As Neptune and Uranus could solve the mysteries of planet formation, we need to go back and study them in depth, says Leigh Fletcher
During the 1980s, the Voyager 2 spacecraft transformed our view of the ice giants, Uranus and Neptune, from mere wandering points of light to complex and beautiful planetary systems. The Voyager data remains a treasure trove of information on the planets’ interiors, atmospheres and diverse satellites. But with only a single encounter recorded for each planet, we haven’t even scratched the surface of these worlds. Out there on the ‘frozen frontier’, discoveries await the next robotic explorers. Today, the international community is lobbying space agencies to mount a mission to these ice giants, the only major class of planet yet to have a dedicated orbital explorer.
The scale of the challenge is matched by the scientific importance of these two worlds. As we gaze out to extrasolar planets, we’ve realised that worlds of a similar size to our ice giants are commonplace. By contrast, the larger gas giants like Jupiter and Saturn seem rare. And yet we still struggle to understand how Uranus and Neptune formed. Giant planet formation is essentially a race between accretion of gas onto an embryonic rock-ice core, and the dissipation of that gas.
To achieve worlds similar in size to Uranus and Neptune requires fine-tuning of this relationship and we need to know much more: How much ice and rock is present in these ice giants? How is material distributed? How have these worlds been cooling since their formation? To understand their evolution, we need a mission capable of measuring gravitational and magnetic fields, directly sampling the planetary composition, and studying the atmospheres and magnetospheres.
The ice giants are the missing link, the piece of the puzzle to help us unlock the mysteries of planet formation. Uranus presents extreme seasons and magnetic fields, a consequence of the cataclysmic impact that tilted it onto its side; whereas Neptune’s powerful meteorology is driven by heat from within that’s absent (or locked away) on Uranus. Uranus has a classical system of satellites and delicate rings. Neptune, conversely, possesses an interloper from the Kuiper Belt: the massive moon Triton. These worlds can’t be effectively studied remotely, we have to visit them with sophisticated new spacecraft in order to reveal terrains that were in total darkness for Voyager 2, 30 years ago.
With such tantalising potential for discoveries, why haven’t we returned to the ice giants since the 1980s? Reaching 20 or 30 AU requires an international effort and some good timing. With today’s rockets and chemical propulsion, we need to slingshot around Jupiter, to get as much mass as possible into orbit around Uranus or Neptune. That means Jupiter must be in just the right place, which happens once every 12–13 years. The next window is in the early 2030s, so we have no time to lose.
ESA and NASA are currently developing plans for planetary exploration over the next decade, and if they see the ice giants as the highest priority, then we can be ready. Because the discoveries could be astonishing and they’re out there, waiting for us.
Leigh N Fletcher is an Associate Professor in Planetary Science at the University
of Leicester
Looking back: The Sky at Night
16 November 1957
In the episode of The Sky at Night screened on 16 November 1957, Patrick Moore looked at what was set to become a new space-based tourist attraction located in central London. On Marylebone Road, next door to Madame Tussauds, the London Planetarium was preparing to open its doors in March 1958, becoming the first planetarium in the UK.
The auditorium was capable of seating over 300 people, who watched shows projected onto the 18m-wide dome above. For the first 40 years of operation, a mechanical star projector showed a view of the night sky as seen from Earth, with the aim of bringing the wonders of the night sky to the heart of a heavily light-polluted city.
As time and technology progressed, the Planetarium moved to keep up. Laser light performances were added in the late 1970s, then in 1995 a new digital planetarium system – Digistar II – was installed, allowing for complex new programmes rendered in 3D. Rather than just looking at stars from a distance, the show could swoop over planetary surfaces or skim past distant nebulae.
But despite a second projector upgrade in 2004, the Planetarium’s days were numbered. In 2006, Madame Tussauds ceased running astronomical shows, leaving the Peter Harrison Planetarium at the Royal Observatory Greenwich as the home of London’s only planetarium.
Looking ahead to the JWST
As the much-anticipated James Webb Space Telescope (JWST) nears its launch date, Maggie and Chris look back at some of the incredible telescopes throughout history that have redefined our understanding of the cosmos. The team also looks at how the JWST could enable astronomers to unravel some of the biggest secrets in the Universe.
BBC Four, 14 November, 10pm (first repeat BBC Four, 17 November, 7:30pm)
Check www.bbc.co.uk/skyatnight for more up-to-date information
