Is Earth getting closer to the sun, or farther away? - Livescience.com

Over the course of the sun's remaining lifetime — estimated at another 5 billion years or so, according to NASA (opens in new tab) — models of how stars evolve over time predict the sun will lose about 0.1% of its total mass before it begins to die, Brian DiGiorgio, an astronomer at the University of California, Santa Cruz, told Live Science in an email.

Although 0.1% may not sound like a lot, "this is a lot of mass," DiGiorgio said.

Because the sun is losing mass, its pull on Earth is weakening, leading our planet to drift away from our star by about 2.36 inches (6 centimeters) per year, DiGiorgio said.

"This is pretty negligible, especially compared to the normal variation in Earth's orbital distance that happens because of its slightly elliptical orbit — about 3%," DiGiorgio said.

Because this is faster than the 365 or so days it takes for Earth to complete an orbit around the sun, the tidal bulge Earth generates on the sun sits ahead of Earth.

The bulge's mass has a gravitational pull associated with it, tugging Earth ahead on its orbit and slinging it farther from the sun, Scharringhausen noted.

(A similar effect is leading Earth's moon to slowly drift away from our planet (opens in new tab).)?

However, these tidal forces have a very weak effect on Earth's orbit: They cause Earth to move about 0.0001 inch (0.0003 cm) away from the sun every year, DiGiorgio calculated.

Given that Earth's distance from the sun may grow by 0.2% over the next 5 billion years, "this dimming corresponds to a 0.4% reduction of solar energy hitting the Earth's surface," he said?

The bigger thing to worry about "is that as the sun evolves over the next 5 billion years, stellar evolution models predict that it will increase in brightness by about 6% every 1 billion years, slowly increasing Earth's temperatures and boiling off the oceans," DiGiorgio said.

"So it is theoretically possible for Mercury to move by the Earth and change its orbit substantially, as it did to Mars in one simulation," DiGiorgio said?

It is also very unlikely that a passing star, planet or other body may perturb Earth's orbit, DiGiorgio said.

"My back-of-the-envelope calculations say that we should only expect a star to come closer than the orbit of Pluto about once every trillion years," DiGiorgio said.

There is a chance it will not puff out enough to reach Earth, meaning our planet may survive and continue to orbit.

However, most estimates suggest the sun will grow enough to swallow Earth, leading the planet to spiral "inwards towards oblivion," DiGiorgio said.

"However, even if the Earth survives, there is no chance that humans would be able to survive with it," DiGiorgio said.

If humans are still around 5 billion years from now and wanted Earth to remain habitable during the sun's expansion, we would have to slowly move the planet outward to around the orbit of Saturn, keeping it temperate enough for life as we know it as the sun continued to output more and more energy.