This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
When scientific papers come with titles like "The serendipitous discovery of a possible new solar system object with ALMA" and "ALMA discovers the most distant object of the solar system" it's easy to feel a little excitement in the air.
The Atacama Large Millimeter/submillimeter Array (ALMA) is a near-brand-new astronomical observatory perched at over 5,000 meters altitude in Chile's Atacama desert. It has an unprecedented combination of sensitivity and spatial resolution for studying wavelengths of electromagnetic radiation that can probe some of the otherwise dimmest, coolest, and most enshrouded objects in the universe.
Now, two reports by Vlemmings et al. and Liseau et al. claim that early ALMA data may have serendipitously detected not just one, but two previously unseen objects in the outer solar system. One of these, nicknamed 'Gna' is suggested by the researchers to most likely be a body of anywhere from 200 to 800 kilometers in diameter lurking somewhere between 12 and 25 times further from the Sun than the Earth (i.e. 12 to 25 astronomical units, AU).
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This would make Gna a substantial addition to the outer solar system, orbiting within Neptune's orbit, and a possible new Centaur object. However, the estimates of distance and size are subject to enormous uncertainty since the detection is based around three 'epochs' of data separated by a few months (only two of which detected the object), during which the mystery object moved across the sky by an amount consistent with something in proximity to the solar system. An alternative explanation would be a much larger body - perhaps a giant planet or even a brown dwarf - some 2,500 to 4,000 AU distant.
The other reported detection is perhaps even more mysterious. With observations some 10 months apart the researchers claim that ALMA has detected a sub-millimeter thermal radiation source that is very close on the sky to the Alpha Centauri AB system (within arcseconds of angle) and apparently moves in near synchrony with those stars as they shift across the sky due to parallax from our viewpoint and their own proper motion through the galaxy.
But, if this object was simply a part of the Alpha Centauri system it would have to have a size in the stellar mass range (perhaps a few tenths the mass of the Sun) and that would make it bright enough in visible light to have been noticed long ago - which it never has been. So it probably isn't a star.
The explanation favored by the authors of the report is that this is either a super-Earth sized planet or a super-cool brown dwarf - somewhere between 300 AU and 20,000 AU from the Sun respectively - and therefore part of the solar system. Although they do also seem to suggest that it could be more like an Extreme Trans-Neptunian Object (ETNO), which might not be so massive.
The possible existence of a super-Earth or brown dwarf in the outer reaches of our solar system, is, to say the least, quite a proposition. Other research has certainly suggested the possibility, but without any direct observational support. If correct it would be a hugely significant discovery about the grand architecture of our planetary system and offer countless new insights to the history and nature of planet formation and evolution.
However, it may be a bit early to crack open the champagne. These data are fascinating but also puzzling. For example, this hypothesized object would be in an orbit strongly inclined with respect to the primary plane of the solar system (the ecliptic) by some -42 degrees - raising the question of how that could occur. Furthermore, the likelihood of the first detection of a massive planetary companion to our solar system also aligning with the direction to the very nearest stars, and in being close step with their apparent motion in the sky, seems a little odd.
Of course, coincidences do happen. And since ALMA is a new observatory the first data will tend to be acquired around known targets, so if you're going to discover new things they will often appear close to familiar things. But this set of circumstances raise at least a question over whether there is something more mundane going on to explain these discoveries. For example, with the processing of the data from this brand-new radio telescope and its ongoing shakedown.
One thing is certain though. It's moments like these that the potential for wonder in the universe can stop us all in our tracks.