The hunt for "Planet Nine" is intensifying, and anyone can join in. You'll need a computer with an Internet connection, plenty of patience, and the determination to hunt for something that would be very dim and might not actually exist.
Planet Nine is the hypothetical planet beyond Pluto that astronomers have been buzzing about for a couple of years. If it's there, it's probably big - larger than Earth, perhaps a "mini Neptune".
A new initiative by Nasa and the University of California at Berkeley, called Backyard Worlds: Planet 9, is crowdsourcing the hunt for Planet Nine. It will use archived observations from Nasa's Wide-field Infrared Survey Explorer (Wise) mission, which scanned the skies for asteroids and other faint objects. It's possible that Planet Nine - or perhaps a "brown dwarf" star or two - is lurking in its speckled images of space.
This planet could be 500 times as far from the Sun as Earth is, but it would still be part of our solar system, with a highly elliptical orbit that never takes it anywhere close to the Sun.
The mystery planet's existence is inferred from the orbits of many smaller bodies in the outer solar system. They orbit the Sun and cluster in a manner that suggests the possible gravitational influence of an unseen, large planet.
Last year, California Institute of Technology (Caltech) astronomer Mike Brown and colleague Konstantin Batygin published a paper in the Astronomical Journal that offered a possible orbit for the hidden planet, which a news release from Caltech referred to as "Planet Nine".
Professor Brown last week said that additional observations of distant, small objects and ensuing calculations and modelling suggest that it is roughly eight times as massive as Earth, and slightly closer to the Sun than previously thought.
He said that it is probably the core of a giant planet that was ejected from the inner solar system long ago. He also speculated that it was likely to have an atmosphere, which would make it broader, warmer and easier to detect.