About 100 tons of cosmic dust falls on the Earth everyday, and this has been happening for billions of years, making the Earth heavier. Why doesn’t this cause our orbit around the Sun to change?
The orbit of the planets is determined by the total mass in the system, and the dominant mass is that of the Sun, about 2×1030 kg. It contains 99.8% of the mass of the Solar System.
Although 100 tons daily seems like a lot to us, on the scale of solar system bodies it is very small. A quick back of the envelope calculation shows that the change in the mass of the Earth over 4.5 billion years is about 1.6×1017 kg (= 100 ton/day × 365.25 days × 4.5 billion years). It is miniscule compared to the mass of the Earth, which is 5.9×1024 kg (about a part in a hundred millions), and negligible compared to that of the Sun. So we will not see noticeable change in the size of the Earth’s orbit.
Dr. Judit Ries