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by
John Whatmough
BEHIND THE SPECULATIONS
Updated 3 Apr 2005
This page represents my most current knowledge of the
properties of the various exoplanets based on articles and papers I have
read. As I hear about new discoveries, I will update this page to
reflect them. This does mean, however, that the artwork and speculations
on the rest of the site may be out of date compared to what is stated
here on this page. Notice that this section has been recently updated to
reflect more recent atmospheric models of Hot Jupiters which suggest
such worlds are rendered dark by layers of sodium gas. The rest of the
site will be updated accordingly as time permits.
INTRODUCTION
What we know about extrasolar planets comes entirely
from the painstaking efforts of astronomers taking measurements of stars
night after night for years. And yet, the information garnered from such
observations seems miniscule when compared with our knowledge of planets
in our own solar system, like Mars or Saturn. What can we hope to
understand about planets whose entire identities currently rest with a
series of data points? How can we come to know worlds still unknown,
save for their effect on the motion of their parent stars?
As it turns out, we can know a great deal. The data
that we have gathered about these other worlds are telling. Their mass
and orbital distance, as well as the properties of their stars, can
reveal much. And, they have kindred in our own solar system for
comparison. Until the next generation of optical interferometers and
space telescopes provide us the first close up pictures, we will not
know whether 51 Pegasi b has moons or if 55 Cancri c is girdled in a
delicate ring system. But we can make speculations and extrapolations.
So, what do these worlds look like? Extrasolar Visions
bases its speculations on the following factors.
THE OBSERVED FACTS
There are many professional
websites that deal with extrasolar planets. One of the most
comprehensive is the
Extrasolar
Planets Encyclopedia, which is what Extrasolar Visions uses for its
chief reference.
The velocity curve of HD 209458 showing the wobble
caused by its planet
Most extrasolar planets have been discovered via the
radial velocity method, which infers a planet's orbital period, orbital
distance, and approximate mass from the wobble of its parent star. So,
in most cases, we have these three pieces of data from which to
determine a planet's properties. In most cases, as well, good data is
available for the parent star itself, information such as spectral
class, mass, radius, and surface temperature. If such stellar data is
available, then the planet's temperature can be approximated. Armed with
a planet's mass and temperature, we can go a long way in our
speculations.
The light curve of HD 209458 dims during the
transit of its planet
Some planets have been discovered by the transit
method, which detects the dimming of a star as a planet passes in front
of it. Transits give us a planet's radius, orbital period, and orbital
inclination (which must be near 90º for the transit to be possible).
Transits also allow astronomers to probe a planet's atmospheric density
by detecting the rate of dimming just as the planet begins to cross the
disk of its star, as well as the planet's chemistry by detecting how the
starlight is absorbed as it passes through the planet's atmosphere.
If a planet is detected both by transit and by radial
velocity, its total density can be determined because both the planet's
radius and mass are known. The planet of HD 209458 has been detected
using both these methods and much of what we know about Hot Jupiters is
based on the properties of this particular world.
Extrasolar Visions contains a great deal of actual
data about extrasolar planets. Unfortunately, the recent pace of
exoplanet discovery has been quite fast. Not only are new discoveries
being made, but data about earlier discoveries are constantly being
refined. I update planetary data on the site as I find out about it, but
it is certainly possible that some planets have out dated information.
If you have a need for the most up to date data,
please use the website of the planet search that discovered the planet
you are interested in or, if all else fails, check out the
Extrasolar
Planets Encyclopedia. Just keep in mind that the field of extrasolar
planet research is still in its infancy and there is still much that is
uncertain. Especially in cases where more than one planet has been found
around a star or where a planet's orbital period is many years, it can
be very difficult to pin down the exact properties of each world. You
may even find that different planet searches have different data about
the same planet. The bottom line is to take any exoplanet data as
approximate.
SIZE
Most of the extrasolar planets discovered so far are
similar in mass to Jupiter and Saturn. Many are even more massive. Like
Jupiter and Saturn, these planets are probably gas giants. Such worlds
would dwarf the Earth.
