NASA Studied Mercury To Calculate Sun’s Loss Of Mass, Gravitational Pull

The sun is about half-way through its lifespan of 10 billion years in its current state, and in all the time it has been around, it has lost some mass. As our familiar star ages and continues losing mass, its gravitational pull on the solar system weakens as well.

A team of researchers from NASA and Massachusetts Institute of Technology (MIT) studied changes in the orbit of Mercury to determine the rate at which solar mass is being lost, which has implications for our knowledge of some other solar parameters and also the stability of the gravitational constant.

In a study published online Thursday, the researchers said the actual rate of solar mass loss, based on observations over seven years, was only slightly lower than what was predicted by theoretical frameworks. But what was significant was the observed value had a lot less uncertainty than the theoretical models, and that allowed the scientists to improve the stability of the gravitational constant, called G, by a factor of 10.

G is usually considered to be a fixed number, but whether it really is a constant or not is still an unanswered question of fundamental physics.

Theoretical models predict the rate of loss of solar mass to be about 0.1 percent of the sun’s mass over 10 billion years. That would cause the sun’s gravitational pull to reduce enough for planetary orbits in the solar system to increase by about half an inch (1.5 centimeters) every year, per AU (one astronomical unit is the distance between the sun and Earth, about 93 million miles or 150 million kilometers).

“Mercury is the perfect test object for these experiments because it is so sensitive to the gravitational effect and activity of the sun,” Antonio Genova from MIT, lead author of the study who is working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement Thursday.

Researchers took data collected by NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft which, in 2008 and 2009, made three flybys of the planet closest to the sun, and orbited it from March 2011 to April 2015.

The perihelion of Mercury’s orbit — the point at which it is closest to the sun — shifts over time, and this phenomenon, called precession, has been known for a long time. This happens for a number of reasons, such as the gravitational pull of other planets, the warping of space-time around the sun due to the star’s own gravity, and the sun’s interior structure and dynamics.

One of those internal structures of the sun is its oblateness, or how much it deviates from being a perfect sphere, given the bulge at its equator. Looking at subtle changes in Mercury’s orbit, the researchers also arrived at an improved estimate of the sun’s oblateness.

“We’re addressing long-standing and very important questions both in fundamental physics and solar science by using a planetary-science approach. 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,” Goddard geophysicist Erwan Mazarico, who was a coauthor of the study, said in the statement.

Titled “Solar system expansion and strong equivalence principle as seen by the NASA MESSENGER mission,” the open-access paper appeared in the journal Nature Communications.