MadSci Network: Astronomy
Query:

Re: The Earths Orbit around the Sun/ Solar Flares

Area: Astronomy
Posted By: Jay H. Hartley, Grad Student,Lawrence Livermore National Lab
Date: Wed Apr 30 20:22:15 1997
Area of science: Astronomy
ID: 861468086.As
Message:

Dear Lossy,

In principle, our orbit is stable, meaning that it doesn't change. There is nothing inherent in the gravitational force that would cause a change. In reality, there are some drag forces from interactions with other planets, the solar wind, asteroids, dust, perhaps even the radiation of gravitional waves, etc., and these will tend to draw energy out of our orbit, making us drift closer to the sun. The drag from the earth's upper atmosphere causes low-orbiting satellites to drop back to earth eventually. In the case of the earth around the sun, the drag forces are so small that we'll stay in basically the same orbit right up to the time, in a few billion years, when the sun uses up its hydrogen fuel and turns into a red giant. At that point, the surface of the sun will expand all the way out to Earth, and the stability of our orbit won't matter one bit.

Until then, we're OK. :-)

The presence of the moon just serves to make our system's total mass a bit heavier, which modifies our orbit a touch. The mass of the moon is about 1.2% of the earth's, which doesn't seem like much, but most satellites in this solar system are a millionth the size of their "host" planet. As small as it is, it is far enough away from Earth (384,000 km) that it moves the center of gravity of the earth-moon system over 4,000 km from the center of the earth. That means that both the earth and the moon basically orbit a point roughly 2,000 km below the surface of the earth. This point is also the one to track when tracing out our orbit around the sun. It is called the "center of gravity" or "barycenter" of the earth-moon system. The motion of the center of the Earth (or the moon) would trace out a wiggle around that path. The presence of the moon does nothing to make our orbit decay faster or anything like that.

As far as solar flares, according to the NASA What Is A Solar Flare? page, a really big flare gives off 10^32 ergs of energy (3.6x10^13 ergs is one kilowatt-hour of energy). Now, that sounds like a huge amount, but it is only one-tenth of the energy emitted by the sun in a second, and by my calculations is 100 million times smaller than the kinetic energy of the earth as it moves in orbit. Add in that only a tiny fraction of a flare's energy is absorbed by the earth, and you can see that flares have basically no effect on our orbit.

Mercury, though a small planet, still has almost as much kinetic energy as earth because it's moving faster. I don't think it has a magnetic field, so even though it is closer it might capture an even smaller portion of a flare's energy. In total, I'm sure flares don't effect it's motion either.

In summary, barring some bizarre catastrophe, like the passage of a roaming planet through our system or a close encounter with another star, the Sun will come to us long before we fall into the sun.

Oh, and the particles in flares and the solar wind have escape velocity, meaning that they just keep on going, never to return to the sun. There, I hope I answered all your (current) questions. Thanks for asking. :-)

To complement Jay's excellent answer:

In fact, when the Sun enters the red giant phase in about 5 billion years, it will begin losing a lot of mass in an enhanced solar wind. This mass loss will decrease the attraction of the Sun on the Earth and so the orbit of the Earth will move out substantially. So while it is true that in the red giant phase, the Sun will swell out to the current orbit of the Earth, the Earth's will probably not be swallowed up. I am not sure that this is of any practical importance...

Marc Herant, astro admin.


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