Mercury lost its crown, and isn’t our Solar System’s hottest planet

In terms of power, the only body of note in the Solar System is the Sun. Planets, in general, reflect, absorb, and re-radiate energy emitted by their parent stars, and the Sun is the only parent star any of the planets have here in our Solar System. To an excellent approximation, the Sun keeps not only Earth, but all the planets at a temperature well above what they’d be without it, which is just a few Kelvin. (Without an external heat source, most planetary temperatures would equilibrate at the same temperature as the cosmic microwave background, or -270 °C / -455 °F.) Therefore, you’d think that proximity to the Sun, or the planetary order of our Solar System, would be the overwhelming factor in determining a planet’s temperature.

Indeed, this works incredibly well for nearly all of the planets in our Solar System. Neptune, the most distant planet from the Sun, is also the coldest planet. Moving inwards:

• Uranus is hotter than Neptune,
• Saturn is hotter than Uranus,
• Jupiter is hotter than Saturn,
• Mars is hotter than Jupiter,
• Earth is hotter than Mars,
• and Venus is hotter than Earth.

Finally, continuing our inward journey, we make it to the innermost planet of all, Mercury. There, we find that its daytime temperature — when its immersed in full sunlight — reaches a whopping 800 °F (427 °C) at maximum. Then, at night, when it faces away from the Sun, the temperature plummets, dropping all the way down to lows of -180 °C (-290 °F). But even at its absolute hottest, Mercury never reaches the temperatures achieved on Venus, which remain steady, day-and-night, at between 440–480 °C (820–900 °F): always hotter than Mercury at its absolute hottest. Here’s the science of how.

It makes sense that the closer you are to a star — or any source of light — the greater the amount of its emitted light will hit you. If two planets were in orbit and one were twice as far away from its parent star as the other, the farther one would only receive one-quarter of the sunlight-per-unit-area as the innermore one would. If a planet were three times as distant, it would receive just one-ninth the sunlight-per-area that the innermore one received.

However, a planet’s various orbital parameters cannot be the only factor that determines how hot they are. If orbital distance were the only parameter that determined a planet’s temperature, then the closest planet to the Sun would inevitably be the hottest, and they would all get progressively cooler as we moved farther and farther away.

From a physical point of view, we can reason as to what those additional properties might be. One important one is a planet’s reflectivity, which scientists call its albedo. The smaller a planet’s albedo is, the better it is at absorbing all of the emitted light (across all wavelengths of the electromagnetic spectrum) that would be incident upon it, while the higher a planet’s albedo is, the better it is at reflecting light. As you can see, by examining the below view of Mercury, it has “dark areas” and “light areas” that have lower and higher albedos, respectively.

As it turns out, Mercury is a particularly low-albedo planet. In fact out of all eight of the major planets in our Solar System, it has the lowest albedo (or lowest reflectivity) of them all, meaning that it’s a better absorber of sunlight than any of the other planets are. There are two different ways, in general, that planetary scientists measure a planet’s reflectivity:

• geometric albedo, which measures what percentage of light, if the light source were coming from directly behind you as you looked at a world, would return to your eye and be perceived as part of the world’s brightness,
• and Bond albedo, which measures the total proportion of electromagnetic energy that gets reflected by the world in question.

On both of these metrics, Mercury is the most absorptive, least reflective planet in the Solar System. Its geometric albedo is only 0.142, meaning that only 14.2% of the light that shines on it contributes to its observed brightness, and its Bond albedo is even lower: ranging from 0.068 to 0.088, meaning that only between 6.8% and 8.8% of the Sun’s incident energy gets reflected by Mercury.

As a result of its proximity to the Sun and its effectiveness as an excellent absorber, Mercury is indeed very hot. Quantitatively, planet Mercury, which completes an orbit around the Sun every 88 Earth-days, achieves a maximum temperature during the day of a whopping 700 Kelvin (427 °C / 800 °F) at its hottest, equatorial locations. Because Mercury rotates very slowly, its night side spends a consecutively long time in the dark, shielded from the Sun; during those times, it gets down to just 100 Kelvin (−173 °C / −280 °F). That low temperature is incredibly cold, and is even far colder than any known naturally occurring temperatures here on Earth.

So if that’s the story of the closest planet to the Sun, Mercury, then what about the next one out: Venus?

