The Young Astronomers
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Scientists using the Hubble telescope’s cosmic origins spectrograph (COS) have revealed that the exoplanet HD 209458b – colloquially known as Osiris – is being blasted with a solar wind powerful enough to blow its atmosphere into space like the tail of a comet.
HD 209458 b Credit: In image
The planet is a gas giant similar to Jupiter though it is slightly less massive (0.69 ± 0.05 MJ / 69%±5% Jupiter’s mass). The planet orbits 100 times closer to its sun than Jupiter orbits Sol, this gives the planet an incredibly short orbit of just 3.5 days. To put this in perspective the innermost planet in our solar system – Mercury - orbits Sol once every 88 days.
Osiris was discovered in November 1999 via detecting a dip in the brightness of its parent star as the planet transited across its disk. This was the first discovery of an exoplanet using the transit method[1].
As it sits so close to its parent star it is subjected to an incredibly high level of U-V radiation, this bombards the planet’s atmosphere heating it to a phenomenal temperatures of up to 1000 degrees Celsius (2000 degrees Fahrenheit)! This kind of temperature is found within the majority of the planet’s atmosphere as ‘heavy’ elements such as silicon have been detected escaping the upper atmosphere and into space. Such elements are usually found deep in the lower areas of gas giant planets so the heating must be transferred down to the lowest sections to bring these atoms to the surface where they can escape. A higher temperature in a planet’s atmosphere means that the particles have more energy and find it easier to break free of their planet’s atmosphere.
Maths Incoming – Its not bad don’t worry!
Lets have a look at an example of the difference in the kinetic energy of a gas particle at room temperature, compared to the temperatures experienced in Osiris’ atmosphere.
For this we must look at one particular equation:
Where:
K.e. = Kinetic energy in Joules
k = Boltzmann’s Constant = 1.38×10-23JK-1
T = Temperature in Kelvin
So at room temperature which is about 22 degrees Celsius = 295K, the kinetic energy of a gas particle (notice it doesn’t matter what the mass of the particle is) is:
which gives a value of:
Comparing to a particle of gas in the atmosphere of Osiris (assuming a temperature of 1000K):
which gives:
As you can see the energy of both particles is very, very small. However, in the atmosphere of Osiris an particle has 3.39 times the kinetic energy compared to a particle at room temperature. It is this excess energy allows the particles to overcome Osiris’ gravity and stream off into space.
Maths Ends Now
As the planet sits so close to its star a huge quantity of material is ‘blown off’ every second and the star’s solar wind blows it away in a rapid stream directly away from the star like the tail of the comet. The direction of the tail is always directly away from the star, and so it doesn’t line up with the star’s direction of motion as you may have expected.
Note: Tangential velocity simply means straight line velocity of the planet at a particular point in its orbit.
As Osiris is very much like Jupiter and its parent star HD 209458 is almost identical to Sol (despite being slightly more massive) it is believed that if Jupiter was located in the corresponding position in our solar system it too would behave in a similar way and we would have a tail owning planet in our own backyard!
Despite this constant erosion of Osiris’s atmosphere astrophysicist Jeffrey Linsky says that “It will take about a trillion years for the planet to evaporate,” so there is no need to worry about Osiris just yet!
The system is located around 153 light years away from Earth and is located in the constellation Pegasus.
[1].The various different ways of detecting Exoplanets will be covered in my upcoming series.
Saturn is the sixth planet out from the sun and is the second of the gas giants, it is also the second largest planet after Jupiter. It has been a popular target for observation for many centuries, and it formed the outer limit of the early solar system as it is the last planet clearly visible to the naked eye.
Saturn has many interesting features: – Despite it being the second largest planet in solar system it is also the least dense. It is so diffuse that if Saturn was placed in a trough of water (it would have to be impossibly large of course! ) it would float, this means that Saturn’s average density is less than 1gcm3.
Saturn also possesses the most spectacular ring system in the solar system. Ten complete ring have been discovered with another two discovered incomplete rings or ring arcs. The most recent ring to be discovered is the Phoebe Ring, this is a massive ring on the exterior of the ring system. It is a tenuous collection of dust particles and is believed to extend from around 59 Saturn radii out to around 300 Saturn radii. It is nearly invisible and is undetectable by the human eye and amateur telescopes. Despite being difficult to detect the ring has been shown to be around 20 times the thickness of the planet itself.Its creation is believed to be a result of micrometeorid and larger impacts on the moon Phoebe (from which the ring gets its name). The moon Phoebe has an average orbital distance Saturn radii which puts well within the ring itself. Material from the ring slowly moves inwards towards the planet due to a process called infalling – this in itself is caused by incoming solar radiation destabilising the ring’s components orbits. This added to Saturn’s gravity is slowly causing the breakup of all the rings as they slowly migrate towards the planet’s upper atmosphere where they are absorbed. Don’t worry though the rings will remain a prominent feature for millions of years yet! If the Phoebe was visible to the naked eye it would make Saturn appear larger in the sky than the full moon!
