Have you ever wondered where the edge of the Solar System is, and what’s out there? New and recent missions are revealing there’s so much more to discover in the outer reaches of our cosmic neighbourhood.
When most people describe the Solar System, they think only of the 8 planets orbiting our Sun and dwarf planet Pluto. 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 Clyde Tombaugh’s discovery of Pluto in 1930 led astronomers to wonder whether 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.”
What has New Horizons discovered beyond Pluto?
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 that originate in interstellar space, but which are now trapped in the heliosphere.
New Horizons isn’t the only spacecraft looking out for particles from the edge of our Solar System.
IBEX at the edge of the heliosphere
Since 2008, the Earth-orbiting Interstellar Boundary Explorer (IBEX) has been imaging the
outer reaches of the heliosphere where the magnetic field of the Sun meets up with the Milky Way’s.
It’s examining energetic neutral atoms, particles that originated inside the heliosphere, but which have interacted with the galactic magnetic field and been scattered backwards.
By mapping these neutral atoms, researchers can then extract details about the structure of the outer heliosphere.
“A big discovery was a structure called the IBEX Ribbon, a structure with two to three times more particles coming out of it,” says David McComas, IBEX’s principal investigator. “The Ribbon has got dimmer over time as the pressure of the solar wind changes.”
IBEX has been operating long enough to watch the heliosphere throughout an entire 11-year solar cycle, observing the Ribbon dim and brighten as solar activity fluctuates.
Astronomers are still pondering what creates the Ribbon, but it’s thought to have something to do with the heliosphere’s interaction with the interstellar magnetic field.
IBEX was only able to observe this strange structure as the spacecraft gives a global overview of the region. But what it cannot provide is detailed information about conditions at any one point.
To take those measurements you need to be in situ. Four spacecraft have visited this region. Pioneer 10 and 11 lacked the power to take any readings, leaving only two explorers: the Voyagers.
As the Voyager probes speed ever further away from Earth they’ve been collecting data, allowing astronomers to construct a 3D map of the heliosphere, showing the various features as the probes pass through them.
When the Voyagers left the Solar System
Voyager 1 began its exit from the heliosphere back in 2004, when it passed through the termination shock, the point which marks the boundary of the Sun’s magnetic field, where its influence begins to lose sway to the surrounding galactic magnetic field.
“The termination shock is where the solar wind meets the interstellar medium and abruptly slows down,” says Ed Stone, project scientist for the Voyager missions.
Like with the IBEX Ribbon, it’s thought the termination shock is linked to the solar cycle, moving inward and outward with solar activity, so that when Voyager 1 passed through the termination shock in 2004 it was 94 AU from the Sun, yet when Voyager 2 passed it three years later, it found the termination shock was only 84 AU out, suggesting the shape of the heliosphere was far more complicated than previously imagined.
With the joint data of these two spacecraft, astronomers have found the space beyond the termination shock is filled with magnetic bubbles created by the Sun’s field folding and twisting as it interacts with the Galactic field, before finally crossing the edge of the heliosphere, known as the heliopause, the point interstellar space begins.
The view of the Voyagers is limited. They are each seeing only one path through the region, albeit in different directions.
“Voyager 1 is in the northern hemisphere of the heliosphere and Voyager 2 is in the south. Voyager 1 is heading out along the meridian – in other words into the interstellar wind – while Voyager 2 is off on the flank of the flow,” says Stone.
Voyager 1 crossed the heliopause in 2012, while Voyager 2 did so in 2018, showing once again how the heliosphere changes throughout the solar cycle.
“We believe Voyager 1 left the heliosphere when it was shrinking and the boundary was moving towards the Sun a little bit. When Voyager 2 left it was expanding,” says Stone.
Both spacecraft are now journeying through the outermost region of our Solar System, where the magnetic field and particles from our Sun begin to meld in with those of the Milky Way.
“Inside the heliosphere the magnetic field comes from the Sun, but outside it’s from the Milky Way,
so there’s an abrupt transition and that’s one of the things we’re measuring,” says Stone.
“Voyager 2 also has a working plasma instrument that can tell us how the solar wind interacts with the interstellar medium.”
Already the pair have turned up some unexpected twists to the heliosphere’s story, such as finding that the interstellar magnetic field is unexpectedly pointing in line with that of the solar magnetic field.
However, this could just be a symptom of the fact that the two spacecraft are still in the turbulent region where the interstellar medium and the solar wind meet.
Stone hopes the Voyagers’ dwindling power supplies will last long enough to pass into calmer interstellar waters.
“We won’t get completely free, but we should see the interstellar magnetic field become smoother and more representative of the Milky Way,” says Stone.
Even if they live to reach pristine interstellar space, it will be another 40,000 years or so before the Voyagers escape the Sun’s gravitational influence and officially leave the Solar System.
Before then, the spacecraft will travel through the Oort Cloud, a vast region 2,000 AU from the Sun where long period comets are believed to originate.
The Voyager team are trying to squeeze as much data out of the pair before they lose power, as they will be our last glimpses of the region for a long time.
Though New Horizons is also sailing out towards the heliopause, it will almost certainly run out of power before reaching it.
There have been several new missions proposed which would explore the dim and distant region at the edge of the Sun’s influence – from probes that will investigate the outer planets and Pluto, to spacecraft whose sole purpose is to look beyond the heliopause – but none is currently scheduled.
Even with our limited view of the Solar System’s vast edge, it is evident that the region is not the empty, inert place it first may seem.
Instead, it is a region of icy boulders, where speeding particles and turbulent magnetic fields connect our star with the expanse of interstellar space beyond.
Ezzy Pearson is BBC Sky at Night Magazine’s News Editor. This article originally appeared in the August 2020 issue of BBC Sky at Night Magazine.