Final answer:
Gravity was a significant source of Earth's heat in its early history through gravitational contraction, a concept suggesting that as the Sun's outer layers fell inward due to gravity, heat was generated. Today, gravity's contribution to Earth's heat is minimal but it remains crucial for maintaining Earth's shape and trajectory, as well as terrestrial heat contributions from celestial impacts.
Step-by-step explanation:
In the early days of the Earth, gravity was a significant source of heat through a process known as gravitational contraction. This idea, proposed by British physicist Lord Kelvin and German scientist Hermann von Helmholtz in the nineteenth century, described how the Sun might generate energy by converting gravitational energy into heat as its outer layers "fell" inward due to gravity. Essentially, they suggested that the Sun could be shrinking, staying hot and luminous as a result. However, this source of heat could only provide energy for up to 100 million years, much shorter than the actual age of the Earth, which is about 4.5 billion years. Therefore, gravitational contraction could not be the ongoing primary source of the Sun's energy.
Today, the amount of heat produced by gravitational contraction on Earth is negligible compared to the heat generated from the planet's interior, largely due to the decay of radioactive isotopes. While gravity contributed significantly to Earth's heat in the past, particularly during its formation when kinetic energy from accretion was converted to thermal energy, its role today is minimal in heat production but critical in maintaining Earth's shape and orbit, as well as the trajectory and accumulation of celestial objects impacting Earth, which can sometimes contribute to heat through kinetic impact.