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We know about Bohr's model and his vague postulate challenging Rutherford regarding discrete orbits and not emitting electromagnetic waves during this.

Extending this idea to our solar system, does the Earth emit gravitational waves around its orbit of the Sun and if not why not, and if yes, will the Earth fall into the Sun?

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  • $\begingroup$ The Earths orbit is astronomically far away from the ground state, if there is such a thing, of a particle in a Newton potential. Also the Bohr atom is obsolete since 1925 or so. $\endgroup$ – my2cts Oct 14 at 23:22
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Extending this idea to our solar system, does earth emit gravitational waves around its orbit to Sun[?]

Yes. In general, any matter distribution with a time-varying quadrupole moment will emit. In practice, this means that you need to have a mass distribution that is not spherically symmetric, but the Earth-Sun system satisfies this.

[A]nd if yes,, will Earth fall on sun?

The rate at which the Earth-Sun system radiates power via gravitational waves is minuscule. If you look at p. 9 of these lecture notes, you'll find the equation for the average power emitted by two orbiting masses with semi-major axis $a$ and eccentricity $e = 0$ is $$ \langle P \rangle = - \frac{32}{5} \frac{G^4}{c^5} \frac{m_1^2 m_2^2 (m_1 + m_2)}{a^5} $$ For the Earth-Sun system, this works out to be about 196 watts of power.

The Earth has so much kinetic energy that this energy loss won't affect Earth's orbit by any perceptible amount over the lifetime of human civilization. You can also look lower down on page 9 for the equation for the lifetime of a circular binary system; the result is $$ \tau = \frac{5}{256} \frac{c^5 a_0^4}{G^3 m_1 m_2 (m_1 + m_2)} \approx 1.07 \times 10^{23} \text{ years}, $$ which is several billion times longer than the current age of the universe. I would say this is not a terribly pressing concern.

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  • $\begingroup$ +1 for "not a terribly pressing concern". I would also assume the Doppler shift and electromagnetic radiation pressure present a much stronger damping mechanism than gravitational waves, although I cannot compute an estimate how much. $\endgroup$ – dominecf Oct 14 at 19:41
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Presuming general relativity is correct, yes the Earth orbiting emits gravity waves. The intensity of this emission is quite low. The intensity of the gravity field the Earth creates is small. The speed the Earth moves around the Sun is small. So the rate of energy being carried away is very small. Thus the stability time of the Earth's orbit (w.r.t. this energy loss) is quite long. Other effects would almost certainly be larger. Tidal drag of the Sun on the Earth, for example. Probably little tugs from the gravity of other planets would be larger than the gravity wave effect. Probably the Earth's orbit will be stable enough for other dire things to happen first, such as the sun getting along in its life span, and drastically changing character. (I'm weaseling on that because I don't recall what is the expected fate of the Sun.)

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  • $\begingroup$ expected fate of the Sun: Turns to a red giant in approximately 5 billion years from now. Opinions are divided as to whether its radius will be greater than or less than the radius of the Earth's orbit. If less, it won't be much less. You will be able to roast marshmallows to a perfect golden brown merely by exposing them to a view the daytime sky for a few seconds. $\endgroup$ – Solomon Slow Jun 21 '18 at 19:40
  • $\begingroup$ "Quite low", "quite long", "almost certainly", "probably" (2x). Be bold! $\endgroup$ – my2cts Oct 14 at 22:27

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