SCIENCE

A planet larger than Earth could be hiding in the cold, dark depths of our solar system. SO EXCITING!!!! (Nat Geo News)

What is a planet, anyway? Use our resources to find out.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

Discussion Ideas

• The good folks over at Caltech probably discovered a planet lurking in the “cold, dark depths of our solar system.” If it’s so dark out there, how did astronomers see it?
  • They didn’t. Like Neptune, its nearest planetary neighbor, Planet Nine can only be viewed with a telescope.

 

• So how was Planet Nine identified?
  • Math! Math is the language of science, and here’s a portion of the text to translate.

 

 

• OK, but I’m not entirely fluent in the language of science. In English, how did astronomers determine there might be a big new planet out there in our solar system? Take a look at the illustration, and watch the video for some help.
  • First, astronomers mathematically calculated the orbits of Kuiper Belt objects. (The Kuiper Belt is a remote region in the solar system containing thousands of icy bodies.) They noticed that “the most distant objects all swing out in one direction, in a very strange way that shouldn’t happen.”
    • “‘It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,’ says Michel E. Brown, who co-discovered Planet Nine. The odds of having that happen are something like 1 in 100, he says. But on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent. ‘Basically it shouldn’t happen randomly,’ Brown says. ‘So we thought something else must be shaping these orbits.’”
  • Then, astronomers hypothesized why these distant Kuiper Belt objects had such weird dispersal. “We realized that the only way we could get them to swing in one direction is if there is a massive planet—also very distant in the solar system—keeping them in place while they all go around the sun.”

 

• The way Planet Nine was (tentatively) identified is a fantastic example of the scientific method. How did astronomers use some of the feedback loops in the illustration above to determine Planet Nine might be out there? Read through this great article on the discovery for some help.
  • Testing Ideas:
    • Gathering data: Astronomers observed the oddball orbits of some Kuiper Belt objects.
    • Interpreting data: Astronomers gathered data about orbital paths, gravitational anomalies, and other “celestial mechanics.”
  • Exploration and Discovery:
    • Sharing data and ideas: Planet Nine is a result of the collaboration between the observational and mechanical aspects of astronomy. “Brown is someone who looks at the sky and tries to anchor everything in the context of what can be seen, and [Konstantin] Batygin is someone who puts himself within the context of dynamics, considering how things might work from a physics standpoint.”
  • Benefits and Outcomes:
    • Satisfy curiosity: Planet Nine helps explain the orbital paths of many of our solar system’s objects, and addresses a question of why one of the most popular types of planets in the galaxy wasn’t found in our own solar system.
  • Community Analysis and Feedback:
    • Publication
    • Discussion with colleagues: Brown and Batygin are hoping some of the world’s most powerful telescopes might be focused to find Planet Nine.

 

• Planet Nine is about 10 times more massive than Earth, and about 2-4 times Earth’s diameter. Is it a big version of our planet? Read our short encyclopedic entry for some help.
  • No, it’s probably more like a “mini-Neptune” than a “super-Earth.” Planet Nine orbits so far away from the sun, it would take between 10,000 and 20,000 years to make one complete revolution.
  • Planet Nine is probably a big blue “ice giant” like Neptune or Uranus. The bulky mantles of ice giants are mostly compressed gases such as methane and ammonia. (The mantles of gas giants, like Jupiter and Saturn, are mostly hydrogen and helium. The mantle of rocky planets such as Earth are, well, rock. Read more about our own awesome mantle here.)
  • Like most ice giants, Planet Nine might have a ring system and at least one moon.

 

• Why would Planet Nine be considered a planet? Read our short definition of a planet for some help.
  • A planet is a large object that orbits a star.
    • Check. Planet Nine orbits the sun.
  • To be a planet, an object must be massive enough for gravity to have squeezed it into a spherical, or round, shape.
    • Check. Calculations about Planet Nine indicate it’s a big ball of ice and gas.
  • Planets must be large enough for gravity to have cleared a path—swept up any rocky or icy objects from its orbit—around the star.
    • Check. This is probably the clincher (and really what sank Pluto). Planets need to be the “gravitational bully” of their neighborhood, and Planet Nine fits the bill.
  • Planet Nine “—at 5,000 times the mass of Pluto—is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown says makes it ‘the most planet-y of the planets in the whole solar system.’”

 

• How did Planet Nine form?
  • “Scientists have long believed that the early solar system began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets—Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. ‘But there is no reason that there could not have been five cores, rather than four,’ says Brown. Planet Nine could represent that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its distant, eccentric orbit.”

 

• When can we see it?
  • Hey, it took us 85 years to see pretty pictures of Pluto!
  • “If it is in the most distant part of its orbit, the world’s largest telescopes—such as the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope, all on Mauna Kea in Hawaii—will be needed to see it. If, however, Planet Nine is [not at aphelion], many telescopes have a shot at finding it.”
  • “I would love to find it,” says Mike Brown, who co-discovered Planet Nine. “But I’d also be perfectly happy if someone else found it. That is why we’re publishing this paper. We hope that other people are going to get inspired and start searching.”

 

 

TEACHERS’ TOOLKIT

Nat Geo: Scientists Find Evidence for Ninth Planet in Solar System

Caltech: Caltech Researchers Find Evidence of a Real Ninth Planet (great article!)

Nat Geo: What is a planet?

(extra credit!) The Astronomical Journal: Evidence for a Distant Giant Planet in the Solar System