Mercury’s proximity to the Sun and small size make
it exquisitely sensitive to the dynamics of the Sun and its
gravitational pull.
Credits: NASA/SDO
Credits: NASA/SDO
Like the waistband of a couch potato in midlife, the orbits of
planets in our solar system are expanding. It happens because the Sun’s
gravitational grip gradually weakens as our star ages and loses mass.
Now, a team of NASA and MIT scientists has indirectly measured this mass
loss and other solar parameters by looking at changes in Mercury’s
orbit.
The new values improve upon earlier predictions by reducing the
amount of uncertainty. That’s especially important for the rate of solar
mass loss, because it’s related to the stability of G, the
gravitational constant. Although G is considered a fixed number, whether
it’s really constant is still a fundamental question in physics.
“Mercury is the perfect test object for these experiments because it
is so sensitive to the gravitational effect and activity of the Sun,”
said Antonio Genova, the lead author of the study published in Nature Communications and a Massachusetts Institute of Technology researcher working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
The study began by improving Mercury’s charted ephemeris — the road
map of the planet’s position in our sky over time. For that, the team
drew on radio tracking data that monitored the location of NASA’s
MESSENGER spacecraft while the mission was active. Short for Mercury
Surface, Space Environment, Geochemistry, and Ranging, the robotic
spacecraft made three flybys of Mercury in 2008 and 2009 and orbited the
planet from March 2011 through April 2015. The scientists worked
backward, analyzing subtle changes in Mercury’s motion as a way of
learning about the Sun and how its physical parameters influence the
planet’s orbit.
NASA and MIT scientists analyzed subtle changes in
Mercury’s motion to learn about the Sun and how its dynamics influence
the planet’s orbit. The position of Mercury over time was determined
from radio tracking data obtained while NASA’s MESSENGER mission was
active. Credits: NASA's Goddard Space Flight Center
For centuries, scientists have studied Mercury’s motion, paying particular attention to its perihelion, or the closest point to the Sun during its orbit. Observations long ago revealed that the perihelion shifts over time, called precession. Although the gravitational tugs of other planets account for most of Mercury’s precession, they don’t account for all of it.
The second-largest contribution comes from the warping of space-time
around the Sun because of the star’s own gravity, which is covered by
Einstein’s theory of general relativity. The success of general
relativity in explaining most of Mercury’s remaining precession helped
persuade scientists that Einstein’s theory was right.
Other, much smaller contributions to Mercury’s precession, are
attributed to the Sun’s interior structure and dynamics. One of those is
the Sun’s oblateness, a measure of how much it bulges at the middle —
its own version of a “spare tire” around the waist — rather than being a
perfect sphere. The researchers obtained an improved estimate of
oblateness that is consistent with other types of studies.
The researchers were able to separate some of the solar parameters
from the relativistic effects, something not accomplished by earlier
studies that relied on ephemeris data. The team developed a novel
technique that simultaneously estimated and integrated the orbits of
both MESSENGER and Mercury, leading to a comprehensive solution that
includes quantities related to the evolution of Sun’s interior and to
relativistic effects.
“We’re addressing long-standing and very important questions both in
fundamental physics and solar science by using a planetary-science
approach,” said Goddard geophysicist Erwan Mazarico. “By coming at these
problems from a different perspective, we can gain more confidence in
the numbers, and we can learn more about the interplay between the Sun
and the planets.”
The team’s new estimate of the rate of solar mass loss represents one
of the first times this value has been constrained based on
observations rather than theoretical calculations. From the theoretical
work, scientists previously predicted a loss of one-tenth of a percent
of the Sun’s mass over 10 billion years; that’s enough to reduce the
star’s gravitational pull and allow the orbits of the planets to spread
by about half an inch, or 1.5 centimeters, per year per AU (an AU, or
astronomical unit, is the distance between Earth and the Sun: about 93
million miles).
The new value is slightly lower than earlier predictions but has less
uncertainty. That made it possible for the team to improve the
stability of G by a factor of 10, compared to values derived from
studies of the motion of the Moon.
“The study demonstrates how making measurements of planetary orbit
changes throughout the solar system opens the possibility of future
discoveries about the nature of the Sun and planets, and indeed, about
the basic workings of the universe,” said co-author Maria Zuber, vice
president for research at MIT.
By Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.
Editor: Rob Garner
Source: NASA/Mercury(Planet)
