What is the coldest planet? Hottest?
The hottest planet in our Solar System is Venus with a temperature of around 462 °C (863 °F). This is because of Venus's thick atmosphere of Carbon Dioxide which causes a runaway greenhouse effect. This seemingly neverending process traps heat from the Sun and so the heat doesn’t escape back into space. The temperature of the atmosphere is measured using satellites such as the Venus Express orbiter learn more about it here. The Surface Temperature was confirmed through the Soviet Venera 13 program!
Here's what Venus looks like you can see the thick clouds of carbon dioxide and sulphuric acid .
The coldest planet is Uranus, which overthrew the previous title holder Pluto. Now you would think that Neptune would have the coldest temperature as it's 1 billion miles farther from the Sun than Neputune. However, scientists have discovered that Uranus is colder with an average temperature of -224°C. They think that this is because Uranus's core is quite cold at around 4000°C whereas Neptune's core is 7000°C this mean's that Uranus's core heats the atmosphere much less than Neptune's this creates the colder temperatures that we observe.
The atmosphere of Uranus is a Duck egg blue. It consists of hydrogen and helium but has ices as you go deeper. No not water ice but, ice made out ammonia, methane and water .
Source: My own Knowledge
How do you measure the temperature of a planet?
To measure the temperature of a planet astronomers usually remotely like an infrared sensor. Although, sometimes through physical exposure by using something like an electronic thermometer which may be on a lander.
Physically measuring the temperature of a planet is challenging, as most of the planets are gas/ice giants so they have no surfaces. Therefore most astronomer find the temperatures of the planets by observing the infrared radiation that they emit. They radiation corresponds to a unit of energy that can be interpreted into a unit of temperature .
The main way scientists measure the temperature of an astronomical body is by the amount of thermal energy (heat) the object radiates. The laws of the universe tell us that all objects greater than Absolute Zero radiate a certain amount of energy.
It is harder to measure a planet's temperature than say a star's, this is because a planet's temperature is affected by many more variables.
A planet's atmosphere and surface composition will reflect thermal radiation and some of the absorbed thermal energy is retained due to the greenhouse effect.
Therefore, astronomers estimate the temperature of a distant planet through complex calculations that account for such variables as the temperature of the nearest star, the planet's distance from the star, the percentage of light that is reflected, the composition of the atmosphere and the planet's rotational habits.
Most of the information here is from my head But the variables are from this link https://sciencing.com/can-astronomers-tell-distant-objects-temperature-is-9560.html.
I recommend you check it out.