9 things we don’t know about the outer solar system
(summarised from the book “Uranus, neptune, pluto and the outer solar system” by Linda I. Elkins-Tanton.)
1. How does the Uranian atmosphere smooth out the seasonal extremes of solar heating?
When Voyager arrived, one pole of Uranus was pointed at the Sun, but the temperature was almost uniform, slightly hotter at the equator.
2. Why is Uranus’s internal heat source so small when compared to other giant planets?
Jupiter, Saturn and Neptune radiate significantly more heat than they receive from the Sun. Uranus doesn’t.
3. What is the source of energy that heats both Neptune’s and Uranus’s upper atmospheres to such high temperatures?
The temperature of the exosphere (the uppermost layer of the atmosphere) of Uranus is 477 Celsius and of Neptune is 327 Celsius. There is no explanation either from solar heating or from internal heating.
4. Why are the radio emissions (100 to 1000 kHz) from Uranus and Neptune oriented and why do they occur in bursts?
This is not true of natural radio emissions from Earth and from Saturn. Could it be some feature of the aurora of these planets.
5. Why do Uranus and Neptune have so much less hydrogen and helium than Jupiter and Saturn?
Hydrogen makes up only 15% of Uranus and Neptune, but it should have been more available in the early outer solar system.
6. How many large bodies are there orbiting near and beyond Pluto, and how large might they be?
The number of bodies >1000 km in diameter in the Kuiper belt has been variously estimated as large as “thousands” or as small as “ten”.
7. What orbits between the Kuiper Belt and the Oort Cloud?
The Kuiper belt has an abrupt outer edge at 49 AU and the inner edge of the Oort cloud could be as far away as 1000 AU. Sedna has a perihelion of 76 AU and an aphelion of 800 AU, but that’s just one body.
8. What are Kuiper belt and Oort cloud objects made of?
For example, why are Kuiper belt objects red in colour? A wide range of compositions is detected, and unexplained.
9. Can the Kuiper belt and Oort cloud be used to understand dust disks seen around other stars?
The Voyager spacecraft detected dust in the Kuiper Belt.