For 72 years Pluto was the only solar system object known that orbited the Sun beyond Neptune. Then Albion was found, and soon thereafter many more; now thousands of such trans-Neptunian objects (or TNOs) are known.

In 2005, my friend Mike Brown and his team announced the discovery of one called Makemake (MAH-kay-MAH-kay, from a Rapa Nui god of creation and fertility). It’s over 1,400 kilometers in diameter, smaller than Pluto, and orbits the Sun on a moderately elliptical path that takes it from 5.7 to 7.9 billion km from the Sun — Neptune is about 4.5 billion km out, for comparison.

That means Makemake is cold, so far from the Sun’s furious heat. Its average temperature is roughly 40K, which is about -230°C or -390°F. That’s cold enough to freeze out nitrogen and oxygen, so Earth’s atmosphere would be solid ice on Makemake. Yikes.

Which is to say that if you saw a hotspot on this tiny, frozen lump out in the deep black you’d be damned surprised. Which is exactly what the team of astronomers were when they observed Makemake using JWST [link to journal paper]. It seems that most of the object is in fact quite cold, but something on or near it is much warmer, about 147K (-126°C or -200°F). That’s still pretty dang cold, but a blowtorch compared to the rest of the object. On Earth, that would be like seeing boiling water on the ground in the middle of winter.

OK, so what’s going on?

First, some basic physics. A long time ago, scientists realized that all objects above absolute 0 in temperature emit light. That light has a range of wavelengths (think of them as colors), peaking in brightness at one specific wavelength and dropping down at longer and shorter wavelengths. For historical reasons (that is to say, it’s a holdover from the way those early scientists figured it out) this is called a blackbody curve. There’s a complicated mathematical function that describes it (go here if you’re curious, though the explanation is terse; other sites go into it more deeply).

Bottom line is that at a given temperature the amount of light emitted peaks at a wavelength that depends on that temperature, and the hotter an object the shorter the wavelength is. You are an object at about 37°C, and your peak wavelength is at about 7 microns, in the infrared. An object at 3,600° peaks in the far red, and would be just visible to the eye. The Sun is 5,500°C and it peaks in green light at about 0.5 microns (though it emits at other colors, too, which blend to make it look white). Hotter objects might peak in the ultraviolet, or even X-rays. 

Colder objects have a longer wavelength peak. At Makemake’s overall temperature, you expect it to peak at about 100 microns (what we call the submillimeter, since a millimeter is 1,000 microns). Astronomers have observed Makemake at wavelengths around there, and found its brightness graphed versus wavelength fits that blackbody curve pretty well. At least, it does near the peak.

But then the astronomers I mentioned above took a gander with JWST, which looks in the infrared. At 18 and 25 microns you’d expect it to be very faint. Instead, they found it to be much brighter, between 4 and more than 10 times brighter! How can that be?

Well, they come up with two scenarios. One is a hotspot on its surface. Most of Makemake is terribly cold, but if there’s a spot on it that’s at roughly 147K, the observations match what you’d predict (by adding together the blackbody curves of an object at 40K plus another at 147K).

[I’ll note that Makemake has a small moon, which the astronomers included in their calculations; it cannot explain the warmer observations so there must be more going on.] 

Why would it have a hotspot?

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