Want to stay on top of all the space news? Follow @universetoday on Twitter
As the distance from the Sun increases, the temperature drops. Many scientists agree that the gravitational influence of the Sun ends about 2 light years from our star. So the answer to ‘what is the coldest place in the Solar System’ would be interstellar space that is 2 light years out. The temperature there is estimated to be 10-20 degrees above absolute zero(10-20 K). On the Celsius scale that would be between a negative 263.15 and negative 253.15. Pretty chilly either way you gauge it.
But within the Solar System itself, surprisingly, the Moon holds the coldest spot, as in 2009 the Lunar Reconnaissance Orbiter measured -240 °Celsius, about 30 °C above absolute zero. This is about 10 °C cooler than Pluto’s dayside, which was measured at -230 °C in 2006. The high rims of the the craters block the Sun from ever reaching the bottoms of the craters, and in this permanent darkness, they stay at a constant frigid temperature.
Back out in the far reaches of the Solar System, the dwarf planet Eris. It is located in the scattered disc of the Kuiper Belt. The average surface temperature there is 42.5 Kelvin (-230.65 C). Neptune’s moon Triton could makes its case as well. The average temperature there is a chilly 38.15 K. The ‘planetoids’ within the Oort Cloud may be slightly cooler at 42 K, but no one is sure. Even closer to the Sun is the dwarf planet Pluto where the average surface temperature is in the neighborhood of 44 K. To put these temperatures in perspective, the average temperature here on Earth is 287.2 K.
Other than interstellar space, the Oort Cloud, a hypothetical ring of icy bodies at the edge of the Solar System, is thought to be the coldest place in our Solar System. It is a vastly unknown or misunderstood entity. In 1950, Jan Oort, a Dutch astronomer, proposed that certain comets have their origins in a vast, extremely distant, spherical shell of icy bodies. This giant conglomeration of objects was soon named the Oort Cloud. It is believed to lie at a distance between 5,000 and 100,000 AU from the Sun. The outer limit of the Oort Cloud is considered to be the end of our solar system. It may contain as many as 2 trillion icy bodies. Occasionally, something perturbs one of these objects. The disruption could be giant molecular clouds, passing a star, or tidal interactions with the Milky Way’s disc. When this happens the object will enter the inner solar system where it is known as a long-period comet. These comets have very large, eccentric orbits and have been observed in the inner solar system only once, each.
Interstellar space is the current answer to ‘what is the coldest place in the Solar System’. As with so many things in astronomy, as technology improves, we may find even colder places or objects.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.