More exoplanet news! A quite a bit nearer than the planet from my previous post, Tau Ceti has been found to have five planets. One of those planets is in the habitable zone, which has excited quite a few people. They shouldn’t be breaking out the champagne quite yet though, as I shall explain.
Tau Ceti has a low metallicity. This means that it is mainly made up of hydrogen and helium, which in turn implies that any planets surrounding it are also made up of the lighter elements, in other words they are likely to be gas giants.
Artists impression of our sun on the left, Tau Ceti on the right. (Picture by R.J. Hall from wikipedia. Licence: Creative Commons Attribution-Share Alike 3.0 Unported)
The way we can tell that Tau Ceti has such low metallicity is by measuring the light come off it using a technique called emission spectroscopy. In chemistry lessons at school, you held different elements in a Bunsen burner flame and noted the colour of their flames was different for each element. Emission spectroscopy is a continuation of that, in which the light from a star is separated into its constituent electromagnetic frequencies (think a prism separating a white light into its spectrum), and the elements are classified from those frequencies. The easiest ones to measure using visible light are hydrogen and iron, so a star’s metallicity is usually described as the (logarithmic) ratio of iron to hydrogen, or Fe/H.
This doesn’t mean that there can’t be rocky planets around Tau Ceti, but it does make it a lot less likely. Also Tau Ceti is surrounded by a debris disk – a disk of dust and general bits of not-quite-planets – which would mean any planets would be constantly barraged by impacts.
The important thing, however, is the existence of planets in our local neighbourhood, which means that the universe is a lot more familiar that it appeared at our first glances of it.
The paper is here.
Land of Gregs 