Unlike the Earth or Mars or Venus, gas giants have no
solid surfaces. If you were to try to land on such a world, you would
descend thousands of miles through the clouds into an atmosphere that
became progressively denser and hotter the further down you went, until
the intense pressures crushed your spacecraft. Most of the mass of a gas
giant is contained in this impossibly dense atmosphere of hydrogen and
helium. Some gas giants may be all gas. Others may have rocky cores,
only a few times the mass of the Earth, buried deep below the crushing
atmosphere.
Extrasolar gas giants can be smaller than Neptune, but
they are not likely to be larger than Jupiter, even if they have many
times Jupiter's mass. Jupiter's radius is a result of an equilibrium
between the outward gas pressure of its atmosphere and the force of
gravity trying to collapse it. As it turns out, adding more mass to
Jupiter would only increase the planet's gravity, causing it to collapse
back to about the same radius it started with. Planets with one Jupiter
mass, or two, or twenty, are all expected to have radii close to that of
Jupiter. In fact, an extremely low mass star, called OGLE-TR-122b, was
recently discovered with a radius very similar to Jupiter's.
Size comparison of the Sun, Jupiter, and the newly
discovered star OGLE-TR-122b
As stellar mass increases, the energy from
thermonuclear fusion reactions in a star's interior pumps energy into
the atmosphere, causing the radius to expand. Stars like the sun are
more than ten times the radius of Jupiter, and some aged super giant
stars would engulf the orbit of Mars if they were to be placed at the
center of our solar system. But below the mass threshold needed for
fusion, substellar objects like gas giants and brown dwarfs will
generally collapse to the size of Jupiter.
But, as with everything, there are exceptions. Gas
giants at short orbital distances, the so called Hot Jupiters, are super
heated by their nearby host stars. This expands their outer atmospheres
giving them radii significantly larger than that of Jupiter. One such
world, HD 209458 b, is known both from radial velocity measurements and
from its transit across the face of its parent star. This enables us to
know both the planet's radius and its mass. HD 209458 b is only
two-thirds the mass of Jupiter, but because of stellar heating, it has
one and a half times the radius of Jupiter. If this planet were to be
moved to a more distant orbit, its atmosphere would cool and condense
back to about the same size as Jupiter's.
Although the atmosphere of a gas giant represents the
bulk of its mass and determines its radius, most of that atmosphere is
hidden out of sight deep within the planet. Only the very uppermost
layers of the atmosphere provide the colors and textures seen on its
surface from space.
CLOUDS AND ROTATION
What are such colors and
textures like? Jupiter is about eleven times the diameter of the Earth,
yet it rotates once every ten hours. This fast rotation pulls the cloud
layer into horizontal bands. Convection from the hot interior of the
planet causes alternating bands to travel in alternate directions. The
Earth's atmosphere behaves in a similar manner, only much of it is
transparent and the wind speed is much less, so there are fewer bands
and the borders between bands are less defined. On Venus, the rotation
is extremely slow and there are no bands.
The cloud bands of Jupiter.
Current theories on how gas giants form indicate that
they should form with fast rotation rates. This is borne out in our own
solar system, where all four of the gas giants rotate faster than the
Earth, causing their atmospheres to be banded. So we should expect
extrasolar gas giants to also rotate quickly. Still, there are probably
exceptions. Planets that have been involved in massive impacts or have
extremely large moons may have lost their rotational energy. Tidal
forces will also slow the rotation of planets close to their stars. Many
of the Hot Jupiters have probably become tidally locked to their stars,
causing their rotational and orbital periods to be the same.
HAZE
Jupiter's cloud bands are vibrant and colorful, easily
seen even with a small telescope. But the bands of Saturn are more
muted. And Uranus' bands are almost entirely invisible. What causes this
difference? All four of the gas giants of our solar system have layers
of haze above the clouds. Depending on the thickness of this haze, the
bands below will either be visible or obscured. Jupiter's haze is thin,
making the cloud bands stand out. On Uranus, the opacity of the haze is
total, making the bands only visible through computer enhancement.
The thickness of the haze layer has a lot to do with
the temperature of the planet and the chemical composition of the
clouds. But small variations in these factors can cause wide variations
in the opacity of the haze. To understand how very similar planets can
have markedly different haze opacity, compare Uranus and Neptune. Both
planets are quite distant, have similar masses, and similar
compositions. Yet Uranus is almost totally featureless, whereas Neptune
has visible cloud bands.
