Surprisingly, radio observations of the planets also turned up unexpected discoveries. While testing their transit radio array using the Crab Nebula as a reference source, two scientists at the Carnegie Institution Department of Terrestrial Magnetism noticed a strange variable signal that repeated each night. First suspecting that it was ignition noise from a nearby farm vehicle, they later realized that they were detecting radio emission from powerful electrical storms on Jupiter, which was at the same declination as the Crab Nebula as it passed through their fixed telescope beam. Mercury, long thought to have one side bathed in eternal daylight, was found to be rotating. Radio observations revealed the greenhouse effect on Venus, causing surface temperatures to reach over 600 degrees Celsius, and detected intense radiation belts around Jupiter, analogous to the Earth’s van Allen belts. The other giant planets were all found to be warmer than can be explained by solar heating alone. Precise pulsar timing measurements disclosed the first known extrasolar planetary system, a precursor to the thousands of extrasolar planets later discovered by ground and space based optical studies.