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Have We Found Another Planet In The Outer Solar System?


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

102 Have We Found Another Planet In The Outer Solar System?
Don't get too excited just yet. Stock image by mozzyb/Shutterstock

Astronomers are debating two new papers, each of which record something that might be the most distant object yet seen in the Solar System. Neither paper has been peer reviewed, and each set of observations is open to alternative explanations, some of them very interesting in their own right.

The Atacama Large Millimeter/submillimeter Array (ALMA) has spent time looking for the infrared glow that might be visible from a large planet orbiting the binary stars of Alpha Centuari, the nearest star system to our own.


In July 2014 and May 2015 ALMA detected radiation 4 to 5 arcseconds (a little more than a thousandth of a degree) from Alpha Centauri B, the fainter star of the pair that lies 4.3 light-years from Earth. After initially designating the object Alpha Centauri D the team, led by Professor René Liseau of Chalmers University of Technology, Sweden, ruled out this possibility.

In a paper submitted to Astronomy and Astrophyiscs and available on ArXiv, they concluded an object this bright in ALMA's wavelengths at such a distance must be a faint star, not a planet. Such a star would be bright enough in visible light to be seen with binoculars. Even allowing for the glare of Alpha Centauri A and B, we would certainly have seen it before.

But the radiation ALMA picked up is also consistent with a distant planet in our own Solar System. The authors acknowledge that finding such an object so far south of the ecliptic (the line the Sun appears to trace through the sky), where most Solar System objects are seen, is unexpected. Others highlight the coincidence of it having similar motion to a star that happens to be in the same direction.

The object could be similar in size to Charon. NASA


A second paper, also submitted to Astronomy and Astrophysics but yet to be reviewed, reports what could be an object in the outer Solar System photobombing ALMA observations of the star W Aquilae. After concluding that an object outside the Solar System was unlikely to fit the observations a Chalmers University team in Sweden, with one of the same authors as the first paper, came up with two possibilities they consider realistic if the finding is not an error.

“The most exciting possibility is that we have observed a planetary body or brown dwarf in the outer reaches of the Oort cloud,” the authors write. This could be a planet-sized object located up to 4,000 times further from us than the Sun passing through the Solar System but not permanently located here. A less dramatic option is an asteroid 220 to 880 kilometers in diameter (140 to 550 miles) located somewhere between Jupiter and Neptune. 

ALMA is terrifically powerful, but has a tiny field of view, making it unsuitable for searching for randomly located objects. To find two outer Solar System objects so close in the sky to stars of interest suggests either these objects are exceptionally common or ALMA is picking up something else.

In most fields of science only specialists would have an opportunity to learn about these developments so early. However, astronomers have a tradition of submitting papers to the website ArXiv prior to publication in peer reviewed journals. This gives other experts an opportunity to comment, potentially providing a different form of peer review and strengthening the final paper. Occassionally, the general public gets advanced notice as a result.


So far astronomers are urging caution. At Slate, Phil Plait writes: “Don't fire up the warp drive just yet folks, there are many reasons to be very skeptical here.” Twitter discussion is equally skeptical, but everyone agrees; if either is real, it's exciting.


spaceSpace and PhysicsspaceAstronomy
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  • Outer solar system objects