 
Though they have similar masses and are both far
from the sun, Uranus and Neptune, appear quite different from each
other.
For reasons currently unknown, Neptune produces far
more internal heat than Uranus. It is thought that this extra energy is
what causes the difference in appearance. But if we only had the mass
and orbital distance of these two planets to go on, we would never know
which happened to be more hazy than the other. So although we can say
that some extrasolar planets will be rendered featureless by thick haze,
we can't know which from the limited information we currently have.
COLOR, ALBEDO AND TEMPERATURE
Uranus may be featureless, but
it certainly has color. Its turquoise hue can be seen even from
Earth-bound telescopes. Neptune is deep blue. Saturn's cloud bands have
the color of butterscotch. Jupiter has bands of white and brown,
highlighted by storms and clouds of red and orange and yellow. Each of
the four gas giants of our solar system is painted in a unique set of
hues. What causes this wide range of color? Cloud composition, unstable
condensates created by stellar radiation, and the absorption of light by
"clear" air. But each of these is, in turn, dependant on temperature. In
Albedo and Reflection Spectra of Extrasolar Giant Planets (Sudarsky
et al. 2000), giant extrasolar planets are divided into five different
classes based on temperature.
CLASS I: Ammonia Cloud Jovians, Analogs of Jupiter
and Saturn
Jupiter receives far less heat from the sun than does
the Earth. Earth's clouds are made of water ice crystals. These crystals
are white, so clouds on Earth appear white. But Jupiter is colder. The
clouds there are mostly made of ice crystals of ammonia, which freezes
at a much lower temperature than water.
But ammonia clouds are still white. So what causes the
other colors of Jupiter's clouds? The brown, red, yellow, and orange
clouds turn out to be made of ammonia ice as well, but ammonia ice
stained by more complex condensates of carbon and sulfur. These
condensates are unstable and break down quickly, so they must be
continually created, probably by ultraviolet radiation from the sun, to
persist. But the process is not well understood.
The Jupiter-like planet of Gliese 777A
Saturn gets its butterscotch color from similar
processes, only its color variations are muted by Saturn's denser haze
layer. It's likely that any extrasolar gas giants at temperatures less
than 150 Kelvin will share this yellow-brown color. Planets like 55
Cancri d and Gliese 777A b fall into this category. Such planets would
have bands of brown, gold, and white and have albedos similar to that of
Jupiter.
Calculations of ammonia cloud jovian atmospheres
suggest that, free of chemical stains caused by UV or other radiation,
such worlds would have albedos of about 0.57 if in orbit around a G2 V
star like the sun. Simulations of Jupiter using the same model and
factoring in chemical stains give an albedo of 0.4, quite close to the
actual value of 0.35.
Note: The albedos given herein are Bond albedos. Bond
albedo gives the ratio of total stellar radiation reflected to that
which is absorbed. Bright planets will have albedos close to 1. Dark
planets will have albedos close to 0. Bond albedo is an important factor
to calculating the temperature of planetary atmospheres.
CLASS II: Water Cloud Jovians
The majority of extrasolar planets have been found at
higher temperatures than the ammonia cloud jovians. What do planets that
are only a bit hotter than Class I look like? Gas giant planets with
temperatures greater than 150 K, but less than 350 K do not exist in our
solar system, but we can extrapolate from the terrestrial planets in
this range. Both the Earth and Mars have white clouds of water ice.
Jupiter also has clouds of water ice in the warmer and deeper layers of
its upper atmosphere. So we can assume that gas giants orbiting at
similar distances to Earth and Mars are likely to have clouds of water
ice as well, so they would appear, by and large, white.
Water
cloud jovians like HD 28185 b are probably covered in bright clouds of
water ice, just like the Earth, and may have Earth-like moons. But,
although we would find the rains on such giant planets familiar, the air
itself is likely to be unbreathable methane.
The colder water cloud jovians may have areas of the
atmosphere, such as the poles, still frigid enough to support ammonia
clouds, although the lower levels of stellar radiation received by the
poles may make any brown hydrocarbon stains fainter than what is seen on
Jupiter. The warmer water giants may form yellow condensates of
sulfurous compounds, as on Venus, and may even have clouds of sulfuric
acid. Where the clouds are at the very top of the atmosphere, the
surface will appear bright white. But regions where the clouds form a
bit lower will be obscured by layers of gas, primarily methane, above
them. Methane scatters blue light weakly, so these deeper cloud regions
will have a slight bluish tinge.
