Why are all planets cold?

Answer 1

Not all planets are. Warmth depends entirely on the atmosphere and how close it is to the star. for example, Mercury's daytime and nighttime are about 600 degrees Celsius apart, being about 300 C and -300 C respectively, and Venus has so much carbon dioxide, that the surface is three times as hot as on mercury, which means venus's surface would feel like a mushy candy bar left in the sun. however, Earth, whithout an atmosphere, would have an average temp of -18 degrees AT THE EQUATOR. this is why greenhouse gasses are important, because that is why we have nice temperatures like an average of 54 degrees Fahrenheit planetwide. Past us, most of the rocky planets ( and dwarf planets) have little to no atmospheres, so they have temperatures very close to absoulte zero. outside our solar system, a very common sight is a 'hot Jupiter' meaning a Jovian planet of around Jupiter's mass that orbits it's sun in less than an Earth Day. being this close has many consequences, one of which being if they are low-density they have a very good chance of being blown apart by solar winds. one such planet has an average daytime temperature of about 2400 degrees C, about 1000 degrees less than it's own parent star.

Answer 2

Planets receive heat from the sun, but their temperatures vary based on factors like distance from the sun, atmosphere, and internal heat sources. Some planets, like Mercury, are extremely hot due to their proximity to the sun, while others, like Neptune, are cold due to their distance from the sun and lack of significant internal heat sources.