmosphere. Strong ultraviolet
light from flares that penetrates to a planet’s surface could damage
organisms or prevent life from arising.
Currently, team members Rachel Osten (STScI) and Brasseur are
examining stars observed by both the GALEX and Kepler missions to look
for similar flares. The team expects to eventually find hundreds of
thousands of flares hidden in the GALEX data.
"These results show the value of a survey mission like GALEX, which
was instigated to study the evolution of galaxies across cosmic time and
is now having an impact on the study of nearby habitable planets," said
Don Neill, research scientist at Caltech in Pasadena, California, who
was part of the GALEX collaboration. "We did not anticipate that GALEX
would be used for exoplanets when the mission was designed."
New and powerful instruments like the James Webb Space Telescope,
scheduled for launch in 2018, ultimately will be needed to study
atmospheres of planets orbiting nearby red dwarf stars and search for
signs of life. But as researchers pose new questions about the cosmos,
archives of data from past projects and missions, like those held at
MAST, continue to produce exciting new scientific results.
These results were presented in a press conference at a meeting of the American Astronomical Society in Austin, Texas.
The GALEX mission, which ended in 2013 after more than a decade of
scanning the skies in ultraviolet light, was led by scientists at
Caltech. NASA's Jet Propulsion Laboratory, also in Pasadena, California,
managed the mission and built the science instrument. JPL is managed by
Caltech for NASA.
The Space Telescope Science Institute (STScI) in Baltimore, Maryland,
conducts Hubble Space Telescope science operations and is the mission
and science operations center for the James Webb Space Telescope. STScI
is operated for NASA by the Association of Universities for Research in
Astronomy in Washington, D.C.
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Contacts
Christine Pulliam / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4366 / 410-338-4514
cpulliam@stsci.edu / villard@stsci.edu
Chase Million
Million Concepts, State College, Pennsylvania
765-914-5336
chase.million@gmail.com
Source: HubbleSite
