The edge of the Solar System

Astronomers are revealing that there is far more to our cosmic neighbourhood than the planets alone, as Ezzy Pearson discovers

The missions improving the view where the Sun’s influence ends

When most people describe the Solar System, they think only of the eight planets orbiting our Sun. Yet in truth, our neighbourhood goes far beyond that. Outside their orbits lies a vast and invisible expanse of space that astronomers are beginning to shine a light on.

Beyond Neptune lies a ring of icy objects left over from the formation of the Solar System, known as the Kuiper Belt. The region extends between 30 and 50 AU (where 1 AU is the distance between Earth and the Sun). Theorists first proposed its existence after the discovery of Pluto in 1930 led astronomers to wonder if other worlds might be hiding out there. It’s only recently that technology has been able to reveal these icy objects en masse, and now telescopes such as PanSTARRS and the upcoming Vera Rubin Telescope scan the sky every night, looking for the distant specks of Kuiper Belt objects (KBOs) moving across the background stars.

Through the years there have been several targeted surveys taking a deep look at specific areas in the region. One of these looked for a potential target for one of the few spacecraft which has ventured into this mysterious space – New Horizons. The probe flew past Pluto on 14 July 2015, before passing by a second KBO, Arrokoth, on 1 January 2019. “We searched for a new target for four years beginning in 2011, culminating in 2014 with the discovery of Arrokoth by Hubble,” says Alan Stern, the principal investigator of New Horizons. “We found dozens of Kuiper Belt objects, but only two were within our fuel reach. We’re now starting a new search using large ground-based telescopes to find a third flyby target.” As well as getting close-up views of these distant worlds, New Horizons has also been using its Long range Reconnaissance Imager (LORRI) telescope to examine several neighbouring KBOs while travelling through the neighbourhood, giving a broader context to the detailed views from the flybys.

“These objects are too small to see in detail from any telescope on Earth or even the Hubble Space Telescope,” says Stern. “With New Horizons we can determine their light curves, their rotational periods and their shape. We can search for satellites and improve our knowledge of how many KBOs have moons. In addition, we can study the microphysical properties of the surface, such as roughness. That can only be done from New Horizons.”

To the Kuiper Belt and beyond

Throughout its journey through the Kuiper Belt, New Horizons has measured the distribution of dust in the seemingly empty space. Eventually astronomers expect the spacecraft will notice these dust levels falling away, signalling its time in the Belt is done. “The Kuiper Belt only runs a finite distance,” says Stern. “By the late 2020s, around 2027, we will exit the Belt. It’s not the end of the mission though. There’s a lot of demand for New Horizons because we’re far out in the Solar System carrying an amazing scientific payload.”

Researchers are already requesting valuable time on the spacecraft to look at the heliosphere, the bubble of gas generated by the solar wind which surrounds our Solar System, kept in place by the Sun’s magnetic field. Already, New Horizons has discovered pickup ions – particles which originate in interstellar space, but which are now trapped in the heliosphere.