Sky & Telescope - December 2023 - 40

BEATRIZ INGLESSIS / S&T
Interplanetary Sneezeguards
of the star's magnetic field. For the Sun, the boundary lies
very close to our star, somewhere between five and 20 solar
radii, or roughly 0.05 a.u. - well within Mercury's orbit. But
for an active red dwarf, the boundary lies much farther out,
several dozen times the star's radius. Trappist-1's corona, for
example, reaches partway into its habitable zone.
Not all red-dwarf planets are cursed, though. Work by
Cohen and others indicates that the calmer red dwarf TOI700
has a mild stellar wind. The terrestrial planets in its
habitable zone - which lies beyond the star's Alfvén surface
- might maintain their atmospheres.
Lay of the Land
One problem with tackling how a magnetic field affects
atmospheric loss is that we don't know the escape rates at
Venus and Earth all that well. " We know more about Martian
atmospheric escape than we do about any other planet, "
says Curry, who's the principal investigator of NASA's
MAVEN mission, a key provider of those measurements.
" And while we take pride in that, I'm also horrified. " There's
never been a mission dedicated to studying Earth's atmospheric
escape, she laments, and the spacecraft data we have
from Venus are limited.
In a quest to change that, Curry is leading an effort
with the Parker Solar Probe to study our planet's fraternal
twin. Parker is whizzing past Venus during a series of flybys
intended to nudge the spacecraft into a tighter solar
orbit. Some of these passes take the craft through Venus's
atmospheric tail. Parker's measurements make previous
instruments sent to Venus look like the first generation of
smartphones, so the flybys present a unique opportunity.
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4
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6
Preliminary results indicate both that there's more stuff in
Venus's outflow and that it's moving faster than expected.
Curry, Brain, and others are also looking forward to
NASA's Escape and Plasma Acceleration and Dynamics
Explorers (ESCAPADE) mission. Set to launch on a Blue Origins
rocket in summer 2024, ESCAPADE will join MAVEN to
provide multi-angle, real-time coverage of the Red Planet's
response to the solar wind. (In a rare instance of cross-disciplinary
work, the agency's heliophysics division is funding
the mission.) NASA has also extended MAVEN's mission
through 2031 to support a planned sample-return mission,
so it might even be possible to send a third orbiter while both
craft are still active.
Meanwhile, Brain, Cohen, and a team of two dozen
scientists from a half-dozen disciplines are tackling the
questions of whether and when a magnetic field matters and
under which conditions a planet can retain its atmosphere.
Their ultimate goal, Cohen says, is to create a landscape of
all the possibilities, so that we can say, " If a planet is here
under these conditions, it should have a sustainable atmosphere,
and the magnetic field maybe helped. And if it's here,
it didn't. " The team recently calculated what happens when
they plop a Mars-like planet around a red dwarf. Preliminary
results indicate the escape rates increase, but the researchers
are still exploring which processes have the greatest impact
and exactly how much loss occurs.
Cohen has also explored whether we could detect an exoplanet's
intrinsic magnetic field. Astronomers have discovered
thousands of worlds around distant suns by detecting the
minuscule dip in the star's light as the exoplanet passes in
front of it. If the planet has a strong self-made field, then the
magnetosphere will create a bubble with a different density
in the space-weather environment. This bubble should
refract the radio emission from the star in a unique way as
the planet transits, potentially revealing the field to radio
telescopes such as LOFAR.
Nevertheless, Cohen thinks that we should keep our gaze
on Venus, Earth, and Mars to understand planetary magnetic
fields and atmospheres. " Understanding why these three
planets are so different will give us 90% of our understanding
about how [terrestrial] planets form in the universe, " he
says. " My strong belief is that we should use our solar system
as the main constraint for any problem we try to solve for
exoplanets. So if something doesn't work for what we know
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for the solar system, I don't think it can work elsewhere. "
" I am sure that there are situations for which the magnetic
5 Venus Radii
Looking down on Venus
PARKER FLYBYS The Parker Solar Probe has buzzed Venus several
times (shown here from above the planet's north pole), each trajectory
taking it tantalizingly close to the planet. Flybys 1 through 5 occurred
2018-2021; flyby 6 happened in August 2023. The seventh flyby, scheduled
for November 2024, will explore the tail region right behind Venus.
40 DECEMBER 2023 * SKY & TELESCOPE
field is really important, and I'm sure that there are situations
for which it is not, " says Brain. " And there are situations that
are kind of right on the hairy edge. . . . For now, I'm kind of
declaring victory by helping the community understand that
this is a question - that it's not settled science. "
¢ When asked if she wants to go to Mars, Science Editor
CAMILLE CARLISLE's favorite reply is, " No way. I mean, have
you seen it? "
Sky & Telescope - December 2023

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