Watching the sunset on another world 150 light years from Earth….
What would it look like?
HD 209458 b (Osiris) is a large exoplanet or extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light-years from Earth’s solar system. First discovered on November 5, 1999 during “Spectroscopic studies.”
Osiris is a 7th magnitude world, visible from Earth with good binoculars or a decent telescope. Osiris (HD209458b), 150 Ly from Earth orbits very closely to its sun. Measured by the Spitzer Infrared Space Telescope on March 23. 2005 it had an atmospheric temperature of around 1,000 degrees Celsius (1,800 degrees Fahrenheit) it’s year lasts just 3.5 Earth days long. Very bad for the aging process!
- Known to have an atmosphere
- Observed to have an evaporating hydrogen atmosphere
- Found to have an atmosphere containing oxygen and carbon
- One of the first two extrasolar planets to be directly spectroscopically observed
- The first extrasolar planet found to have water vapour in its atmosphere. (April 2007)
(Sunset On HD 189733 – Osiris)
Osiris doesn’t actually have as surface on which to sit peacefully watching an alien sunset but for the purposes of this post we will pretend it does. What it does have however is an atmosphere, which when passed through by the light from its star (sun) – it does this every so often when Osiris passes between Earth and its own sun (transits)– allows the scientific types here on sunny Earth to figure out in a scientific way mostly beyond the understanding of the average Earthling, exactly which colours its sun would set in, should Osiris have a surface – as already stated it doesn’t but… and we were spending a happily romantic evening sitting on the (make-believe) surface.
I said it HASN’T got one!!! So don’t try it!! It won’t be any good for you!!
In the image to the right Osiris’s sunset can be seen as it would appear if having travelled at light speed to reach it, you were floating 6,200 miles/10,000 km above the planetary surface – preferably not in a deckchair unless you’re wearing a heavy duty space suit😉 a space station would do nicely.
The Osirisian sun- HD 209458– it’s star, is, much like Earth’s Sun, white – yes I know…The Sun is yellow…but if there was no atmosphere it would look white – not that we would see it because without an atmosphere life on Earth would have survived for approx. 20 seconds before exploding into instant extinction.
An alien sunset on Osiris really does look alien – unlike Earth’s. Osiris’s atmosphere consists of sodium which when starlight (sunlight) zips through it, absorbs the red light. (think prisms and colours of rainbows when white light is split) This results in the remaining starlight appearing blue. It makes perfectly good sense if you were paying attention in your school science lessons. If you weren’t then it’s your own fault and you should be thoroughly ashamed of yourself!!
As back home on Earth the blue light from the star is scattered (“Rayleigh scattering,” a mechanism also responsible for the Earth’s sky being blue) creating a progressive change through the blue end of the spectrum through to a pretty green and later a deep, dark shade of green as its star dips further beneath the horizon.
In such colours you should be able to get a good view of Osiris’s sun without going instantly blind in which case you will notice it looks oddly flat around the southern half. The same effect occurs when we watch the sun setting from Earth. This is the consequence of diffraction (light bending) and nothing to worry about..
Whilst you are relaxing in the Osirisian sunset you can reflect on the little snippet of mythology attached to it’s title… Osiris was an Egyptian god, usually identified as the god of the afterlife, the underworld and the dead. He was classically depicted as a green-skinned man with a pharaoh’s beard, partially mummy-wrapped at the legs, wearing a distinctive crown with two large ostrich feathers at either side, and holding a symbolic crook and flail. His green skin is symbolic of new birth. This image is based on New Kingdom tomb paintings.
AND FOR COMPARISION PURPOSES, THE SUNSET ON HD 189733 b
Sunset on HD 189733 b looks like an especially awesome Earth sunset when the sky is very clear and there is only a small amount of dust in the atmosphere. HD 189733 b is much closer to its star (sun) than Earth is to our Sun so its star looks considerably bigger 25 times larger than Earth’s sun when viewed at sunset compared.
HD 189733 b’s sun is orange and nowhere near as hot as Earth’s Sun and consequently is coloured orange as oppose to the white of our Sun. Instead of undergoing a gradual change of colour as it sets, this alien sun transforms straight from its original orange colour to deepest red in the thick layers of the lower atmosphere.
HD 189733b discovered in 2005 is an extrasolar planet of similar size to Jupiter. It orbits a star (it’s sun) in a binary system called HD 189733 in the constellation of Vulpecula in two Earth days. The aging process on this world would be terribly fast in our terms and those wrinkles would appear in a frighteningly super-smart short time!! The star system itself one of the closest planet-star systems known making it extremely hot. It is located near the Dumbell Nebula, approximately 62 light years from Earth and is known as a “Hot Jupiter.”
HD 189733b shares many similar characteristics as HD 209458b (a.k.a. “Osiris) Although HD 189733b’s atmosphere isn’t thought to be evaporating like Osiris’, atmospheric gases extend far beyond it and out into space. This is significant in that starlight can also pass through meaning that scientists have been able to figure out that the atmosphere contains water and methane resulting in the probability that HD 189733b may have a blue hue, reminiscent of Uranus.
The atmosphere also contains iron, silicate and aluminium oxide particles. These would seem to collect in HD 189733b’s upper atmosphere, forming a thin, hazy, reflective cloud in the exosphere. This leads to the natural conclusion that the weather on HD 189733b is hot and cloudy.
Thanks to the Spitzer telescope it was discovered in February 2007 for the first time that the atmosphere of exoplanet HD 209458b (Osiris) is relatively dry, thick and dusty. Osiris even contains grains of sand (silicates).
Although it is not suitable for the existence of alien life it is an exciting step on the road to discovery of such worlds.
Prof. Frédéric Pont