Sky and Telescope - November 2017 - 11

If solar physicists could detect gravity waves, then they could learn how
fast the core rotates. (Acoustic waves
pass through the core too quickly to be
sensitive to its spin.) But every potential
detection of gravity waves, going back
to the 1970s, has not held up against
further investigation.
Eric Fossat (Côte d'Azur Observatory, France) and colleagues now say
they've unearthed this shy signal. The
team dug through more than 16 years
of data from the joint ESA/NASA Solar
and Heliospheric Observatory (SOHO)
spacecraft, launched in 1995 in part to
look for g modes reverberating through
the Sun. Counterintuitively, to ﬁnd
them, the researchers focused on acoustic waves that had moved through the
core. Gravity waves change the Sun's

internal structure and density, so their
presence should affect the sound waves'
travel time.
Careful analysis of more than
34,000 eight-hour chunks of acousticwave data uncovered what looks like
the collective imprint of many gravity waves, Fossat's team reports in the
August Astronomy & Astrophysics.
The gravity waves' frequency suggests that the core rotates every seven
days. That's about four times faster than
the radiative zone above it, and four to
ﬁve times faster than the surface. Work
using Kepler data suggests that many
other stars' cores also rotate at different speeds than their outer layers. If the
SOHO detection proves true, then it'll be
one of the spacecraft's top discoveries.

IN BRIEF

■ CAMILLE M. CARLISLE

Saturn's magnetic axis aligns surprisingly
well with its rotational axis. As NASA's Cassini conducted its dramatic Grand Finale
dives between Saturn and its rings, the
magnetometer onboard measured the planet's magnetic field. Its tilt is much smaller
than 0.06°, which was the lower limit obtained prior to the Grand Finale. Magnetic
and rotational axes are misaligned in other
magnetically active planets; astronomers
think such tilts help sustain those planets'
internal dynamos. Saturn's good alignment
suggests we don't understand its internal
structure as well as we thought we did.
The wobble of a misaligned magnetic field
would also have revealed the rotation of the
planet's core, and without that misalignment, scientists are still unsure of the true
length of Saturn's day.

EXOPLANETS

Hot Jupiter Might Have Ozone-like Layer

E XOPL A NE T: ENGINE HOUSE V F X / AT-BRISTOL SCIENCE CEN TRE / UNIV ERSIT Y OF E X E TER

Artist's concept of WASP-121b and its host star

ASTRONOMERS HAVE potentially

detected a stratosphere in the atmosphere of the hot Jupiter WASP-121b.
In a stratosphere, temperature increases
with altitude - unlike the troposphere
below it, where the temperature drops
the higher you go. Earth has a stratosphere because of ozone, which absorbs
the Sun's ultraviolet rays and heats up
this atmospheric layer above it.
By watching the hot Jupiter pass
behind its yellow F-type star with the
Hubble and Spitzer Space Telescopes,
Thomas Evans (University of Exeter, UK)
and colleagues teased out light that
came from the exoplanet. Although
they couldn't resolve individual spectral
lines in this light, the researchers were
able to study the shape that multiple,

mushed-together lines made. Among
the shapes they uncovered is a bump in
the exoplanet's spectra that they think
is emission from water.
We should only detect emission
from water molecules if the part of the
upper atmosphere they're in is hotter
than the gas that lies below - if the
gas below were hotter, the water would
absorb some of its glow, an effect not
seen here. Thus, the team concludes in
the August 3rd Nature that WASP-121b
has a stratosphere. That result implies
it has some sort of ozone-mimicking
compound in its upper atmosphere,
perhaps vanadium oxide (VO), although
the team couldn't determine whether
VO is present.
■ CAMILLE M. CARLISLE

¢ BY DAVID DICKINSON

Goodbye, LISA Pathfinder
The LISA Pathfinder mission came to a
quiet end, as planned, on July 18th. LISA
Pathfinder launched in December 2015, entering an orbit 1.5 million km (1 million miles)
sunward of Earth. The gravitationally tranquil
environment enabled the spacecraft to keep
two small cubes in freefall, nearly motionless
with respect to each other. This technology
demonstration was key to the upcoming
Laser Interferometer Space Antenna (LISA)
mission, which will listen for low-frequency
gravitational waves such as those generated
by merging supermassive black holes.

Saturn's Mysterious
Magnetic Field

Sprites in Space
Six of "the world's smallest spacecraft,"
dubbed Sprites, piggybacked into lowEarth orbit on June 23rd. The spacecraft,
each slightly larger than a postage stamp,
weigh 4 grams each and contain solar panels, sensors, and communication equipment. Two of the Sprites were mounted on
larger satellites named Venta and Max Valier; Max Valier carried another four Sprites
in its cargo container for deployment as
independent satellites. While communication with the Max Valier satellite has failed,
halting the release of its Sprites, several
ground stations have made contact with
the Sprite mounted on the Venta satellite.
The achievement is a small milestone for
Breakthrough Starshot, which helped create the microsatellites as a test for building
future interstellar travelers.

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017