4.2. A new member of the solar system: an ETNO?

=[snip]=

In addition, bound motion at a distance of, say,
1000 AU (the aphelion of Sedna) would result in an orbital
motion of less than 4000 yr−1, but the apparent motion in the sky
would be dominated by the annual parallax (∼ 20000). Curiously,
when we examined a total of 766 925 known solar-system objects
for being within 150 around α Cen at the time of observation,
we found no source down to the limiting V-magnitude
of 26.0.

Again, a low-albedo, thermal Extreme Trans Neptunian
Object (ETNO), such as the hypothetical super-Earth of Trujillo
& Sheppard (2014), would be consistent with our flux data.

=[snip]=

For reasons of sensitivity (or rather, lack thereof), TNOs on
highly eccentric orbits have traditionally been firstly detected
when close to their perihelia. Further away, there would have
remained unseen (e.g., Sheppard et al. 2011). However, a sizable
population of such bodies is expected to exist at large distances
from the Sun. It is clear, therefore, that ALMA with its high
submm sensitivity provides presently the only existing means
to detect TNOs far from their perihelia, where temperatures are
merely some tens of Kelvin. There must be a vast reservoir of
objects between, say, roughly 100 and 1000 AU, of which we
hitherto have seen only a tiny fraction.

5. Conclusions

Within ten months between 2014 and 2015, ALMA imaging
observations revealed a new source in two of the bands, at 0.74 mm
(Band 8) and 0.87 mm (Band 7) respectively, whereas the noise
was too high in the other bands. Staying within 500· 5 of both
α Cen A and B, this object essentially shared the high proper
motion of α Cen. With a spectral slope of 2, its submm-SED
appears thermal. However, simple arguments convince us that
this object cannot be an ordinary star. We argue that the object is
most likely part of the solar system, in prograde motion, albeit at
a distance too far to be detectable at other wavelengths, viz. an
ETNO (>100 AU), a hypothesized Super-Earth (∼ 300 AU) or
a super-cool brown dwarf (∼ 20 000 AU).