Is space between the planets hot or cold? Two rivals were both right
Joseph Fourier guessed that the space between planets was about as cold as Earth's poles. In 1959 Sydney Chapman argued it was blisteringly hot, around 10,000 degrees. The gas out there is so thin that it never settles to a single temperature, so each was right: dust near Earth sits close to room temperature while solar wind particles run scorching.
For centuries people imagined the gap between worlds as empty. Robert Boyle, apparently the first to print the word interplanetary in 1691, contrasted air with the ether or vacuum beyond. As late as 1898 the astronomer Charles Young called it a vacuum more perfect than anything humans could make. The picture changed in the early 1950s, when Ludwig Biermann, studying comet tails, proposed that the Sun constantly blows its atmosphere outward at supersonic speed.
That outflow, the solar wind, is a plasma of charged particles mixed with dust, gas and cosmic rays. Near Earth there are about 5 particles per cubic centimetre, occasionally up to 100, while sea-level air holds some 29 billion billion. The wind thins with the square of distance from the Sun. Dust near Earth's orbit averages about 283 kelvin, a mild 10 degrees Celsius, falling to around 165 kelvin in the outer asteroid belt.
Thin as it is, the plasma behaves in striking ways. It conducts electricity and drags the Sun's magnetic field outward, so the field at Earth's distance is roughly 100 times stronger than a simple vacuum calculation predicts, about 7 nanotesla. Worlds without magnetic fields, like the Moon, take the wind directly on their surfaces; lunar soil has been collecting solar particles for billions of years, which makes Moon rocks a record of the Sun's output. Earth and Jupiter deflect the flow with their magnetospheres, although some leaks in to feed aurorae and the Van Allen radiation belts.
You can see this medium on a dark night. Zodiacal light, a faint cone along the path of the planets after sunset or before dawn, is sunlight scattered by dust between Earth and the Sun. Gegenschein, fainter still, glows opposite the Sun where dust beyond our orbit reflects light back. The whole region ends at the heliopause, which the Voyager probes crossed around 120 astronomical units out.
Source: Interplanetary medium
