With New Horizons' flyby of Pluto, spacecraft launched by humans have now visited the nine biggest planets (dwarf or otherwise) orbiting the sun.

Publications such as the New York Times have called Pluto "the last of known worlds to be explored" and  "the end of an era of planetary exploration."

Alan Stern, principal investigator of the New Horizons mission, has called the mission "the last picture show."

That may sound a bit depressing to those of us captivated by our first close-up glimpses of Pluto.

But don't worry – it isn't really the last picture show. Not by a long shot.

"There's a lot left to explore," says Rebecca Ghent, a planetary scientist at the University of Toronto and at the U.S.-based Planetary Science Institute, a non-profit corporation dedicated to solar system exploration.

Her opinion is shared by other Canadian planetary scientists. Here are some solar system destinations left to look forward to.

1. Eris, the most massive object after the eight planets, and other Kuiper Belt kin

New Horizons showed that Pluto is the largest object in the outer region of the solar system known as the Kuiper Belt.

A Kuiper Belt object called Eris, discovered in 2005, was previously thought to have a wider diameter than Pluto. That was one of the main reasons for Pluto's demotion from planet to dwarf planet in 2006. New Horizons measured Pluto as being two kilometres wider than thought, pushing it past Eris, but there's more than one way to measure size.

"Eris is without doubt the more massive of the two," says Brett Gladman, an astronomer at the University of British Columbia who studies objects in the Kuiper Belt.

In fact, it's about 25 per cent more massive, probably because it's denser and rockier than Pluto.

So we may have just visited the biggest object in the Kuiper Belt.

"But we haven't been to the most massive object."

Other unexplored dwarf planets in the Kuiper Belt include Makemake, a reddish world that has frozen nitrogen, ethane and methane on its surface and Haumea, one of the fastest rotating large objects in our solar system, which has two known moons.

Eris and Dysnomia

Eris, seen in an artist's conception with its moon Dysnomia, is the most massive object in the Kuiper Belt and has never been visited. (Caltech/NASA)

2. Smaller icy worlds

Dwarf planets may not even be the most interesting objects in the Kuiper Belt, though. Gladman thinks we could learn even more from some of the smaller icy chunks in that region – one of which New Horizons may visit in 2018, if it gets the funding.

"Pluto can be seen as the last planet (or ex-planet), but really it's pretty much the first of a whole swarm of bizarre objects," says Phil Stooke, associate professor of planetary geology at Western University. "The surprises about Pluto are showing us just how bizarre these things are."

Kuiper Belt object

The Hubble telescope found two objects in the Kuiper Belt that are potentially reachable by NASA’s New Horizons spacecraft in 2018. (NASA, ESA, and G. Bacon (STScI))

3. Mysterious Sedna

The solar system doesn't end with the Kuiper Belt.

In 2004, astronomers announced the discovery of a reddish, planet-like object that never gets closer to the sun than three billion kilometres outside the Kuiper Belt. The object, named Sedna, has such an elliptical orbit that it is sometimes as far away as 150 billion kilometres from the sun.

"Sedna is suspected of being captured by the Sun from a cloud of objects orbiting a different star as they came close together long ago," Stooke says. 

The astronomers who discovered Sedna suggested it could be the first object from the inner Oort Cloud, a region beyond the Kuiper Belt which is thought to contain trillions of icy objects that have never been seen by humans.


Sedna is a reddish, planet-like object that never gets closer to the sun than 3 billion kilometres outside the Kuiper Belt. The object, named Sedna, has such an elliptical orbit that it is sometimes as far away as 150 billion kilometres from the sun. (NASA/JPL-Caltech)

4. Asteroids and comets

So far, we've had good looks at about a dozen asteroids, says Stooke. Most recently, the Dawn spacecraft has given us beautiful images of Ceres, the largest object in the asteroid belt between Mars and Jupiter.

But asteroids are very diverse.

"There are big differences," Stooke said in an email, "and some types, like metal asteroids, we have never seen."

Asteroids also come in a wide range of colours. For example, Bennu, the target of the upcoming OSIRIS-REx mission, which Ghent is involved in, is extremely dark. The mission will visit and take samples from the asteroid.

The asteroid's colour can affect its orbit, and could potentially also help predict what the asteroid is made of, which would be valuable to those interested in mining them.

Because of that, Ghent suggests it may be useful to visit asteroids of different colours.

Comets are also very varied, and we've only visited about half a dozen, most recently 67P/Churyumov-Gerasimenko, target of the European Space Agency's Rosetta mission.

Osiris Rex

Asteroids come in a variety of colours. Bennu, the target of the upcoming OSIRIS-REx mission is extremely dark. The spacecraft, which launches in 2016, will visit and take samples from the asteroid. (NASA's Goddard Space Flight Center Conceptual Image Lab)

5. Venus

It may surprise you to see Venus on this list, as it's our nearest neighbour and has been visited by spacecraft since the 1960s. But, believe it or not, to date we have fewer detailed images of its surface than we do of Pluto, says Ghent.

Venus Mariner

NASA's Mariner 10 took this colour-enhanced image of Venus as it flew past in 1973. Venus's thick and cloudy atmosphere mean that we now have better photos of Pluto's surface than Venus's. (NASA)

The only ones that exist are a few photographs taken by the Soviet Venera landers between 1975 and 1982.

"Mostly what we have is radar data," Ghent said, who has put in a proposal for an instrument for a future NASA mission to Venus.

That's because Venus is shrouded in thick clouds that only radar can penetrate, and even our radar data so far is not very detailed.

Venera 13 lander image

This panorama image, taken by the Venera 13 spacecraft on March 1, 1982, is one of the few images ever taken on the surface of the planet, Earth's nearest neighbour. (NASA History Office)

There are many other objects in the solar system that have already been "visited" in passing, but may be worthy of more detailed studies using orbiters or landers.

NASA has some missions in the planning stages, including one to Jupiter's moon Europa, which has an ice-covered ocean, and Saturn's moon Titan, the only body in our solar system other than Earth known to have lots of liquid on its surface.

Ghent said solar system exploration is more than just taking photos during a flyby.

"We have initial pictures of every [large] body," she said, "but we haven't really touched the tip of the iceberg in terms of explorations."