Ignoring condensates, these planets would have albedos
as high as 0.81 around sun-like stars. Thus, planets like 47 Ursae
Majoris b or upsilon Andromedae d would be bright with bands of white
and muted blues.
CLASS III: Clarified Jovians
For planets between 350 and 900 Kelvins, clouds of
water ice cannot form, and there is no other stable condensate
available. Such worlds would have entirely clear skies, rendering the
entire planet a deep blue for the same reason that the sky on Earth is
blue.
Although the air around us may appear transparent, it
actually scatters blue light very weakly. The more air you look through,
the bluer things seem. When you look into the sky, you are seeing the
black of space filtered through miles of slightly blue air. The net
affect is a blue sky. This phenomenon is known as Rayleigh scattering.
upsion Andromedae c is mostly blue
and cloudless, but may be cool enough at the poles to sustain some
clouds of water ice.
The upper atmospheres of gas giants are mostly methane
gas, which weakly scatters blue light just like air on Earth does. If
you were looking down into a vast ocean of methane, it would appear
blue. This is exactly what we would see if we looked at a cloudless gas
giant, a deep blue world highlighted with white clouds wherever the
temperature fell low enough. Such planets may even have "ice caps" of
white clouds at the cooler poles.
A Hubble portrait of Mars. Note the bluing around
the edges of the Martian disk |
True color photos of Saturn's northern hemisphere
as seen by Cassini |
The blue color of Rayleigh scattering can be seen on
planets of our own solar system. Mars and Jupiter appear bluer around
the edge of their disks because at the edges light must travel through
more of the atmosphere to reach the viewer. A dramatic example of
Rayleigh scattering can be seen in recent pictures of Saturn taken by
Cassini. Right now it is winter in Saturn's northern hemisphere, and the
shadow of the planet's rings cause even less warmth to reach the
planet's surface. The net effect of this winter cooling is that it is no
longer warm enough for clouds of ammonia to form in the north. The
resulting clear skies make the entire northern hemisphere of the planet
appear blue.
55 Cancri b is at a temperature where the entire
atmosphere is too hot for clouds to form, rendering the planet a deep
blue.
So, surprisingly, very warm planets would look quite
similar to their more distantly orbiting cousins, like Neptune. Class
III extrasolar planets appear blue because it is too hot for clouds to
form, where Neptune appears blue because it is too cold for clouds to
form. Because of this deep blue coloring and lack of reflective clouds,
these worlds would have low albedos of around 0.12 around sun-like
stars. 55 Cancri b and rho Coronae Borealis b are examples of clarified
jovian planets.
CLASS IV & V: Hot Jupiters
The first extrasolar planet found orbiting a sun-like
star was 51 Pegasi b. Its discovery was a triumph, but its observed
properties sent shockwaves through the astronomical community. For 51
Pegasi b was almost as massive as Jupiter, but it orbited at only 0.05
AUs from its sun, far closer than Mercury. At the time, theories of
planet evolution allowed giant planets to form only at distances greater
than several AU, at about the distance of Jupiter from the sun. To have
51 Pegasi b orbiting so close to its star made astronomers re-examine
their theories. It wouldn't be so bad if 51 Pegasi b was a rare case.
But the discovery of planets orbiting 55 Cancri, tau Bootes, and many
other sun-like stars have shown these so called "Hot Jupiters" are quite
common, although it should be noted that current planet detection
methods do favor the discovery of close in massive worlds.
51 Pegasi b's surface temperature has been calculated
at over 1300 Kelvin. Other Hot Jupiters could reach over 1500 Kelvin.
And these calculations only take into account heating by the nearby
parent star. Like Jupiter itself, Hot Jupiters are probably radiating a
great deal of heat internally. Tidal forces caused by the close
proximity of their parent star would heat such planets even more,
although many Hot Jupiters were probably tidally locked by their stars
long ago, in which case such stellar tides would no longer be
significant.
What would a Hot Jupiter look like? Such worlds would
be so hot that their lower atmospheres expand and become incandescent,
glowing faintly red. This glow might even be visible from space,
especially on the planet's night side.
