Astronomers have used seven years’ worth of data about the movement of Mercury to estimate how much mass the Sun is losing and using this, they were able to work out how far and how fast planets will have drifted away from it by the end of the Sun’s life.
Over the next 5 billion years, the Earth’s orbit will shift outwards about 75,000 kilometers (46,600 miles). Given that our planet is about 149.6 million kilometers – or 1 Astronomical Unit (AU) – from the Sun, this won’t matter much. Each planet will move between 1.4 and 1.6 centimeters per AU per year. The scientists estimate that the Sun will lose 0.1 percent of its mass over 10 billion years; this is the first time that the value has been constrained using observations, reducing the uncertainty.
The study, published in Nature Communications, was not just about this drift, though. The researchers' goal was to study the most subtle gravitational effects to test if the predictions of both Newtonian mechanics and Einstein’s general relativity are correct. So far there are no deviations.
The team was able to estimate that if the gravitational constant – the value used to show the force between two objects caused by gravity – is not a constant, its yearly change has to be smaller than a part in 25 trillion. This is an improvement by a factor of 10 on the previous estimate, which used the motion of the Moon. It also provides better constraints on the strong equivalence principle, the idea that it’s not possible to distinguish between an accelerated frame of reference or a gravitational field.
“We’re addressing long-standing and very important questions both in fundamental physics and solar science by using a planetary-science approach,” co-author Erwan Mazarico, a geophysicist at the NASA’s Goddard Space Flight Center, said in a statement. “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.”
This interplay is, obviously, at its most visible in the orbit of the closest planet to the Sun. Mercury’s perihelion (its closest point to the Sun) shifts every year due to the gravitational attraction of the other planets and our own star, deforming space-time around it. Smaller contributions to this shift, technically known as precession, depends on the structure and movements inside the Sun.
“Mercury is the perfect test object for these experiments because it is so sensitive to the gravitational effect and activity of the Sun,” lead author Antonio Genova, a Massachusetts Institute of Technology researcher working at NASA’s Goddard Space Flight Center, added.
The data was collected by the NASA’s MESSENGER mission, which first flew over Mercury in 2008 and then orbited the planet from 2011 until it was sent crashing down on Mercury in 2015.