ASTRONOMERS PROBE ALIEN SKIES
Astronomy (March 2002, pp. 18-19)
On June 6, 1761, while observing a rare transit of Venus across the disk of the Sun, Russian
astronomer Mikhail Lomonosov detected the first planetary atmosphere beyond Earth. Now, 241
year later, a team of astronomers has used the Hubble Space Telescope to probe the atmosphere
of a distant world 150 ly outside our solar system as the planet traversed the face of its host star.
Canadian astronomer Dave Charbonneau of Caltech led the team, which discovered
sodium in the atmosphere of a planet orbiting the solar-type star HD 209458. While the
discovery comes as no surprise – Charbonneau and his colleagues expected to find sodium – it
points to an exciting future when astronomers will be able to study the chemical (and perhaps
life-sustaining) characteristics of extra-solar planets.
The planet belongs to a bizarre class known as hot Jupiters. It contains 70% the mass of
Jupiter, yet races around its host star every 3.5 days at a mere 6.7 million kilometers – less than
1/20-th the Earth-Sun distance (i.e., 0.05 AU). At that distance, the star bakes the upper cloud
decks to a blistering 2,000o F.
In 1999, Charbonneau’s team co-discovered the gas giant planet, which regularly transits
its star. With the hope of being able to detect atmospheric gases like sodium, the team observed
three separate transit events in April and May 2000 with Hubble’s imagining spectrograph. It
found that the amount of starlight absorbed by sodium atoms varied as the planet ventured in and
out of transit. Moreover, the team has since determined that sodium in the atmosphere of HD
209458’s planet was indeed stealing some of the star’s light as it crossed the star’s disk.
“Their discovery of the sodium spectral line in the atmosphere of the planet orbiting HD
209458 is absolutely secure. I have no doubt that this result is correct,” says University of
California, Berkeley astronomer Geoff Marcy, who leads the team that has discovered the
majority of the 80 or so planets that have been found outside the solar system.
Charbonneau expected to find sodium because it’s easy to detect and because it is common
in the atmospheres of low-mass stars and brown dwarfs. But surprisingly, his team found less
sodium than expected. He suspects that either high cloud decks are blocking the star’s light
before it can reach lower, sodium-rich layers, or the sodium has combined with other molecules
to form sulfides. “Observations in the next year will allow us to determine while is the dominant
effect,” says Charbonneau.
Charbonneau’s team, which includes Tim Brown of the National Center for Atmospheric
Research, Robert Noyes of the Harvard-Smithsonian Center for Astrophysics, and Ron Gilliland
of the Space Telescope Science Institute, has applied for more Hubble time to look for water
vapor in the planet’s atmosphere. In addition, ground-based transit observations could detect
methane (CH4) and carbon dioxide (CO2). “Now that we have this tool, we can look in other
portions of the spectrum for a whole host of other atoms and molecules we expect to be there,”
says Charbonneau.
Comments
1. Astronomers have long suspected that planets exist around other stars. It wasn’t until the
1990’s, however, that the technology advanced to the point that these extra-solar planets could
be detected. To date the existence of these planets (and 80+ have been discovered with more
discoveries being announced monthly) has been inferred indirectly. That is, they have not
been seen directly through a telescope or in a photograph.
Instead, astronomers have detected these planets using two different methods:
a)
Velocity effects due to the planet’s gravitational tug
on its parent star.
b)
Light effects when the planet passes in front of its
parent star.
Astronomers are working on technology that will allow them to detect extra-solar planets
directly. There’s a race underway among different research teams who are developing different
techniques. It’s only a question of time before one of the teams is successful.
In the past few months, astronomers were successful in photographing a brown dwarf
positioned 20 to 30 AU from its parent star. That’s the same distance as Uranus and Neptune is
from the Sun.
A brown dwarf is not a planet but then again it’s not a star. It forms in the same
manner as does a star by a collapsing cloud of gas and dust in interstellar space;
however, it has a mass less than 100 times that of Jupiter, meaning that the core
temperature is not high enough to sustain thermonuclear reactions (i.e., hydrogen
burning) of a star like the Sun. In essence, a brown dwarf is a failed star. Astronomers
believe that are billions upon billions of these “star wanna be’s” populating the Milky
Way Galaxy.
It is just about as difficult to photograph a brown dwarf that close to its parent star as it
is a planet. This means that astronomers are right on the verge of photographing a
planet. So…stay tuned!!
2.