More massive than most Hot Jupiters,
HD 73256 b's large gravity prevents its atmosphere from leaking into
space, despite the extreme temperatures. The planet is not hot enough
for silicate clouds to form at the top of the atmosphere. Instead, the
clouds are buried deep beneath an obscuring layer of sodium rich gas,
darkening the entire planet.
But there is another factor. At such high
temperatures, alkali metals become vapor. The most common of these is
sodium, which is gaseous above about 1150 K. Normally, sodium absorbs
narrow ranges of yellow light. But, in the high pressure atmosphere of a
Hot Jupiter, atoms of gaseous sodium constantly collide with hydrogen,
causing the absorption ranges of the sodium to widen. This effect is so
pronounced that significant amounts of sodium in a Hot Jupiter's
atmosphere can darken the entire planet, causing it to look a dark
grayish brown. Recently, the presence of sodium gas has been detected in
the atmosphere of HD 209458, giving some confirmation to the idea that
Hot Jupiters are indeed dark worlds. Such dark worlds are called Class
IV and have extremely low albedos of about 0.03 around sun-like stars.
The gravitational pull of Osiris, the planet of HD
209458, is too weak to prevent super heated hydrogen from escaping into
space, forming a comet-like tail behind the planet.
It is likely that the hottest and least massive of the
Hot Jupiters also experience another unusual feature: atmospheric
evaporation. On extreme Hot Jupiters, particles at the surface may be
heated enough to escape the planet's gravity altogether and leak into
space. Such planets would be enshrouded in tenuous envelopes of bluish
gas far above their main surfaces. Just this sort of extended gas
envelope or "corona" has been found around the transiting Hot Jupiter of
the star HD 209458.
51 Pegasi b may be hot enough and of
low enough mass to allow silicate clouds to form. Such clouds would
float high above the sodium haze that would otherwise render the entire
planet dark and unreflective.
For worlds that are hotter than 1500 K or that are a
bit cooler but significantly less massive than Jupiter, clouds of
silicates can form close to the top of the atmosphere. Instead of dark
worlds, these Class V planets are nearly as bright as the ammonia cloud
jovians. But on these worlds, the clouds are vaporized rock and it rains
sand. Some confirmation of the brightness of Class V giants was found in
2005, when the infrared signature of HD 209458 b and another transiting
Hot Jupiter, called TrES-1, was detected directly. The Bond albedo of
the 0.75 Jupiter mass TrES-1 was calculated at around 0.3, indicating
that part of the planet must be shrouded in clouds of silicates. Such
worlds, with atmospheres hot enough to vaporize sulfur, sodium, and
possibly even lead, would be totally unlike anything in our own solar
system.
Methane Cloud Jovians, Analogs of Uranus and Neptune
And what about distant Uranus and Neptune? Far colder
than Class I giants like Jupiter and Saturn, the clouds of these planets
are dominated by methane, not ammonia. Except for the occasional icy
wisp, it is too cold on either Uranus or Neptune for clouds to form at
the surface. However, clouds do form at lower depths where the
atmosphere is warmer. We see little of such clouds because they lie deep
below layers of obscuring methane. Methane absorbs red light and
scatters blue, thus the two outer planets are dominated by blue and
turquoise hues.
Astronomers have not yet found planets with large
orbital distances and comparatively small masses like Uranus and
Neptune. But it is assumed that such worlds nevertheless exist. If they
do, they probably have much in common with these two frigid worlds.
TIDAL LOCKING
As previously mentioned, Hot Jupiters have a high
probability of being tidally locked. For an example of tidal locking, we
need look no further than our own moon. Because it rotates at the same
rate as it orbits the Earth, the Moon always presents the same face to
us. The far side of the moon was totally unknown until probes were sent
to beam back pictures and data to us. The reason we only ever see one
side of the moon is that tidal forces long ago altered the rotation of
the moon to match its revolution. For each orbit the moon takes, it
rotates once in the same direction. The moon is slowly returning the
favor. The Earth's day is gradually slowing. In the distant future, the
same side of the Earth will always face the moon.
