New clues in search for Planet Nine | Science News for Students

New clues in search for Planet Nine

Hints about the distant world’s location and brightness raise hopes of actually spotting it
Aug 1, 2016 — 7:00 am EST
planet 9

This illustration shows Planet Nine, a hypothetical new member of the solar system whose location researchers are trying to pin down.


There may be a ninth planet lurking in the fringes of our solar system. More clues about where to search for it are coming from the Kuiper (KY-pur) belt. That’s a band of icy debris beyond Neptune. New calculations suggest the mystery planet might be brighter — and a bit easier to find — than once thought.

Evidence for a “Planet Nine” is scant. The orbits of six distant objects in the Kuiper belt seem to line up. Their oval orbits all point in roughly the same direction and lie in about the same plane. This suggests that the gravity of a hidden planet has herded them onto similar courses. Such a planet would need to be about five to 20 times as massive as Earth.

Mike Brown and Konstantin Batygin announced this clue in January. Both are planetary scientists at the California Institute of Technology in Pasadena. Now they’ve used the evidence to describe Planet Nine in more detail. Their work also homes in on where it might be hiding.

Their results appear in the June 20 Astrophysical Journal Letters.

On average, Planet Nine would likely be some 500 to 600 times farther from the sun than Earth is, Brown and Batygin say. Its orbit would be highly stretched and tipped by about 30 degrees relative to the rest of the solar system. This would take it well above and below the orbits of the eight known planets. And right now, it likely would be near its farthest point from the sun — possibly as far as 250 billion kilometers (155 billion miles) away.

But evidence that it exists is still slim. “The argument that a planet is there is not ironclad,” cautions Renu Malhotra. She’s a planetary scientist at the University of Arizona in Tucson. “I think it’s worth studying. There’s enough there to not ignore this evidence,” she adds. “We just shouldn’t get depressed if the planet’s not there.”

Malhotra and her colleagues have been looking for other evidence of a ninth planet. And they think they’ve found another clue: The orbits of those same six frozen Kuiper belt objects all take a similar amount of time. Such synced orbits usually hint at a gravitational link among all the bodies involved. But these distant worlds are too small to affect one another, says Malhotra. That suggests there’s another, more massive world pulling all of them in line. Her team reported its finding in the same journal.

The evidence from Malhotra’s team would put Planet Nine, on average, about 665 times farther from the sun than Earth is. It would be at least 10 times as massive as Earth and circle the sun once every 17,117 years.

The synced orbits are “very intriguing and very interesting,” says Scott Sheppard. He’s a planetary scientist at the Carnegie Institution for Science in Washington, D.C. “But they need more of these objects” to show the syncing isn’t just due to chance, he adds. In 2014, Sheppard and Chad Trujillo suggested that a ninth planet could explain the orbits of a dozen worlds (including those six) in the Kuiper belt. Trujillo works at the Gemini Observatory in Hilo, Hawaii.

“We want to discover more of these smaller [bodies], which are more numerous and can lead to the big one,” says Sheppard. He and Trujillo are hunting for remote Kuiper belt objects with telescopes in Chile and Hawaii. They’ve had some success in adding a few more distant lumps of ice to the collection. And the orbits of these new discoveries show hints of being in line with the others, Sheppard says.

If new objects help astronomers zero in on Planet Nine’s location, there’s a chance they could even see it directly. Its chilled atmosphere — colder than about ‒220° Celsius (-364° Fahrenheit) might contain only hydrogen and helium gases. These gases are good at reflecting light, report Jonathan Fortney and his team in the June 20 Astrophysical Journal Letters. Fortney is a planetary scientist at the University of California, Santa Cruz.

“We expect the planet, if it’s there, to be a kind of mirror,” Fortney says. “We think it would be bright with a whitish hue.” Depending on its size, Planet Nine might be even bright enough to be detected by the Dark Energy Survey. This project is scanning for new galaxies and supernovas in the southern sky. However, it also can spy on objects closer to home.

“The real problem is knowing where to look,” Fortney says.

Brown and Batygin think they’ve narrowed it down to a large patch of sky near the constellation Orion. Planet Nine, if it’s there, will be hard to find, Batygin warns. “But the reward is an expansion of our planetary family.” 

Power Words

(for more about Power Words, click here)

astronomy    The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

atmosphere   The envelope of gases surrounding Earth or another planet.

constellation    Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

dark energy    A theoretical force that counteracts gravity and causes the universe to expand at an accelerating rate.

debris   Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft. 

degree   (in geometry) A unit of measurement for angles. Each degree equals one three-hundred-and-sixtieth of the circumference of a circle.

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gravity    The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

helium     An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -458 degrees Fahrenheit (-272 degrees Celsius).

hydrogen    The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

Kuiper belt     An area of the solar system beyond the orbit of Neptune. It is a vast area containing leftovers from the formation of the solar system that continue to orbit the sun. Many objects in the Kuiper belt are made of ice, rock, frozen methane and ammonia.

Neptune    The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.

observatory     (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

plane     (in geometry and space) A flat surface (like a sheet or piece of paper) that extends in all directions, infinitely, without end. It has no height (or depth), much the way a line also has  just two dimensions.

planet    A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

planetary science    The science of other planets besides Earth. A person who works in this field is known as a planetary scientist.

solar system   The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

supernova    (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.


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R. Malhotra, K. Volk and X. Wang. “Corralling a distant planet with extreme resonant Kuiper belt objects.” Astrophysical Journal Letters. Vol. 824, June 20, 2016, p. L22. doi: 10.3847/2041-8205/824/2/L22.

M.E. Brown and K. Batygin. “Observational constraints on the orbit and location of Planet Nine in the outer solar system.” Astrophysical Journal Letters. Vol. 824, June 20, 2016, p. L23. doi: 10.3847/2041-8205/824/2/L23.

J.J. Fortney et al. “The hunt for Planet Nine: atmosphere, spectra, evolution, and detectability.” Astrophysical Journal Letters. Vol. 824, June 20, 2016, p. L25. doi: 10.3847/2041-8205/824/2/L25.