The Dark Star: The Planet X Evidence (12 page)

9. Brown Dwarfs

In
the last chapter, we looked at various types of binary bodies that have been
proposed over the years. Some of these are dwarf stars, the 'black' and 'red'
versions of which are quite large, and generate light independently. To be
circling around the sun and yet to have evaded detection, these types of dwarf stars
would have to be located practically halfway to the nearest star - light years
away.

So,
we looked at arguments for a smaller body, and I advocated a smaller, darker
body which is several times the mass of Jupiter and orbits the sun in an
eccentric manner. This body is similar to the type of planet proposed by John
Matese and John Murray. Currently, this massive planet is remarkably faint, and
has probably not been detected directly (although there is always the
possibility that it has been spotted, but misunderstood to be a more distant
star). The tenth planet is a dark, distant body which reflects very little
light back to us from the sun at the sorts of distance we are talking, despite
its size.

Is This Dark Star, As I Call It, A 'Brown Dwarf'?

Brown
dwarfs are neither stars nor planets, but something in between. For a long
time, their existence was just theoretical. This was because they don't shine
with the intensity of stars and are, for the most part, 'dark' objects in the
sky. Stars and planets form through the accretion, or clumping together of
matter. Stars form in stellar nurseries, alongside other sibling stars, often
in quite close proximity.

This is why there are so many binary star systems. They then
spread out, like young birds leaving the nest. They carry with them immense
discs of material which swirl around the star, gradually clumping into planets
and other bodies. These planets can be very diverse, in terms of both size and
properties.

It
makes sense that there should exist a huge range of different shaped objects in
the galaxy, from the smallest comets, through the range of planets, through a
range of these 'brown dwarfs' and on through an equally large range of
different sized stars. In the galaxy, variety is the spice of life.

Until
fairly recently, our knowledge of stars and planets was pretty straightforward.
Stars shone, emitting light by hydrogen fusion processes, and planets were dark
objects orbiting them. This was simply common sense. No one spent too many
sleepless nights worrying about what would happen when an object, undergoing
the process of gas accretion to form a star or planet, would end up with mass
somewhere in between.

Brown
dwarfs are those bodies which have insufficient mass to begin the internal
nuclear processes that fire stars. The smallest are about 12 times the size of
Jupiter, the largest about 80 times, which is still less than a tenth of the
mass of our sun. The more massive the dwarf, the brighter it will appear.

They
are capable of emitting their own light and heat, even though via by a
different set of processes as compared to the sun's nuclear fission. Also,
their ability to create light and heat depends very much on their age. Even
very small brown dwarfs are quite bright to begin with, but their luminosity
quickly drops away with advancing age. One could say that they age quickly; the
flower of their youth is dissipated through intense activity early on.

It is thought that there is a "50:50" chance that a
brown dwarf might exist between us and our nearest star.
¹
Brown
dwarfs tend to be about the same size as Jupiter, despite being many times
heavier. They are denser, and also hotter and more active. They have hydrogen
cores, like the gas giants, and can spin as quickly as once per hour. They
radiate most of their energy in infrared light.

There are still many gaps in our knowledge about these objects,
because their inherent dark properties make them difficult to observe directly,
particularly the older ones that have used up their light-emitting fuels. There
is some debate as to whether brown dwarfs form more or less like stars, or
whether they are more characteristic of planets ejected from emerging star
systems in dense stellar nurseries. The current thinking is that they form like
stars do, but tend to get pushed about before they accrete enough matter to
become proper stars.

There is a critical size of about 80 Jupiter masses, where such a
body can sustain hydrogen fusion by the action of temperatures and pressures
generated by its own gravity.
¹
Then a star is born. The formation of
planets is less well understood, and the emerging discoveries of extrasolar
planets are challenging astrophysicists to revise their theories.
Nevertheless, when a planet is forming, up to several Jupiter masses in size,
then it remains simply that: a planet. As the object's mass increases further,
things start to get more complicated.

The concept of brown dwarfs has been bandied about for some time,
although no reliable astronomical data has been available until quite recently.
Carl Sagan once wrote about Harlow Shapley, an astronomer working in the 1950s.
Shapley had suggested that brown dwarfs (or 'Lilliputian stars, as he called
them) would have warm surfaces upon which astronauts could survive and explore.
²
We now know this to be quite untrue: Brown dwarfs are warm versions of Jupiter.
This massive planet has no surface, only an immense gaseous atmosphere, full of
clouds and storms. I discussed this with an expert on brown dwarfs some years
ago, who stated in passing that “while brown dwarfs are not inhabitable, they
might have moons that might be habitable.”
³
These mini failed stars
might harbour life on their own systems of planets.