A graphic showing Saturn and two of its moon's in relation to the Phoebe ring Image credit: NASA/JPL-Caltech
The other main rings in the Saturnine system are (travelling from the outer atmosphere) D, C, B, A, F – All of these are found in the main ring system – G, E – these are found outside the main ring ‘belt’. There are numerous divisions or gaps in the rings, small separations are called gaps with larger ones called divisions. Some of the main divisions are: – The Cassini division between rings A & B and the Roche division between rings A & F. Some of the gaps include the Encke and Keeler gaps both are found within A ring.
This an image was created from a number of Cassini images and shows Saturn's main ring system Credit: NASA
This image is only here to demonstrate the diversity of the rings. For a detailed look at them I advise you to open the image in another tab of your browser, unfortunately site limitations prevent me from doing it full justice directly in the post.
The rings are held in place and maintained by some of Saturn’s 63 moons – these are fittingly called shepherd moons. Each of the shepherd moon’s gravity helps keep the rings orbit stable and helps to maintain its structure. In some cases particular orbital resonances - orbital distance ratios – have created the many gaps in the ring and it is also this phenomenon that prevents these gaps from ‘closing’. Sometimes shepherd moon work in pairs, an example Pandora and Prometheus - This pair is responsible for maintaining the G ring.
We are all familiar with the general layout of our Solar System, four inner rocky planets and four outer gas giants with an asteroid belt separating the two groups.
The Solar System that we now inhabit is, and has been stable for a considerable period of time (for at least a few billion years) though a new study has concluded that the early Solar System may have had at least one other inhabitant.
It has long been known that the early Solar System was a very changeable and violent region of space. It would have taken many millions of years for the gravitational tugs and pulls of the developing planets to allow our current planets to settle into stable orbits. During this time, many planetesimals (small bodies larger than asteroids that over time collided to form the planets that we see today via accretion) would have had their orbits destabilised. Some would have crashed into the sun or the developing gas giants or even, in at least a few cases but perhaps quite commonly, they could be thrown out of the Solar System entirely. However, what about the planets we know today? Could they have suffered similar events?
Uranus for one carries a scar of such an occurrence. Its axial tilt is so extreme it orbits the sun ‘on its back’ at nearly 90o to the plane of the ecliptic. This can be explained if a planetoid roughly the mass of Mars collided with Uranus in the early Solar System, so clearly such collisions can and did occur, but what about orbital disruptions?
New simulations conducted at the Southwest Research Institute inColoradoby David Nesvorny may provide an answer.
He conducted over 6000 simulations of this phase of the Solar System’s history with various different configurations of planets and orbits to try to determine which was most likely to produce a Solar System resembling the one we inhabit today. The results are quite surprising.
Using a four gas giant system at the start of the simulation, only 2.5% of simulations will produce a system with four at the end point, with the vast majority loosing at least one giant to gravitational distortions that would eject such it from the Solar System like is possible with the smaller planetesimals. Such a small percentage of ‘successful’ simulations indicates that it is highly unlikely for our Solar System to have a four gas giant starting scenario. Such systems appear too unstable for all planets to remain undisturbed as they develop.
However, other versions of the simulations produce much more favorable results. A situation where the Solar System formed with five giants is roughly 10 times as likely to have an end point similar to the current layout of our cosmic backyard.
This world would have been similar to the icy giants Uranus and Neptune and comparable to them in terms of mass. During the violent settling period of the early Solar System it would have suffered several gravitational encounters with other bodies leading to a final encounter with Jupiter around 600 million years after the birth of the Solar System that threw it into interstellar space never to return.
An artist's Impression of what the ejected planet may have looked like Credit: Southwest Research Institute
Such a scenario may seem outlandish to some, though it does carry weight. The recent detection of many times the expected number of free planets drifting the stars indicates that events such as this may indeed be commonplace in the Universe as a whole.
Though this research does pose more questions. If gas giants seem to be rather easily ejected in the early days of Solar Systems’, what about smaller earthlike worlds, and those similar to Mars?
Generally these are ignored in most research as their gravitational influences a much smaller than those of the much more massive gas giants, and thus have much smaller effects of Solar System evolution. However, research is now shifting to how the effect and are effected by the changing nature of forming Solar Systems.
David Nesvorny admits that he is as of yet not fully convinced that this lost world actually existed and further research will be needed to fully understand the development of the early Solar System though we are definitely getting closer.
You can read more here.
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