The Planet with a Tail – Recovered and Updated

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:


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.

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