One of the interesting things that happened in the original discoveries of extra-solar planets
is the discovery of “hot Jupiters.” These are planets with the mass of Jupiter (300 M ) but
in Mercury-like orbits (i.e., 0.05 to 0.5 AU). This was a completely unexpected result.
Astronomers believe that planets with the mass of Jupiter must form at a large distance from
their parent star along the “snow line.” In the Sun’s case the “snow line” (i.e., the location
in the solar system where water changes from liquid to solid) is at 5 AU, precisely the
location where we find Jupiter today.
So, if the “hot Jupiters” formed out at the parent star’s “snow line” or beyond then somehow
they must have move or migrated from that distant location to right next to the parent star.
Exactly what caused this migration is currently a topic of active research in planetary
science.
Jupiter is the most massive planet in the solar system with
318 times the mass of Earth. It is believed to have been the
first of the planets to accrete from the solar nebula.
Snow Line
1 AU
Jupiter
Earth
5.2 AU
In order for it to grow to its enormous size, astronomers
believe that Jupiter along with Saturn, Uranus, and
Neptune formed at the snow line where they accreted large
amounts of ice along with rock.
Since it was the first of the planets to form, it became the
most massive (i.e., it had no competition for the ice and
rock). It also formed when there was lots of gas remaining
in the solar nebula because Jupiter swept up some of this
gas in order to form its extensive atmosphere that is tens of
thousands of kilometers thick.
1 AU
A “hot Jupiter” is found within 1 AU of its parent star.
This is well within the snow line. The question arises:
Snow Line
How did such a large rock and ice
planet form next to the parent star
so far from the snow line?
“Hot
Jupiter”
Most astronomers think that it didn’t. Most think it
formed at the snow line then moved (i.e., migrated)
from its distant position to right next to its parent star.
In fact, there’s evidence in our own solar system that the giant planets Jupiter, Saturn, Uranus,
and Neptune formed at (or near) the snow line then migrated to the their current orbital positions.
During the final stages of planetary accretion, Jupiter migrated inward toward the Sun while
Saturn, Uranus, and Neptune migrated outward away from the Sun. In Neptune’s case it may
have migrated as much as 7 AU from where it was at the time it formed where it’s gravitational
field scattered the Kuiper Belt objects including Pluto.
Atoms and molecules produce light (i.e., electromagnetic radiation), the colors or
wavelengths of which are unique to the type of atom or molecule. For example, the
hydrogen atom produces a red light at a wavelength of 6563 Å (1 Å= 1 x 10-8 cm is called
an Angstrom). This color is an identifying fingerprint for hydrogen. If you see this color
in the atmosphere of a planet or a star then you know that hydrogen is present there.
Sodium was detected in the atmosphere of the “hot Jupiter.” Sodium produces a yellow
light at two wavelengths that are side by side. It is a particularly easy color for
astronomers to detect. As the planet transited in front of HD 209458 light from the parent
star shone through the planet’s atmosphere exposing the presence of the two sodium
wavelengths.
Actually what happened is that the yellow sodium light originated in the photosphere of
the parent star then diminished (absorbed) by the star’s overlying cooler atmosphere. As
the “hot Jupiter” transited the star, its own, even cooler atmosphere, which also contains
sodium, absorbed more of the light from the star making the colors more diminished still.
When the “hot Jupiter” completely passed in front of the star, the colors returned to their
normal brightness level thereby signaling the presence of sodium in the planet’s
atmosphere. The amount of dimming can be used to estimate the quantity of sodium in
the planetary atmosphere.
Water (H20), methane (CH4), carbon dioxide (CO2), and oxygen (O2) all have their own
unique set of light wavelengths (colors). Astronomers are now searching the
atmosphere of the “hot Jupiter” for these wavelengths in an effort to detect these
molecules within the planet’s atmosphere.
HD 209458
“Hot Jupiter”
·
·
Prior to transit
During transit
Intensity of sodium yellow wavelengths
·
After transit
3.
The article states the following:
The planet belongs to a bizarre class known as hot Jupiters. It contains 70%
the mass of Jupiter, yet races around its host star every 3.5 days at a mere
6.7 million kilometers – less than 1/20-th the Earth-Sun distance (i.e., 0.05 AU).
At that distance, the star bakes the upper cloud decks to a blistering 2,000o F.
Calculate the orbital speed (km/s) of the planet in its orbit about its parent star HD
209458.
Circumference of a Circle
D  2  R
V 
D
t
Orbital Velocity or Speed
D  2  R  2  6.7 x10 6  4.21x10 7 km
D
4.21x10 7
V  
 139 km/s
t
3.5  24  60  60
Convert the planet’s cloud deck temperature to Celsius (oC) and Kelvin (K).


