New life-sustaining planets? Don’t get too excited.

Sorry to be a killjoy, but I dislike misleading article titles, like “Another Earth Just 12 Light-Years Away?” The article, from Science Express yesterday, itself is fine, but just because a planet exists in a star’s habitable zone does not and should not imply another Earth. OK, I’m off my soapbox. Here’s the interesting bits:

τ Ceti (Tau Ceti, or HD10700) is a fairly nondescript star, about half as luminous as our Sun. It’s only about 12 light years away, and can be seen with the naked eye (in the constellation Cetus). Recent research indicates as many as 5 planets orbiting this star with two in the habitable zone. However, these astronomers are seriously pushing the capabilities of their systems to be able to resolve this.

By Cetus_constellation_map.png: Torsten Bronger derivative work: Kxx (Cetus_constellation_map.png) [CC-BY-SA-3.0 ( or GFDL (, via Wikimedia Commons.

What I think is more interesting than the potential discovery of the planets themselves is the study done on the characteristics of the noise inherent in these measurements. I won’t go into detail in this particular case, but I certainly appreciate their attention to study properties that people often just assume are stationary and Gaussian. To understand what I’m talking about, it helps to know a little bit about the methods available for detecting planets around other stars.

Detecting Planets–the absolute basics

Exoplanets cannot be detected directly, but rather only by their influence on their sun. There are two basic ways to determine if a planet or planets are orbiting and what their properties may be: (1) look for doppler shifts in the star’s spectrum due to orbital tugging by the planet, and (2) look for [very] faint changes in luminosity due to a planet transiting the face (transit photometry). Both of these methods require extremely high-precision measurements, usually stacked over very long time scales. A very neat demo from our friends at Wolfram Mathworld demonstrates this ‘orbital wobble’ due to the orbiting planet. As perhaps a subset of the Doppler method, astronomers can observe the radial velocity of the star, which will subtly change based on planetary locations. The Doppler method works because the planet isn’t really orbiting the star–the planet and star are mutually orbiting a common center of mass.

τ Ceti

To give you an idea of how small these signals are, the standard deviations on the recorded radial velocities of τ Ceti were about 1.7 m/s. Please appreciate that we’re looking for changes in radial velocity of a star 12 light years away on the order of a few metres per second.

So after some serious Bayesian modelling of data from three observatories, the reasearch group determined that a five-body Keplerian solution is “clearly favoured by the data.” (For those familiar, it was a relative posterior probability of 0.937, versus 0.063 for the next closest and 10e-7 for the next. Moreover, the 0.063 probability solution was also a 5 body solution, but with a 315 day periodicity instead of 168.)

The planets, named HD10700b through HD10700f, orbit their star once every 14, 35, 94, 168, and 642 days (Sols) respectively. The first three are probably incapable of hosting life (too hot), but the last two are thought to be within the star’s habitable zone–where liquid water can exist at the surface.

The Curmudgeon Is Back

OK, now here’s why I don’t like the sensational article titles: the above is all we know. We can estimate that they are rocky and we can estimate their masses, but whether there is water or an atmosphere, or anything else needed for life, we don’t know…yet. Fortunately there are ways to estimate these, but unfortunately because we are looking more or less at the axis of rotation of the system (in other words looking at the solar system from the top instead of the side), current methods of planetary analysis are unlikely to work. This doesn’t mean that people much more clever than me aren’t working on the question though, and I (among many) believe that our first interstellar probe should probably head the 12 light years to τ Ceti. Maybe my grandchildren will get to see the data…

For some further reading:


About faradaysheadache

Research Geophysicist with the US Geological Survey.
This entry was posted in Astronomy, Science, Technology. Bookmark the permalink.

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