The moon is in a 1:1 tidal lock. For each orbit, it
rotates once. Jupiter has tidally locked Io, Europa, Ganymede, and
Callisto in the same way. The sun has similarly locked the rotation of
Mercury, but not completely. Mercury is in a 3:2 tidal lock, rotating
three times every two orbits. The closer a planet is to its star, the
more likely it will become tidally locked over time. Hot Jupiters are so
close to their stars that it is likely that many of them have been
locked by their stars. In the skies of such worlds, the star would never
move, but would be forever fixed and motionless.
Locked gas giants would have slow rotation rates. Most Hot Jupiters orbit
their stars in just a few days. Assuming these worlds are locked, their
rotations would also be a few days. Slow rotation means it's likely that
Hot Jupiters have little or no magnetic fields. Tidally locked worlds
might also have cooler interiors than non locked worlds, as they would
no longer feel tidal stresses from their star. The atmospheres of such
worlds would also be affected.
If a tidally locked world had a thin atmosphere that
was unable to distribute heat evenly across the planet, then the frigid
temperatures on the perpetually dark side would cause the atmosphere to
freeze out entirely. But new atmospheric models suggest that Hot
Jupiters have atmospheres thick enough to distribute heat relatively
evenly across the planet. What's more, the intense heating at the
starward pole, the spot on the planet that receives the most radiation
from the star, would drive extremely powerful winds. The planet itself
may rotate only once every few days, but the upper atmosphere may rotate
much faster, further distributing heat to the dark side.
The planet Venus is in exactly this situation. Its
rotation is very slow. In fact, the planet is rotating slowly
backwards, once every 243 days. Yet the planet's atmosphere
circulates around the planet once every four days in the opposite
direction. It is currently believed that Venus' rotation has been
slowed, and eventually reversed, not by tidal locking, but by friction
against it thick atmosphere.
Though calculations indicate that most Hot Jupiters
are probably tidally locked, there are some factors that would prevent
locking. Hot Jupiters with high orbital eccentricity, would not have
circular enough orbits to allow 1:1 locking, although other locking
ratios would be possible. Mercury, for instance, is in a 3:2 lock
instead of a 1:1 lock due to its orbital eccentricity. The gravitational
influence of other massive planets may also hinder locking. Finally,
extremely young Hot Jupiters may not have had enough time to become
locked.
SEASONS -- ECCENTRIC GIANTS
Many of the extrasolar planets discovered so far
circle their stars in highly elliptical orbits. These orbits take their
planets in close to their parent stars only to be flung out and away
from them. Slowly these planets reach their maximum distance only to
fall back again towards their stars. The further away the planet gets,
the cooler it becomes. Over the course of the year of such worlds, the
heat received from the star can vary widely.
Due to its near circular orbit and low axis tilt,
Jupiter has no observable seasons. But seasonal variations have been
observed on both Saturn and Uranus. On Saturn, Cassini found that the
axis tilt and the shadow of the planet's rings conspire to plunge much
of the winter hemisphere in frigid darkness. Too cold for ammonia clouds
to form, Saturn's winter hemisphere looses its butterscotch hue and
becomes blue.
A similar situation is seen on severely tilted Uranus.
In the late 1990's, the northern hemisphere of Uranus was coming out of
a twenty year season of darkness. The Hubble Space Telescope detected
bright clouds forming in the warming atmosphere. The clouds and banding
discovered by the HST were in stark contrast to the featureless globe
found by Voyager 2 a decade earlier, proving the dynamic nature of the
planet.
All of the seasons in the solar system, from the
familiar rhythms of the Earth to the ring shadowed winters of Saturn,
are caused primarily by axis tilt. In northern summer, the north pole of
the Earth points towards the sun, heating up. In winter, the north pole
points away from the sun, cooling down. Storms erupt during the spring
on Earth, brought on by the warming of one pole and the cooling of the
other. Dust storms caused by spring on Mars can grow so large that they
engulf the planet.
We would expect extrasolar planets with significant
axis tilts to have similar seasonal variations. We don't currently know
the axis tilt of any extrasolar planet, but it is likely that some will
have tilts extreme enough to have seasonal variations something like
Earth's or Saturn's. Water ice clouds on a tilted clarified jovian
planet may migrate south during northern summer only to return north in
the winter. During these seasons, the planet might appear half-blue,
half-white. The white clouds of more distant water cloud jovians may
become darker in the hemisphere experiencing winter. Storms like the
Great Red Spot on Jupiter may erupt as winter transitions into summer.
