by Leslie Mullen
11 March 2010
A dark object may be lurking near our
solar system, occasionally kicking comets in our direction.
Nicknamed "Nemesis" or "The Death Star," this undetected object
could be a red or brown dwarf star, or an even darker presence
several times the mass of Jupiter.
Why do scientists think something could be hidden beyond the edge of
our solar system? Originally, Nemesis was suggested as a way to
explain a cycle of mass extinctions on Earth.
The paleontologists David Raup and Jack Sepkoski claim
that, over the last 250 million years, life on Earth has faced
extinction in a 26-million-year cycle. Astronomers proposed comet
impacts as a possible cause for these catastrophes.
Our solar system is surrounded by a vast collection of icy bodies
called the Oort Cloud. If our Sun were part of a binary system in
which two gravitationally-bound stars orbit a common center of mass,
this interaction could disturb the Oort Cloud on a periodic basis,
sending comets whizzing towards us.
An asteroid impact is famously responsible for the extinction of the
dinosaurs 65 million years ago, but large comet impacts may be
equally deadly. A comet may have been the cause of the Tunguska
event in Russia in 1908. That explosion had about a thousand times
the power of the atomic bomb dropped on Hiroshima, and it flattened
an estimated 80 million trees over an 830 square mile area.
While there's little doubt about the destructive power of cosmic
impacts, there is no evidence that comets have periodically caused
mass extinctions on our planet. The theory of periodic extinctions
itself is still debated, with many insisting that more proof is
Even if the scientific consensus is that
extinction events don't occur in a predictable cycle, there are now
other reasons to suspect a dark companion to the Sun.
A recently-discovered dwarf planet, named
Sedna, has an extra-long
and usual elliptical orbit around the Sun.
Sedna is one of the most distant objects
yet observed, with an orbit ranging between 76 and 975 AU (where 1
AU is the distance between the Earth and the Sun). Sedna's orbit is
estimated to last between 10.5 to 12 thousand years.
Sedna's discoverer, Mike Brown of
Caltech, noted in a Discover magazine article that Sedna's location
doesn't make sense.
"Sedna shouldn't be there," said
Brown. "There's no way to put Sedna where it is. It never comes
close enough to be affected by the Sun, but it never goes far
enough away from the Sun to be affected by other stars."
Perhaps a massive unseen object is
responsible for Sedna's mystifying orbit, its gravitational
influence keeping Sedna fixed in that far-distant portion of space.
"My surveys have always looked for
objects closer and thus moving faster," Brown said to
Astrobiology Magazine. "I would have easily overlooked something
so distant and slow moving as Nemesis."
John Matese, Emeritus Professor of
Physics at the University of Louisiana at Lafayette, suspects
Nemesis exists for another reason.
The comets in the inner solar system
seem to mostly come from the same region of the Oort Cloud, and
Matese thinks the gravitational influence of a solar companion is
disrupting that part of the cloud, scattering comets in its wake.
His calculations suggest Nemesis is between 3 to 5 times the mass of
Jupiter, rather than the 13 Jupiter masses or greater that some
scientists think is a necessary quality of a brown dwarf.
Even at this smaller mass, however, many
astronomers would still classify it as a low mass star rather than a
planet, since the circumstances of birth for stars and planets
The Oort Cloud is thought to extend about 1 light year from the Sun.
Matese estimates Nemesis is 25,000 AU away (or about one-third of a
light year). The next-closest known star to the Sun is Proxima
Centauri, located 4.2 light years away.
Richard Muller of the University of California Berkeley first
suggested the Nemesis theory, and even wrote a popular science book
on the topic. In his view, Nemesis is a red dwarf star 1.5 light
years away. Many scientists counter that such a wide orbit is
inherently unstable and could not have lasted long - certainly not
long enough to have caused the extinctions seen in Earth's fossil
But Muller says this instability has
resulted in an orbit that has changed greatly over billions of
years, and in the next billion years Nemesis will be thrown free of
the solar system.
Binary star systems are common in the galaxy. It is estimated that
one-third of the stars in the Milky Way are either binary or part of
a multiple-star system.
Red dwarfs are also common - in fact, astronomers say they are the
most common type of star in the galaxy. Brown dwarfs are also
thought to be common, but there are only a few hundred known at this
time because they are so difficult to see. Red and brown dwarfs are
smaller and cooler than our Sun, and do not shine brightly.
If red dwarfs can be compared to the red
embers of a dying fire, then brown dwarfs would be the smoldering
ash. Because they are so dim, it is plausible that the Sun could
have a secret companion even though we've searched the sky for many
years with a variety of instruments.
NASA's newest telescope, the Wide-field Infrared Survey Explorer
(WISE), may be able to answer the question about Nemesis once and
Finding Dwarfs in the
WISE looks at our universe in the infrared part of the spectrum.
Spitzer Space telescope, WISE is hunting for heat. The
difference is that WISE has a much wider field of view, and so is
able to scan a greater portion of the sky for distant objects.
WISE began scanning the sky on January 14, and NASA recently
released the mission's first images. The mission will map the entire
sky until October, when the spacecraft's coolant runs out.
Part of the WISE mission is to search for brown dwarfs, and NASA
expects it could find one thousand of the dim stellar objects within
25 light years of our solar system.
Davy Kirkpatrick at NASA's Infrared Processing and Analysis Center
at Caltech found nothing when he searched for Nemesis using data
from the Two Micron All Sky Survey (2MASS).
Now Kirkpatrick is part
of the WISE science team, ready to search again for any signs of a
companion to our Sun.
Kirkpatrick doesn't think Nemesis will be the red dwarf star with an
enormous orbit described by Muller.
In his view, Matese's
description of Nemesis as a low mass object closer to home is more
"I think the possibility that the
Sun could harbor a companion of another sort is not a crazy
idea," said Kirkpatrick. "There might be a distant object in a
more stable, more circular orbit that has gone unnoticed so
Ned Wright, professor of astronomy and
physics at UCLA and the principle investigator for the WISE mission
said that WISE will easily see an object with a mass a few times
that of Jupiter and located 25,000 AU away, as suggested by Matese.
"This is because Jupiter is
self-luminous like a brown dwarf," said Wright. "But for planets
less massive than Jupiter in the far outer solar system, WISE
will be less sensitive."
Neither Kirkpatrick nor Wright think
Nemesis is disrupting the Oort cloud and sending comets towards
Earth, however. Because they envision a more benign orbit, they
prefer the name "Tyche" (the good sister).
Regardless of what they expect to find, the WISE search won't focus
on one particular region of the sky.
"The great thing about WISE, as was
also true of 2MASS, is that it's an all-sky survey," said
Kirkpatrick. "There will be some regions such as the Galactic
Plane where the observations are less sensitive or fields more
crowded, but we'll search those areas too. So we're not
preferentially targeting certain directions."
We may not have an answer to the Nemesis
question until mid-2013. WISE needs to scan the sky twice in order
to generate the time-lapsed images astronomers use to detect objects
in the outer solar system.
The change in location of an object
between the time of the first scan and the second tells astronomers
about the object's location and orbit.
"I don't suspect we'll have
completed the search for candidate objects until mid-2012, and
then we may need up to a year of time to complete telescopic
follow-up of those objects," said Kirkpatrick.
Even if Nemesis is not found, the WISE
telescope will help shed light on the darkest corners of the solar
system. The telescope can be used to search for dwarf planets like
Pluto that orbit the Sun off the solar system's ecliptic plane.
The objects that make up the
are too small and far away for WISE to see, but it will be able to
track potentially dangerous comets and asteroids closer to home.