October 20, 2021

I, Science

The science magazine of Imperial College

Having started life as a hypothetical planet, it was only recently that Neptune’s precise path through space was confirmed, as it completed its first full orbit since its discovery in 1846. To celebrate this occasion, NASA released some stunning anniversary pictures, taken by the Hubble Space Telescope.

At the giant planet’s great distance of 30 times further away than Earth, one Neptune year (which corresponds to one full orbit around the Sun) takes approximately 165 years. To put this in perspective, one season on the surface of the blue-green planet lasts for roughly 40 years!

Image caption: A complete view of Neptune. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

These four images were taken of Neptune across its 16-hour day at roughly 4-hour intervals, therefore giving a complete view of the planet. The characteristics of the planet are almost completely due to its abundance of methane: The distinctive aqua blue colour is given by methane absorption of red light, and the picture above shows clouds of methane ice near the planetary poles.

The discovery of Neptune was a great astronomical achievement, at the time nearly doubling the size of the known solar system. It’s often used as an important case study in the historical and philosophical study of science, and has influenced widely-respected scientific theories such as Newton’s theory of gravitation. Its neighbouring planet, Uranus, was discovered in 1781 by astronomer William Herschel. He began to track its orbit across the sky and, over a number of years, found that there were serious differences between the observed data and what Newton’s theory predicted. Due to the large distances and huge lengths of time involved, it took many years before French astronomer Bouvard predicted that there must be another planet influencing the orbit to explain its highly unusual behaviour.

However, it took a further twenty years for Le Verrier and Couch Adams to each independently predict the positioning of the still-unnamed Neptune on the sky. This was later verified by German astronomer Galle at the Berlin Observatory, who discovered Neptune less than 1 degree away from Le Verrier’s predicted positioning on the sky in 1846. Interestingly, Galileo Galilei narrowly missed out on the credit for Neptune’s discovery – in 1612, he recorded his observation of a new, star-like object – Neptune. A month or so later he noticed that this object had moved relative to the other stars, but failed to classify it as a planet.

Image caption: This image is a composite of three colour images taken by Hubble, showing bright clouds in Neptune’s atmosphere. A composite of 48 further images were used to reveal many of Neptune’s 30 orbiting moons. Image credit: NASA, ESA, and Z. Levay (STScI)

The method behind Neptune’s discovery has enabled present-day exoplanet searches to become increasingly accurate – the tugging of an orbiting planet on other planets and the host star (or ‘sun’) itself is a very commonly-used way of identifying whether or not the star is orbited by one or more planets. It’s thought that planets of approximately Neptune’s mass are very common throughout our Milky Way, and hundreds of them have now been confirmed by NASA’s Kepler mission, launched in 2009.