Hottest Planet In The Solar System

What is the hottest planet in the Solar System?

Of all the planets in the Solar System which one is the hottest? The answer may not be what you expect. Venus is the hottest planet in the Solar System, not Mercury! One would probably assume that the closer a planet is to the Sun, the hotter it will be. There is some truth in that; planets closer to the Sun receive a proportionally higher amount of electromagnetic radiation from the Sun so these closer planets will have a greater potential for higher temperatures.

But it must not be overlooked that there are other factors involved which determines the temperature of a planet besides just its proximity to the Sun. The atmosphere of a planet, or the lack thereof, is probably the greatest factor, along with the magnetic field and gravitational profile of a planet – the magnetic field is a factor in trapping solar wind radiation from the Sun, and the gravitational force of a planet can determine whether it has any atmosphere to speak of at all.

Another factor is the rotation of a planet in relation to its orbit around the Sun – a tidally locked planet to the Sun with no atmosphere will have different temperature extremes on the daylight and nightside of the planet. On the other hand, if a planet has a thick atmosphere and is tidally locked to the Sun, there will be temperature extremes but they will be less intense and the temperature will be distributed more evenly.

So it is quite apparent that a number of different factors need to be taken into consideration to determine how hot or cold a given planet will be. As for the hottest planet in our Solar System, there are just two contenders for this title – Mercury and Venus, and Venus wins. Why Venus is the hottest planet in the Solar System is what we will be exploring in greater detail in the next sections.

How Hot Is Mercury

Is Mercury the hottest planet?

Mercury is the closest planet to the Sun making a complete revolution approximately every 88 days – its distance varies from the closest approach of about 28.5 million miles to the farthest of around 43.5 million miles. Since Mercury is significantly closer to the Sun than Venus, one would think that Mercury would also be the hottest of the two planets.

But remember, as we discussed previously, there are a number of other factors involved in determining the temperature than just how close a planet is to the Sun. And of all the factors mentioned, I think especially in this case the planet’s atmosphere is by far the most important.

Mercury really has no significant atmosphere, only a very tenuous exosphere near its surface. Without any atmosphere to speak of it can not trap and hold the heat from the Sun as well; a very large proportion of it is radiated back into space.

Mercury is tidally locked to the Sun in a 3 to 2 spin-orbit resonance – this simply means that for every 2 revolutions that it makes around the Sun, Mercury will make 3 complete rotations. It makes a sidereal rotation about every 58 days, but the actual length of the Mercurian day, from sunrise to sunset to sunrise again, is about 176 days due to the changing angle of incident light from the Sun as Mercury changes its relative position to the Sun as it revolves around it. That means the length of one night on Mercury is equal to one Mercurian year of about 88 Earth days, and the length of one day on Mercury is also equal in length to one Mercurian year!

The effect that this, along with almost no atmosphere, has on its temperatures is to cause extremely hot temperatures on the daylight side and extremely cold temperatures on the night side of the planet. These temperatures range from a minimum of -280 degrees Fahrenheit at night to a maximum of about 800 degrees during the day along the equator, while at the polar regions of Mercury temperatures are consistently below -130 degrees Fahrenheit.

Mercury is the smallest official planet in the Solar System – Pluto is now classified as a dwarf planet – and has a volume of about 2.76 times that of the Moon and a mass of about 3 times as much. The diameter of Mercury is approximately that of the length of the United States from coast to coast, not counting Hawaii or Alaska of course. The surface gravity on Mercury is about .38g, a little over one-third that of the Earth.

How Hot Is Venus?

Venus is the hottest planet in the Solar System

Venus is the second planet from the Sun making a complete revolution approximately every 225 days and orbits the Sun in a very nearly circular orbit with a close approach of about 66.78 million miles and the farthest distance of around 67.69 million miles. It is very close in size to the Earth, both in volume and mass, with a surface gravity of about .904g, slightly over 90 percent that of the Earth. In fact, it is so close in size to the Earth it has often been called Earth’s sister planet.

But that is largely where the similarity of Venus to Earth ends. The Venusian day is an extremely long one – about 243 days since the same side is closely locked in orientation with the Sun, which is the longest rotation period of any planet in the Solar System. Venus has an extremely thick atmosphere, much thicker than Earths atmosphere – the Venusian atmosphere is made up of about 96.5 percent carbon dioxide. It is so thick that it perpetually covers Venus with a thick layer of clouds, forever hiding its surface from any observations from Earth or space.

The Soviets landed successful probes on Venus with the Venera space probe series – in 1975 Venera 9 and 10 landed on Venus and transmitted the first images from its surface, which were in black and white. In 1982 Venera 13 and 14 landed on Venus and transmitted the first color images from its surface. One very interesting thing the Soviets found from these images is that, despite receiving about twice the amount of sunlight as Earth, only about 5 percent of it reached the Venusian surface. This illumination on the surface of Venus appeared very similar to a gloomy cloudy day on Earth.

Venus is much hotter than Mercury with a maximum temperature of about 932 degrees Fahrenheit on the day side and a minimum temperature of around 842 degrees on the night side – this relatively small temperature variance is caused by Venus’s very thick carbon dioxide-rich atmosphere. The average Venusian temperature is around 860 degrees Fahrenheit.

Venus Is The Hottest Planet In The Solar System

Venera space probe on the surface of Venus

We have now determined that Venus is, in fact, the hottest planet in the Solar System with an average temperature of around 860 degrees Fahrenheit and a range from 932 degrees Fahrenheit for the maximum temperature on the day side to a minimum temperature of 842 degrees Fahrenheit on the night side. This is significantly more than the maximum temperature of 800 degrees Fahrenheit on the surface of Mercury. Why is this the case since Mercury is quite a bit closer to the Sun than Venus? As we shall discuss, and you probably know by now, the answer is quite simple.

Mercury is almost twice as close to the Sun as Venus so it receives approximately 4 times the electromagnetic energy from the Sun(this varies proportionally to the square) per unit area as Venus does. Mercury, with no significant atmosphere, is also much less reflective than Venus; that is Venus reflects a much greater amount of electromagnetic radiation from the Sun than Mercury does.

The answer as to why Venus is hotter lies in its atmosphere – Venus has an extremely thick atmosphere mostly of carbon dioxide(about 96.5 percent) whereas Mercury has only an extremely thin exosphere near its surface – its atmosphere is practically nonexistent. This huge amount of carbon dioxide in the atmosphere of Venus traps the heat, causing what is known as a runaway greenhouse effect – a somewhat lesser effect of this type is feared on Earth as carbon dioxide levels gradually rise which would cause sea levels to rise, flooding many coastal cities. So that’s the answer – Venus is the hottest planet in the Solar System and its extremely thick carbon dioxide-rich atmosphere is the reason why.

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