In the case of eccentric giants, extrasolar planets
with highly elliptical orbits, seasons would affect the entire planet
simultaneously, instead of having opposite affects in opposite
hemispheres. As the planet travels close to the star, the temperature
would increase worldwide. As it drifts out away from its star, the
entire planet would cool down. A highly eccentric planet might be devoid
of water ice clouds when it is close to its star, only to gain more and
more cloud cover as it became more distant. At its greatest distance,
such a planet might be totally covered in clouds, and those clouds might
be cold enough for brown hydrocarbon stains to form.
HD 80606 b has one of the most eccentric orbits known
for an exoplanet, traveling from 0.03 AU to 0.84 AU from its star. This
extreme orbit takes it from Hot Jupiter temperatures close to the star
to water cloud jovian temperatures at the most distant point in its
orbit. Every 112 days, the surface of the planet may transition from a
cloudless deep blue to the brilliant white of global water ice clouds
and back again.
THE STAR -- TEMPERATURE VS
ORBITAL DISTANCE
The most commonly noted parameters of an exoplanet are
its mass and its orbital distance. This is fine when assuming that the
planet's star is much like the sun. But many of the exoplanets orbit
stars that are brighter or dimmer than the sun. This means that the
effective temperature of such planets may be greater or less than what
they would be if they were orbiting in our own solar system. And in the
end, temperature is more important to understanding a planet than
distance.
An excellent example of this is found in the Gliese
876 system. Here, astronomers have discovered two gas giant planets. One
orbits at a mean distance of 0.13 AU, the other at 0.23 AU. Both of
these orbits fit easily within the orbit of Mercury, so one could assume
that the inner planet is a Hot Jupiter and that the outer is probably a
blue clarified jovian mostly devoid of clouds.
The problem is, the parent star of this system is a
red dwarf, both dimmer and cooler than the sun. Calculating the
effective temperatures of the star's planets reveals them to be
relatively cool. Temperature wise, the inner planet is in a loosely
Earth-like orbit, and the outer is equivalent to Mars. Given this, both
worlds are probably water cloud jovians.
RINGS AND MOONS
We really don't know which planets will have rings or
moons, and will probably not know until they are directly imaged.
However, all four of the gas giants of our own solar system posess
systems of both rings and moons, and all of the other planets save
Mercury and Venus have moons. So it seems likely that many exoplanets
will have rings and most will have moons.
Unfortunately, we can know little about extrasolar
rings other than what their composition would be if they exist. At high
temperatures, they would be dusty or composed of relatively dark rocky
material. Farther out, they would become more icy and brighter, like the
rings of Saturn. The size and complexity of such rings would depend on
how the rings had formed and how old they were. Dust rings formed by
outgassing moons or multiple small collisions would be dark and tenuous.
The destruction of a large moon might generate prominent bright rings.
These sorts of events could happen independent of the mass or orbital
distance of the host planet. So other than the material they are
composed of, the nature of extrasolar ring systems is almost entirely
speculative. Certainly, many planets possess rings. But which ones, we
cannot know based on the current data.
The rings of 55 Cancri d.
Extrasolar moons are just as speculative. But the
example of Mercury and Venus does offer us limits. Given stellar tidal
forces and slow rotation rates, planets close to their stars probably
lack moons of any permanence. Hot Jupiters probably formed far from
their stars and may have once had a retinue of moons, but these moons
would have long ago been lost or destroyed as their parent world drifted
closer to their star. Today, such Hot Jupiters may possess the
occasional captured asteroid, but the orbits of such companions would be
unstable and would eventually lead to their destruction. Just like their
ancient predecessors, the captured moons would either break up within
the planet's Roche limit or impact the planet's surface.
But what about those more distant planets that are
lucky enough to retain a family of moons? What would they be like? Two
factors can give us a clue. One is the mass of the planet. In general, a
more massive planet will have the potential for more massive moons.
Jupiter has four large moons that would easily qualify as planets in
their own right if they were orbiting the sun directly. Saturn has a
host of smaller moons and one large moon, Titan, that rivals the big
moons of Jupiter. Uranus and Neptune both have smaller moons, but many
of them are still massive enough to be spherical and one, Neptune's
Triton, is larger than Pluto. Given this sort of progression, it seems
safe to assume that some of the more massive exoplanets might have moons
as massive as Mars or even the Earth.