Comparatively, Venus is a very different world from Mercury on several different metrics. Venus is about twice as far from the Sun, on average, as Mercury is, and as a result, takes approximately 225 Earth-days to orbit the Sun. Whereas Mercury’s average distance from the Sun is 57.9 million km, Venus’s is 108.2 million km. As a result, Venus receives just 29% of the Sun’s energy-per-unit-area as Mercury does. Venus is also a slowly-rotating planet, and in fact rotates even slower than Mercury, spending more than 100 consecutive Earth-days at a time bathed in sunlight and then an equal amount of time in darkness.

In terms of reflectivity, Venus is on the opposite extreme end of the spectrum: it’s the absolute most reflective planet in the Solar System! In term of both its geometric albedo and its Bond albedo, no other planet comes close to being as reflective as Venus is.

• Venus’s geometric albedo is 0.689, meaning that 68.9% of the illuminated light that shines on Venus gets returned to the observer who views it.
• Venus’s Bond albedo ranges between 0.76 and 0.77, meaning that 76-77% of the energy that is incident on Venus gets reflected by the planet.

You would think — because it receives far less energy from the Sun per unit area than Mercury, and because it’s far more reflective than Mercury — that Venus would indeed be much colder than Mercury is. And yet, when we measure the temperature of Venus, we find a surprise: Venus is the same temperature at all times, day or night, at an incredible average of 735 Kelvin (462 °C / 863 °F), making it even hotter than Mercury!

This strange occurrence did more than just puzzle astronomers when they first discovered it, it actually mortified them! Venus wasn’t large enough to generate its own heat, and yet it was hotter at Venusian midnight than at Mercurian high noon. It received less than 30% of the sunlight than Mercury did while being far less absorptive, and yet, always exceeded even the maximum temperature that Mercury ever achieved. How was such a think even possible?

This was an observation that cried out for an explanation, and so scientists began contrasting the two innermost planets. Comparing these two worlds, there are four very stark differences that stand out:

1. Mercury is much smaller than Venus,
2. Mercury is about twice as close to the Sun as Venus,
3. Mercury is much less reflective than Venus, and
4. Mercury has no atmosphere, while Venus has a very thick atmosphere.

As far as absorbing and radiating heat goes, it turns out that size doesn’t matter very much. Planets absorb sunlight based on their cross-sectional surface area — proportional to their radius squared — and radiate it away in the exact same proportion. If Mercury were double its size or Venus were half of its size, neither one would have its temperature change by any appreciable amount. This difference is completely irrelevant.

The proximity to the Sun should favor Mercury, and by a substantial amount: Mercury receives nearly four times the energy-per-unit-area that Venus receives. In terms of reflectivity, Mercury absorbs around 90% of the Sun’s energy that strikes it, whereas Venus absorbs less than a third of the Sun’s incident energy.

Therefore, none of those first three differences could possibly account for Mercury losing its crown to Venus as the Solar System’s hottest planet.

Instead, something very important must be going on with that final major difference: the fact that Mercury has no atmosphere at all, as far as we can tell, whereas Venus definitely possesses one. This is not some new discovery that was only made with the dawn of the Space Age, but rather goes all the way back to our ability to measure planetary transits: when the innermore planets of Mercury and Venus appear to have their disks move across the face of the Sun by passing directly in between the Sun and Earth.

The first observed transit of Venus goes all the way back to the year 1639, with the first observed transit of Mercury occurring even earlier: in 1631. As soon as Kepler worked out his laws of planetary motion, the prediction of transits became possible, and in fact it was Johannes Kepler himself who made these predictions. While Mercury makes very frequent transits, Venus only makes two closely-spaced transits more than a century apart. Kepler successfully predicted that the next transit of Venus, subsequent to the 1639 one, would occur in 1761.

While observing that very transit, Mikhail Lomonosov indeed observed an “arc” of light outside of the disk of Venus: confirming that Venus must, indeed, have an atmosphere. In all the transits, before and since, no one has ever seen such an arc around the planet Mercury.

Every planet, including Mercury and Venus, don’t just absorb light from the Sun, although that’s certainly part of the planetary story of their energy balance and temperature. In order to achieve equilibrium, each planet must then re-radiate that energy back into space: typically in the form of heat.

For a practically airless, atmosphere-free world like Mercury, all of the absorbed heat get re-radiated immediately back into space. Once the heat gets emitted by the surface, none of it becomes trapped, and instead all of it just free-streams out back into the Universe, preventing the day side from becoming hotter than the maximum determined by incident sunlight and absorption, and enabling the night side — because it spends so much time, continuously, not receiving any sunlight — to cool down to such extremely low temperatures.