The
term “brown dwarf” was first used by Jill Tarter of the SETI institute, in her
1975 PhD thesis. She used it in order to correct the use of the previous term
“black dwarf” which was deemed inappropriate because it had already been used
to describe the end phase of a fully evolved star as it cooled from the white
dwarf stage.
¹

Brown
dwarfs are very difficult to find. They glow only faintly, emitting most of
their radiation in the infrared bands. This is because they are below the 0.08
solar-mass stellar limit, and fail to ignite as stars in their own right. Instead,
they emit radiation from energy left over from their formation.

During
the life-span of a brown dwarf, the younger they are, the brighter they appear.
So, we have a better chance of discovering brown dwarfs that have just formed.
As they get older, they start to appear more like Jupiter, only much more
massive. In general, a brown dwarf's luminosity is expected to be about a
hundred thousandth of the sun's.
⁴
Its spectral characteristics are
different than those of very cool stars, unusually showing an absorption line
of the short-lived element lithium.

Contrary
to the description implied by its name, brown dwarfs appear red, actually very
red. A brown dwarf was discovered in the Solar vicinity by Maria Theresa Ruiz
of the European Southern Observatory in 1997, a discovery which offered the
potential for much better study of these elusive objects. She called it KELU-1,
the term for 'red' in the language of the indigenous population of central
Chile.

Although
it is located at a distance of 33 light-years, its visual magnitude is 22.3,
which is the sort of brightness projected for Murray's proposed brown dwarf in
the Oort cloud. This sets a precedent for discovery of an Oort cloud
planet/brown dwarf.
⁵
If my thesis is correct, however, and Murray's
planet is now more closely bound to the sun, then it should be significantly
brighter than this object.

The
best known brown dwarf, and one which we can actually look at through an
Earth-bound 60-inch telescope, is Gliese 229B, discovered in 1995. This one is
in a binary system along with the low-mass red dwarf Gliese 229A, at a distance
of just 19 light-years from the sun. The separation between the brown dwarf
and its companion star is about the same as that between the sun and Pluto. Its
luminosity is about one-tenth of the faintest star. Its spectrum has large
amounts of methane and water vapor.

Methane
could not exist if the surface temperature were above 1500K.

Astronomers consider its temperature to be about 900K (compared to
Jupiter's 130K), its mass to be between 20 and 55 Jupiters, and the age of the
binary system to be between 1 and 5 billion years old. It has a smoggy haze
layer deep within its atmosphere, essentially making it “much fainter in
visible light than it would otherwise be”. It is possible that the ultra-violet
light from its companion star changes its atmospheric properties from those of
an isolated brown dwarf, such as KELU-1.
¹

In November 2000, a team of scientists analyzed a dim object of
60-90 Jupiter masses, which has been found at just 13 light years distance from
the sun.
⁶
Depending on its actual mass, it might be a high-mass
brown dwarf or a low-mass star. The lack of a lithium signature indicates the
latter.

This star/brown dwarf lies alone in space, and is the nearest such
object spotted so far. Scientists have speculated that more such objects
probably await discovery, perhaps even closer to us than this one. Our
knowledge of our own backyard with respect to its resident stars is still very
much incomplete.

So brown dwarfs emit visible light, albeit faintly, but are cool
enough to retain a planet like atmosphere! Stars and planets no longer appear
to be entirely different entities. Imagine living on a moon of a brown dwarf:
the 'dark star' which your moon is orbiting around would be emitting red light
and heat, yet it would appear like Jupiter as far as its size and atmospheric
consistency went. Rather like Jupiter on fire, perhaps!

Your moon would not only be warmed by the intense infrared
emitted from the brown dwarf, but also by its tidal effects (like Io and Europa
are warmed by the otherwise cool Jupiter), and by its ambient red light. If
your moon was terrestrial, in other words had aqueous oceans and a
nitrogen-rich atmosphere, might the emergence of life there be entirely
possible? Without the dangerous ultra-violet radiation and cosmic rays emitted
by the sun, one could argue that this sort of environment is actually
preferable to the environment on Earth!