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Posts Tagged ‘Absorption Line Spectrum’

This story caught my attention last week – the discovery of water on an extrasolar planet which is as Neptune-sized. The exoplanet in question – HAT-P-11b, was discovered by the telescope Kepler in 2009 using the “transit method”. You can read more about that method here.

The star itself, HAT-P-11, is about 122 light years away in the constellation Cygnus. The planet has been determined to be about the size of Neptune, which means it is about five times the size of the Earth. Such relatively small planets can, to date, really only be found by the transit method, as the doppler shift method which discovered the first several hundred exoplanets is not currently sensitive enough to detect planets as small as Neptune.



Astronomers have discovered water in the atmosphere of a Neptune-sized exoplanet for the first time.

Astronomers have discovered water in the atmosphere of a Neptune-sized exoplanet for the first time.



A team has used the Hubble Space Telescope to observe the absorption spectrum produced as HAT-P-11b passes in front of its parent star. The light from the star, as it passes through the gases in the atmosphere of the planet, will have certain wavelengths removed, this is the principle behind the absorption spectrum that I explained here. The scientists have found that wavelengths corresponding to water vapour have been removed from the star’s spectrum, showing that water vapour exists in HAT-P-11b’s atmosphere.

This is tremendously exciting, as finding water is, with our current understanding, vital to finding life. This discovery shows that planets harbouring water are definitely out there, probably in their millions or billions. The next step will, hopefully, be finding water on a planet which is in the habitable zone, but this discovery is definitely a step in the right direction.

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When we look at objects giving off light (or reflecting light) we find three types of spectra. They are produced in different ways, and can actually tell us about the physical properties of the materials producing the spectra. In many ways it was the development of studying and understanding the spectra of astronomical objects that led to the development of astrophysics as opposed to the more traditional astronomy. This happened from the mid 1800s.

The three types of spectra are called “a continuous spectrum” (or continuum emission), “an emission line spectrum” and “an absorption line spectrum”. They look like the following


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UPDATE

You can read about these three types of spectra in more detail in my new book – see http://www.springer.com/astronomy/popular+astronomy/book/978-3-319-09927-9


A continuous spectrum

When Newton did his famous experiment with a prism and sunlight, he noted that the Sun produced a “rainbow” of colours. This is a continuous spectrum. (However, as I will discuss in a future blog, if he had been able to produce a more detailed spectrum he would have noticed some subtleties on this continuous spectrum). So, light from the Sun, and any star, produces a continuous spectrum.

We also get a continuum spectrum from a hot solid, so for example the light produced by incandescent light bulbs is a continuum spectrum. These kinds of bulbs give off light by a very thin coil of metal, the filament, (usually tungsten) getting extremely hot from having an electric current passed through it. When the filament gets to thousands of degrees, it gives off light.


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An emission line spectrum

If, instead of looking at the spectrum of the Sun we were to look at the spectrum of an object like Messier 42 (the Orion nebula), we would notice a very different kind of spectrum. Rather than being a continuous spectrum, we would see a series of bright lines with a dark background. We would also see an emission line spectrum if we were to look at the spectrum from one of the fluorescent light sources which are now replacing the incandescent lights in houses.


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UPDATE

You can read about these three types of spectra in more detail in my new book – see http://www.springer.com/astronomy/popular+astronomy/book/978-3-319-09927-9

An absorption line spectrum

An absorption line spectrum is in some ways the converse of an emission line spectrum. Rather than seeing a series of bright lines on a dark background, one sees dark lines on a continuous spectrum.


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Putting it all together

The diagram below shows how the three types of spectra can be produced. If we observe a continuum source (such as a star of an incandescent light) with nothing between us and that source then we will see a continuous spectrum.


20130705-110330.jpg


If, instead, we look at a gas cloud then we will see an emission line spectrum. This is why the Orion nebula has an emission line spectrum, because we are seeing the emission from the gas cloud from which the stars have formed and still are forming. The lines are in particular places on the spectrum which depends on the composition, pressure and temperature of the gas, as well as whether it is moving towards us or away from us.

If we look at the same gas cloud but with a continuum source behind the cloud then we will see an absorption line spectrum. The dark lines are in exactly the same places (at the same wavelengths) as for the emission line spectrum, but are dark rather than bright.

This was observed by physicists as early as the 1850s. In fact the diagram above is known as Kirchhoff’s radiation laws, after Gustav Kirchhoff (1824-1887), a German physicist of the time. He and Robert Bunsen (he of the eponymous burner) did important spectroscopy work in the 1850s and 1860s. But it was actually not until the 1920s that physicists properly understood the physics of these three different kinds of spectra. I will explain this physics in a series of future blogs.

UPDATE

You can read about these three types of spectra in more detail in my new book – see http://www.springer.com/astronomy/popular+astronomy/book/978-3-319-09927-9


My book, "The Cosmic Microwave Background" includes a sketch of the first ever absorption spectrum seen of the Sun, and of why stars have different colours.

My book, “The Cosmic Microwave Background” includes Fraunhofer’s sketch of the first ever absorption spectrum seen of the Sun, and an explanation of why stars have different colours.



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