Final answer:
Sir Isaac Newton was the first to propose the gravitational force that holds planets in their orbits. However, the concept of a force causing both our weight and planetary motions was initially suggested by Galileo Galilei. Newton was the first to propose a precise mathematical form for the motion of celestial bodies.
Step-by-step explanation:
The scientist who detected the need for a force that holds planets in their orbits was Sir Isaac Newton. However, he was not the first to see that the same force was responsible for our weight and the motion of planets. That idea traces back to Galileo Galilei. Furthermore, some of Newton's contemporaries, including Robert Hooke, Christopher Wren, and Edmund Halley, made progress toward understanding gravity. Yet, it was Newton who proposed an exact mathematical form indicating that the motion of celestial bodies should follow conic sections (circles, ellipses, parabolas, and hyperbolas). This was a notable achievement, as it was known for some time that moons, planets, and comets adhered to these courses, but no one had proposed a mechanism that led them to track these paths and not others.
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