5 o
5
F  32  2000  32  1093 C
9
9
K  o C  273  1093  273  1366 K
o
C
Calculate the approximate radius (km) of the “hot” Jupiter.
1
3
 3  0.70  1.90 x10
 3 M 
  
R  
4  1330
 4   

27
1
3

  6.204 x10 7 m  6.204 x10 4 km

where M is the mass of the planet (kg) and ρ is the average density (kg/m3).
Assume the “hot Jupiter” around HD 209458 has the same density as Jupiter:
MJupiter = 1.90 x 1027 kg
ρJupiter = 1330 kg/m3
FOURTH PLANET OF A PULSAR
Sky & Telescope (March 2002; p. 24)
The first planets ever discovered beyond the solar system (in 1991) were orbiting not a
normal star but a fast-spinning millisecond pulsar 1,000 ly away in Virgo. So smoothly
and perfectly does the pulsar spin that the tiny motions induced in it by the orbiting
planets cause measurable speedups and slowdowns in the arrival times of its radio pulses.
This method of detecting planets is closely analogous to the method used for normal
stars: looking for slight, periodic shifts in the colors in their light (called redshifts and
blueshifts).
But the pulsar method is more sensitive. The giant planets that have been found orbiting
dozens of Sun-like stars weigh from 50 to 3,000 times Earth’s mass, enough to provide
fairly strong gravitational tugs. On the other hand, the three objects that were found
orbiting the pulsar PSR B1257+12 have minimum masses of only 3.4, 2.8, and 0.015
times Earth. The last is about equal to the Moon. (Note: The actual masses are
probably a little larger since the planet’s orbits are probably titled by some unknown
angle to our line of sight.)
Now it turns out there’s a fourth object orbiting PSR B1257+12. Aleksander Wolszczan
(Pennsylvania State University), who discovered the first three, and two colleagues report
that after 11 years of timing the pulsar with the Aercibo radio telescope in Puerto Rico,
they have clear evidence that an even smaller body with a minimum mass of 0.005 Earth
(a third of the Moon) swings far outside the other three in an eccentric orbit with a period
of about 3.5 years. This gives the system the most planets known to orbit any star beyond the
Sun.
A pulsar is a tiny, super-dense neutron star left by a supernova explosion. The planets almost
certainly could not have survived the explosion not to mention the subsequent spin-up of the
pulsar to 161 rotations per second by matter spiraling in from a now-vanished companion
star. So astronomers assume the planets formed later.
Only one other pulsar, B1620-26, shows solid evidence of having a planetary companion.
And that’s a multi-Jupiter-mass giant in a much larger orbit around a close binary consisting
of a neutron star and a white dwarf located in a globular cluster. The companion may have
been drifting freely in space before the close pair happened to pick it up.
The article says the following about the presence of a planet around a pulsar, a truly
unexpected and puzzling finding.
A pulsar is a tiny, super-dense neutron star left by a supernova explosion.
The planets almost certainly could not have survived the explosion not to
mention the subsequent spinup of the pulsar to 161 rotations per second by
matter spiraling in from a now-vanished companion star. So astronomers
assume the planets formed later.
First of all, the pulsar is produced in a tremendous supernova explosion of a star much
more massive than the Sun. This explosion is of such ferocity that any planets present
prior to the explosion would be destroyed by the explosion. A supernova can release as
much energy as all of the other stars combined in a galaxy (i.e., 100-150 billion stars).
Also, the pulsar spins up (i.e., increases the rate at which it rotates) as it accretes material
from a companion star. The inflow of this material from the companion would have had a
catastrophic effect upon any planets. These are the reasons astronomers assume the
planets formed after the formation of the pulsar.
Globular clusters are approximately a few thousand light years in diameter and contain
100,000 to 1,000,000 stars. They are old structures dating 10-13 billion years in age. The
Milky Way Galaxy has 150 or so of these clusters orbiting its center Sgr A*. The origin
of these clusters is currently a major field of research in astrophysics.
The density of stars in a globular cluster is greater than in the Galaxy’s spiral arms where
the Sun and Earth reside. It is believed that from time to time stars in globular clusters
actually collide and merge (i.e., the so-called “blue stragglers”). The article says the
following about a pulsar-white dwarf system with a large Jupiter-like planet:
Only one other pulsar, B1620-26, shows solid evidence of having a planetary
companion. And that’s a multi-Jupiter-mass giant in a much larger orbit
around a close binary consisting of a neutron star and a white dwarf located
in a globular cluster. The companion may have been drifting freely in space
before the close pair happened to pick it up.
In other words some astronomers think that there was collision between stars that merged
the Jupiter-like planet with the pulsar-white dwarf system. This, of course, is speculation.
Astronomers are still researching how planets appear around pulsars.