18 | New Scientist | 29 August 2020News
BILLIONS of years ago, there
may have been two suns in
our solar system. If so, that
could explain how the solar
system caught its outermost
objects, including the
hypothetical Planet Nine.
Our solar system is far bigger
than just the space occupied
by the eight official planets: it
extends out to the Oort cloud, a
belt of icy objects that sits about
2000 to 100,000 times as far
from the sun as Earth does, and
is only loosely gravitationally
bound to the sun.
How the Oort cloud formed
is a mystery, as is the origin of a
proposed extra planet that may
reside about 500 times as far
from the sun as Earth does.
Nicknamed Planet Nine, the
existence of this world could
explain the orbits of certain
objects far out in space. Amir
Siraj and Avi Loeb at Harvard
University have come up with
a way to solve both of those
mysteries at once.
The sun, like most stars,
was almost definitely born
in a cluster with many siblings.
If it had a companion starorbiting with it billions of years
ago, that would have made it
much better at catching space
rocks like Planet Nine and those
in the Oort cloud.
“If the sun had a companion,
which is not unusual because
more than half of all sun-like
stars have a binary companion,
that would have produced a very
important effect on the solar
system because the sun and its
companion would act as a sort
of fishing net,” says Loeb.He and Siraj calculated that
the sun being part of a binary
system in its youth, exerting
more gravitational pull over
a larger area, could increase
the chances of our solar system
having the sort of Oort cloud we
see now by a factor of about five
compared with if it has always
been a lone star. When it comes
to capturing Planet Nine, the
likelihood increased by a factor
of around 20 (The Astrophysical
Journal Letters, doi.org/d63g).
If Planet Nine exists,
researchers suspect that it
didn’t form where it seems to
be now. Instead, it was probably
either ejected from the inner
solar system shortly after it
arose or it was filched from
another passing star.
“There is no simple way
to make Planet Nine,” says
Konstantin Batygin at the
California Institute of
Technology, one of the
researchers who spotted early
hints that this distant world
may exist. “This is something
that could potentially work.”
Once the sun and its
companion gathered all theseobjects, the sibling would have
been pulled away by another
passing star, a process that is
common in young star clusters,
says Loeb. This would have
also blown away most of what
was then the Oort cloud,
resulting in the relatively
sparse belt that we see now.
Evidence for this idea
could come from our search
for proof of Planet Nine’s
existence if we find many other
worlds like it in the solar system.
“If there was a companion star
to the sun, it wouldn’t have just
captured one Planet Nine, it
would have captured a bunch of
those objects just like it captured
a lot of Oort cloud objects,” says
Loeb. “When I go to the kitchen
and find one ant, I know there
are more ants walking around.”
Just like those ants are a
hint at the state of the kitchen,
the objects in the outer solar
system could hint at the sun’s
distant past and the possibility
of a long-lost companion. ❚An artist’s concept
of a world in a binary
star systemSolar systemLeah CraneDETL
EV
VAN^ RAV
ENSWAA
Y/SCIENC
E^ PHOTOLIBRA
RYThe sun’s lost sibling may
have nabbed Planet Nine
MORE of Greenland’s ice was
lost in 2019 than in any year
since measurements began.
The ice sheet lost 15 per cent
more ice than in 2012, the year
the previous record was set.
“It is worrying, but not that
much surprising,” says Ingo
Sasgen at the Alfred Wegener
Institute in Bremerhaven, Germany.
The finding is partially based on
data from the GRACE satellites that
launched in 2002. They revealed
just how much of Greenland’s ice
is disappearing due to melting and
glacier flow into the ocean. There
was less ice loss than usual in 2017
and 2018, but a new record decline
of around 530 gigatonnes followed
in 2019 (Communications Earth &
Environment, doi.org/d626).
The expectation is that ice
loss will accelerate as warming
continues, but the satellite record is
too short to show such a trend. It is
clear that weather in Greenland is
becoming more variable, however.
In some years, such as 2019,
there are more high-pressure
systems over Greenland, drawing
in warm air from North America
that causes lots of melting. In
others, such as 2017 and 2018,
low-pressure systems are more
prevalent, producing lots of snow.
Even in high-snow years, the ice
sheet still loses mass due to global
warming, as glaciers flow faster
and dump more ice in the sea.
The loss of all Greenland’s
ice would add at least 6 metres
to global sea level. It is thought
that this would happen gradually
over millennia rather than suddenly.
However, the process might already
be unstoppable. Some studies
suggest we have triggered positive
feedbacks that mean the decline
will continue even if the planet
stopped warming today. Sasgen
thinks that we don’t know enough
to say for sure that we have passed
the point of no return. ❚Climate changeMichael Le PageGreenland lost
a record amount
of ice in 2019“ If the sun had a
companion, which is not
unusual, they would act
as a sort of fishing net”