Before the 1960s, scientists believed that Mercury took 88 days to complete its orbit around the Sun. It was also assumed that Mars always faces the Sun in the same direction.However, using observations collected by the radio telescope housed within the Arecibo Observatory, researchers could invalidate those theories. They discovered that Mercury takes only 59 days to complete its orbit around the Sun.While Mercury isn’t tidally locked, it does turn; and it completes three rotations in every two orbits.The first binary pulsar — a highly magnetized rotating star orbiting another — was first discovered at the Arecibo Observatory by Joseph Taylor and Russell Hulse in 1974.The more exciting detail about this discovery was finding out that the orbit of the two stars was shrinking at a rate of 1 centimetre per day. The shrinkage was attributed to the loss of orbital energy due to gravitational radiation — or gravitational waves — predicted by Einstein’s General Theory of Relativity.This discovery was crucial for testing the theories of gravity and led to both the scientists winning a Nobel Prize in 1993.For a long time, scientists hypothesized that there had to be more planets in the universe than just the ones that existed within the solar system. But, evidence to prove that theory was scarce. That is, until 1992. That’s when the Arecibo Observatory was able to capture the very first exoplanet. In subsequent years, it was able to discover an entire planetary system around pulsar PSR 1257+12. Past attempts at capturing the surface of Venus were made using optical cameras. These observations could not penetrate through the planet’s top layer of thick clouds, leaving what lay under the surface a complete mystery.However, since Arecibo did not rely on optical images but radio waves, it was able to break through the haze and create the first-ever radar maps of Venus’ surface. This discovery led to the prospect that it may even be possible to map the surface of Venus at resolutions down to 2 kilometers. The US National Aeronautics and Space Administration (NASA) subsequently agreed to upgrade the telescope and provide funds for a 420 kilowatts (kW) transmitter for a closer look.Observations from the Arecibo Observatory also led to the detection of the first-millisecond pulsar, PSR B1937+21. It was discovered in 1982 by DC Backer, Shrinivas R. Kulkarni, Carl Heiles, MM Davis, and WM Goss.Spinning at roughly 641 times per second, it remains the second fastest-spinning millisecond pulsar of the approximately 200 discovered after that. NASA may have caught the first-ever photos of ice on Mercury poles, but that only happened in 2014.Observations from the Arecibo Observatory had already detected water on the North and South of the planet closest to the Sun over two decades earlier in 1992.It showed that the ice persists in Mercury's shadowed craters despite the high temperatures of nearly 426 degrees Celsius on the planet’s surface.