But this kind of speculation should be taken for what
it is, speculation. After all, the Earth's own moon dwarfs the moons of
Uranus even though Uranus dwarfs the Earth. It seems quite possible that
some exoplanets will have only tiny asteroid like companions despite
their great mass, where others may actually be double planets, where a
significant portion of the mass we can detect is actually contained
within a supermassive moon. There may very well be worlds as massive as
Neptune or even Saturn in orbit around some of the more massive
exoplanets that have been discovered. There is just no way of knowing.
One aspect of potential extrasolar lunar systems can
be calculated, however. The maximum distance for which a lunar orbit
will be stable around a planet is the point at which the gravity of the
planet begins to give way to the gravity of the star. This gravitational
sphere of influence is called the Hill sphere. The more massive a planet
is, and the more distant it orbits its star, the larger its Hill sphere
will be. Despite Jupiter's great mass, the Hill sphere of Neptune is
actually larger, due to the eighth planet's greater distance from the
sun.
The Hill sphere puts constraints on the likelihood a
given planet will be able to capture wandering asteroids or planetoids.
The Hill sphere of Jupiter is 0.355 AU, where the Hill sphere of 51
Pegasi b is a mere 400000 kilometers. Thus it is much less likely for 51
Pegasi b to capture an asteroid than Jupiter.
Assuming that a planet has a moon as massive as Io,
Ganymede, Mars, or even Earth, what would such moons be like? This
question brings us to the second factor, the effective temperature of
the parent planet. Fortunately, our own solar system offers ample
examples of small bodies at various temperature ranges. Moons of
clarified jovians and water cloud jovians would be barren and rocky like
Mercury or the moon. If they are massive enough, such moons might
possess atmospheres of carbon dioxide and nitrogen. Water cloud jovian
moons may hold frozen lakes of water ice on their surfaces. Earth mass
moons of water cloud jovians may even be capable of maintaining thick
atmospheres, oceans of water, and life.
Farther out, moons would begin to freeze and contain
more and more water ice like the moons of Jupiter. Water ice gradually
replaces rock as the primary component of moons around planets of Saturn
like temperatures. Recent discoveries on Saturn's moon Titan reveal a
world with mud flats and rivers much like the Earth, but where water ice
takes the place of stone and liquid methane takes the place of flowing
water. At even greater distances from the star, methane freezes solid
and nitrogen becomes liquid. Neptune's moon Triton has nitrogen geysers
that explode over plains composed of methane and water ice.
Moons as massive as the Earth will be tectonically
active for billions of years due to heat stored in their molten cores.
But smaller moons will tend to be inert. Yet many of the moons in our
own solar system are far from quiet, despite their small mass. The ice
geysers of Triton have already been mentioned. Saturn's moon Enceledus
appears to be contorted by tidal stresses and few craters dot its
relatively young surface. Europa as well is cracked and craterless and
probably possesses an under ground ocean, miles thick, kept liquid by
friction generated by the tidal forces of its sister moons and its giant
parent, Jupiter. Another moon in from Europa brings us to Io, the most
volcanically active object in the solar system.
An Earth-like moon orbits a water cloud jovian.
Given all these examples, it stands to reason that
extrasolar moons in close orbits to their parent planets will be
geologically active due to tidal forces just like their kin in our own
solar system. There will surely be a multitude of barren and cratered
lumps of rock or ice orbiting the various exoplanets. But around some of
them, there may also be rocky worlds as massive as Mercury covered in
lava flows and sulfur seas due to the gravitational influence of their
clarified jovian host planets. Perhaps somewhere there are moons as
massive as Earth with frozen surfaces above deep oceans of water through
which toxic fumes bubble from undersea fumaroles, eventually reaching
the surface to form thick atmospheres of sulfur and carbon dioxide. And
perhaps, where conditions are perfect, there are moons where water can
exist as liquid in the open air and form oceans, where the radiation
from the parent star and planet is kept at bay by a strong moon-wide
magnetic field, and where the atmosphere is in a strange state of
imbalance indicating that here, a chemical process has been set in
motion that uses organic material to generate energy. A chemical process
called life.
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