March 8, 2014
Two studies based on Wide-Field Infrared Survey Explorer (WISE) data were published in Astrophysical Journal. One found no evidence of "Planet X", but the other found 3,525 stars close to our solar system.
Objects in green part of the graph are detectable by WISE, but smaller objects could still hide in the red area. Credit: Penn State University
In November 2013, NASA released combined data from Wide-Field Infrared Survey Explorer (WISE) and NEOWISE survey missions that resulted in infrared images for 747 million objects in the sky – asteroids, stars and galaxies.
WISE mission lasted six months from 2010 to 2011 in which the telescope did two scans of an entire sky in infrared. With two scans six months apart, when Earth was on two opposite sides of its orbit around Sun, we can see apparent movement of stars. It's like putting a thumb in front of your face and looking at it with one eye at the time – it "moves" more when it's closer and less when it's farther away. It's same with objects in the sky.
NEOWISE is basically WISE telescope reactivated in 2013 to identify potentially hazardous near-Earth objects while also finding more about previously known asteroids and comets.
Combined data from these two surveys were basis for two new studies. One was looking for large objects beyond Pluto's orbit, most notably "Planet-X". The other study focused on detecting stellar objects up to 500 light-years away from us.
"Planet X" is the hypothesised gas giant planet or possibly even brown dwarf star that is orbiting Sun beyond the orbit of Pluto. It is called "X", roman numeral for "10", because at the time its existence was suggested, Pluto was considered the ninth planet.
A recent study based on WISE data found no such gas giant or small star. Nothing the size of Saturn or larger is orbiting the Sun at a distance out to 10,000 astronomical units and nothing showed up similar to Jupiter or larger out to 26,000 astronomical units. Astronomical unit is Earth's distance to th Sun, 149 million km.
"The outer solar system probably does not contain a large gas giant planet, or a small, companion star," said paper author Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University, University Park, US.
It is possible that a smaller gas giant is orbiting the Sun at those distances, but we simply can't see it at the time. The reason "Planet X" hypothesis came to be was to explain a regular timing associated with mass extinctions on Earth. It was thought a large Jupiter-like planet sweeping through comets in outer reaches of solar system could have sent some in our general direction, causing said extinctions. The idea that planet caused mass extinctions was ruled out previous to this study, but possibility remained that "Planet X" is the cause for irregular comet orbits.
The real discovery, however, are 3,525 stars and brown dwarfs within 500 light-years of Sun which were found in the second study. These distances practically represent our neighbourhood if we keep in mind that Milky Way Galaxy is 100,000 light-years across.
"We're finding objects that were totally overlooked before," said lead author of the second study, Davy Kirkpatrick of NASA's Infrared and Processing Analysis Center (IPAC) at the California Institute of Technology, Pasadena, US.
Some of those stars are really close. One was discovered 20 light-years away in the constellation Norma and a pair of brown dwarfs only 6.5 light-years away – a system that was found a year ago in another study based on WISE data. You'd think we'd know our own backyard a bit better, but some of these stars are too faint to detect, even for WISE.