However, the story is vastly different on Venus. Each quantum of infrared radiation — the re-radiated heat — has got to get through Venus’s atmosphere before it can be re-released into space. Venus, in stark contrast to Mercury, doesn’t merely have a thick atmosphere, but the thickest atmosphere of any solid-bodied world in the Solar System. Only the four gas giants have a thicker atmosphere than Venus does.

It may be useful to think about Venus in analogy to Earth. Here on Earth, we have a planet with an atmosphere: one that’s thin, but substantial. Over the last several decades, we’ve learned in gory detail — and some of us are learning the hard way — that Earth’s atmosphere plays a very intricate role in regulating the temperature of our planet. If Earth would have had no atmosphere at all, then based on our distance from the Sun and our planetary albedo, the average temperature across Earth’s surface would have been much cooler than it actually is: right around a paltry 255 Kelvin (-18 °C / -1 °F), or approximately the temperature of the Antarctic continent.

Yet the atmosphere we have raises our average temperature significantly: by about 33 °C (59 °F), transforming us from what otherwise would have been a frozen world into one that’s temperate, with liquid water on its surface, and hospitable to an enormous variety of life forms across the globe. It’s only due to the blanket-like effect of the clouds and atmospheric gases that we have that lift our planet’s climate into the zone where life-as-we-know it has thrived for so long. In the right amounts, the heat-trapping effects of an atmosphere can be the best thing to ever happen to a world.

We’re most familiar with our own planet’s atmosphere, and for a long time, we assumed that Venus — which is of similar size to Earth — might have a similar atmosphere to Earth’s. In some ways, it is similar. Earth’s atmosphere is dominated by nitrogen, at 78% of our present atmosphere, and Venus’s atmosphere is similarly nitrogen-rich. If you were to add up all of the nitrogen in Venus’s atmosphere, you’d find that, by mass, it was a little bit over three times the amount of nitrogen in Earth’s atmosphere: thicker and denser, but not absurdly so.

Only, nitrogen isn’t the dominant component of Venus’s atmoshere; carbon dioxide is. In fact, 96.5% of Venus’s atmsophere is composed of carbon dioxide, most of which was released by volcanic activity over Venus’s history. All told, Venus’s atmosphere is approximately 92 times as thick as Earth’s is, and many of its gases, including the dominant carbon dioxide, are relatively efficient absorbers of re-radiated infrared heat.

And that’s not the full story. Not only does Venus possess an atmosphere many times the thickness of Earth’s, loaded with huge amounts of those infrared-absorbing gases like carbon dioxide, but Venus is shrouded in several thick layers of highly reflective clouds. These hazes, composed largely of sulfuric acid, typically extend for more than 20 km in thickness, and encircle the planet Venus at speeds from 210 to 370 km/hr, trapping the vast majority of the radiated heat and efficiently transporting it all across the planet.

As a result, the long nights on Venus provide no escape from the heat, as the trapping and thermalizing effects of the cloud layers keep the surface of Venus at an inhospitably high temperature. Things are so severe that if you examined the operational time of every lander that’s ever touched down on Venus’ surface, the longest one barely remained operational for even 2 hours. It’s no wonder: temperatures on the Venusian surface are hot enough to melt lead!

The same blanket-like effect of the clouds and atmospheric gases that lift our planet’s climate into the temperate zone, where life-as-we-know it has thrived for so long, have transformed Venus into the hottest world in the Solar System. Early on in the Solar System’s history, with a cooler Sun and back before the accumulation of so many volcanic gases, Venus likely possessed a much thinner atmosphere. Back in the Solar System’s infancy, Venus was probably similar in temperature to Earth’s today, and Mercury was indeed, for a time, the Solar System’s hottest planet.

However, a runaway catastrophe — driven by the catastrophic addition of greenhouse gases to its atmosphere — created the permanent inferno that’s existed on our sister world for billions of years. While Earth isn’t at risk of the same fate, Venus stands as both the hottest world in our Solar System and a cautionary tale of what an out-of-control greenhouse effect can do. Greenhouse gases, at these extremes, have even caused Venus to usurp Mercury’s one-time crown as the hottest planet in the Solar System. Now, at last, we finally understand why.

This article was first published in March of 2022. It was updated in October of 2026.

This article is featured on Big Think.