Planetary Transits
By Denis Sullivan and Tirikatene Sullivan
Planetary transit events have been important historically, but they have recently again important. In the past, transits of the planet Venus across the face of the Sun have been scientifically important, while the current interest is in using transit events to detect and obtain information about planets outside our solar system. This article briefly explores the background to these phenomena and also provides a description of at Mauna Kea Observatory in Hawaii where we obtained transit data on a so far unique planet.
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Venus transits
The transit of Venus event that will occur on 8 June this year (2004)
involves the planet Venus moving along its orbital path and crossing
our line of sight to the Sun. These solar system events occur because
the orbit of Venus is closer to the Sun than the Earth, and the orbital
planes of the Earth and Venus are roughly aligned; but the events are
also relatively rare due to the fact that the orbital planes are not
the same. Transit events occur in pairs (separated by 8 years) and
these events repeat at intervals of about 120 years.
Venus transits were of significant scientific interest historically as
they were an important part of one method of measuring the distance
between the Sun and the Earth. If we know this distance (the
astronomical unit, or AU) then we can deduce the size of the whole
solar system and also measure the distances to the nearest stars and
beyond.
The combination of Newton's laws of motion and gravity enable us to
accurately determine the relative sizes of the various planetary
orbits around the Sun, by measuring their respective orbital periods
(their years). The relationship used is more commonly known as
Kepler's third law of planetary motion (which Newton actually used in
reverse to formulate his more comprehensive theories). However, if we
want to know the actual size of the solar system then we require the
dimensions of one of the orbits in terms of distances measured on
Earth. For the Venus transit events, the effect of parallax means
that observers at different positions on the Earth will see slightly
different transit paths across the face of the Sun. These can be
quantified by accurately timing the various ingress and egress events.
All the information gathered can be combined to provide a value for the AU.
Modern methods use radar to measure the Earth-Venus distance at
closest approach and proceed to the AU from there. But, because of
their historical importance (and their rarity) Venus transits are of
some interest.
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Extrasolar planet transits
There are now over 100 known planets around stars outside our solar
system (extrasolar planets). Virtually all of these planets have been
detected by observing radial velocity variations (``speed wobbles'')
of the host star. We can only observe the total light from these
remote stars, but by carefully monitoring the wavelength dependence of
this radiation (using a very stable spectrometer) astronomers can now
detect the small line of sight velocity variations in the star caused
by the orbiting planet.
However, if the orbital plane of the planet is suitably aligned
towards our observing direction, not only will the observed velocity
variations be at their largest, but the planet will periodically cross
in front of the star and produce transit events. As distant observers
we do not have the resolution to see the planetary disk against the
stellar disk background (as we can for the Sun and Venus), but if we
continuously monitor the brightness of the star we should observe a
reduction in intensity during the transit that is proportional to the
ratio of the planet/star disk areas (which depends on the square of
the ratio of the radii).
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Jupiter crossing in front of our Sun would produce an approximate 1%
decrease in light for a distant observer, while an Earth or Venus
transit event would produce about a 0.01% decrease. Due to the
effects of our atmosphere, measuring brightness changes using
ground-based telescopes is limited to a precision of no better than
about 0.1% at best, so from the ground we could detect a gas-giant Jupiter
transiting in front of sun-like star but there is no hope of
seeing an Earth-sized planet transiting such a star.
A number of transit events of gas giant planets around distant stars
have been detected to date, and a number of satellite missions are in
the planning stage to make possible the detection of an Earth around a
distant sun.
However, to date there is only one extrasolar planet that has been
detected by both the radial velocity method and the transit
method. This planet goes by the name HD 209458b as it is orbiting the
relatively bright star HD 209458. This star is similar to the Sun and
is at distance of about 150 light years from us.
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The extrasolar planet HD 209458b
The presence of this planet was first revealed by the radial velocity
variations of its host star. However, in September 1999 two
successive transits, 7 days apart were also detected using a small
telescope in Colorado. The orbital period of the planet is about 3.5
days and so at any particular observing site one can only observe (at
night) every second transit. Such a small orbital period also means
that the planet is very close to the star.
