The mysteries at the edge of the solar system keep growing

Unanswered questions are mounting about Uranus and Neptune.

Why it matters: The two cold, distant worlds could hold the key to figuring out why Earth ended up being so hospitable to human life — and understanding similar worlds beyond our solar system.

• "The more we learn, the more we realize how little we actually did know about these two amazing planets," Kathleen Mandt, a planetary scientist at Johns Hopkins' Applied Physics Lab, told me.
• Uranus and Neptune are the two major planets in the solar system that haven't been visited by dedicated missions, leaving major holes in how scientists understand the solar system.
• For years, planetary scientists have called on NASA and other funding agencies to make these worlds a priority for future missions, and now these planets could be explored in the not-too-distant future.

What's happening: A study published last month used data from the Hubble Telescope and other observatories to try to answer the question of why these icy giants appear to be different colors.

• The researchers found Uranus has a thicker methane haze layer in the middle of its atmosphere than Neptune has, giving it a distinctive greenish hue.
• “There’s still an awful lot of uncertainty,” Oxford University's Patrick Irwin, an author of the new study, told the New York Times. “We don’t really know what the particles are made of. The only way to really know what’s going on is to drop a probe into these deep atmospheres.”

The big question: How — and where — did both of these planets form? An answer could help scientists understand the origins of Earth's life-sustaining water.

• It's still not clear where Uranus and Neptune formed in the disk of gas and dust surrounding the Sun during the early moments of the solar system.
• If Uranus and Neptune migrated through the early solar system along with Jupiter and Saturn, which underwent their own complicated dance, the worlds could have flung comets and water-laden asteroids toward Earth, delivering water to our planet.
• "We don't understand the interior structure of these planets," Mandt said. "We don't know how they formed with so much less hydrogen and so much more of the heavier elements than Jupiter and Saturn."

The intrigue: Unlocking the mysteries of the ice giants will also allow scientists to learn more about worlds far beyond our own solar system.

• Many exoplanets found by planet-searching missions appear to have a similar mass and radius to Uranus and Neptune.
• But because scientists know very little about the structures of the two ice giants, it's difficult to understand — or even guess at — the nature of those far more distant worlds.
• "There are 1,500 planets that we're trying to categorize without knowing how our own two work," NASA planetary scientist Amy Simon told me.

What to watch: So far, the only mission to visit Neptune and Uranus from close range was a quick flyby of the worlds made by Voyager 2 in the 1980s, but that could change in the relatively near future.

• A major report from the scientific community laying out the priorities for planetary research in the coming decade put a Uranus mission at the top of the list for NASA.

Yes, but: A dedicated mission to Neptune may need to wait a little longer.

• It's not that Uranus is more interesting than Neptune, experts say. Prioritizing Uranus is actually a more practical choice — the planet is much closer, and it's easier to get a spacecraft out there using current technology.
• "Just getting out to either one is difficult, but Neptune is really, really hard," Simon said.