The fact that one can observe both transits and radial velocity
variations means that this system is similar to the astronomer's
"eclipsing single-lined spectroscopic binary star". A lot of
physical information can be deduced about these systems. For the
star-planet system the 1% drop in brightness during transit means
that the radius of the planet is one tenth that of the star. Spectral
data from star indicates that it is similar to our Sun, so the planet
is somewhat bigger than Jupiter - a gas giant. Further, the
existence of transits means that the planet's orbital plane is pretty
much in our line of sight, so the radial velocity variations can
provide an accurate planet/star mass ratio. The planet mass is
somewhat smaller than Jupiter. We have a bloated gas giant due to
radiative heating from the host star. And there is more, which you
can discover by downloading the paper **here**.
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Extrasolar planet transit measurements at Mauna Kea Observatory
As part of a coordinated Whole Earth Telescope (WET) collaboration
campaign, we were observing at Mauna Kea in Hawaii for the first 2
weeks in November 1999. We had transported our special observing
instrument (a high-speed three channel photometer) from NZ to use in
combination with Mauna Kea's 0.6 metre telescope and observe
pulsating white dwarf stars. On 15 November 1999 visiting Harvard
astronomers told us of a predicted extrasolar transit event for that
night. We successfully observed most of the transit with our
equipment, while they used the University of Hawaii 2.2 metre
telescope to monitor the event. Our combined data formed the basis of
paper published in 2000 in the (international) Astrophysical Journal
that confirmed and extended the discovery data.
A year later,we were again back at Mauna Kea for more WET observing
(this time for 3 weeks!) and we also obtained two more HD 209458
transit events. The data for all 3 transits we observed are graphed
in the first figure along with model fits. The second figure provides
a pictorial representation of the planetary event and shows how the
light curve shape is obtained.
Three separate extrasolar planet transit events monitored by us at
Mauna Kea Observatory in Hawaii on the dates indicated. Also included
are model fits to the data. The November 1999 observations formed
part of a paper published in the Astrophysical Journal that presented
confirming and extending data on the transit following its discovery
in September 1999. The observations made a year later in November
2000 were accomplished inspite of cloud cover (12 November) and the
object getting close to the horizon (19 November). Note that the
precision of the 1999 data and most of the 19 November 2000 data is
better than 1 part in 1000. This was possible because of the
excellent astronomical site, a high-quality photometric instrument and
careful telescope guiding.
A pictorial representation of the transit of the extrasolar planet
HD 209458a passing in front of its host star. The graph below the
picture shows the predicted light decrease according to two models of
the event: (a) the dashed red curve corresponds to a stellar disk that
is uniformly bright, and (b) the solid blue curve represents what you
would see for a stellar disk that exhibits "limb darkening". This
effect means that because the star is actually a 3 dimensional sphere,
light emitted towards the direction of the observer from near the edge
is less intense since it effectively comes from a cooler part of the
stellar atmosphere. Note that the light loss during transit provides
direct information about the relative sizes of the planet and star.
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Observing in an open dome at Mauna Kea Observatory, Hawaii
Mauna Kea Observatory is one of the best observing sites in the world
as it on the top of an inactive volcano on the big island of Hawaii,
some 4200 metres above sea level (higher than Mt Cook). It is a
magnificent astronomical site (when it is clear), but the observing
conditions are arduous since the atmospheric pressure is about 60% of
the sea level value. Astronomers sleep (during the day) further down
the mountain in accommodation (Hale Pohaku) at about 2800 metres.
The observatory on Mauna Kea is something like a technology park with
very large (and very expensive) telescopes such as the 10 metre twin
Kecks, the 8 metre Japanese Subaru and the international 8 metre
Gemini North telescope. In constrast, we were at the "low-tech" end of
things. We were using the University of Hawaii 0.6 metre telescope and
had to work in an open dome. However, when combined with our high-speed
3 channel photometer we had an excellent system for measuring the planetary
transit light curve of the relatively bright star HD 209458.
For our WET work with the much fainter white dwarfs a bigger telescope
would have been very welcome, but the possibility of extended coverage
from the unique geographical position of Hawaii was more important
than aperture size.
We have selected a number of pictures to give a flavour to working in
an open dome at the top of Mauna Kea. Most people would have an image
of sun and beaches when thinking of Hawaii, but we can assure you that
you get a rather different perspective on things when working in an
open dome at night on the top of Mauna Kea.
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