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Only 12 light years distant, two super-Earths orbit in Tau Ceti's habitable zone


December 20, 2012

Artist's conception of the five-planet Tau Ceti system (Photo: J. Pinfield/RoPACS/University of Hertfordshire)

Artist's conception of the five-planet Tau Ceti system (Photo: J. Pinfield/RoPACS/University of Hertfordshire)

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Our stellar neighborhood is becoming crowded courtesy of some newly discovered real estate. Astronomers have uncovered evidence buried in the noise of apparently empty data showing that five super-Earths are orbiting the nearby Tau Ceti – a star chosen as one of the targets in the pioneering 1960 Project OZMA search for extraterrestrial life because of its strong similarity to the Sun. Better yet, the two outermost of Tau Ceti's planets appear to be in the star's habitable zone, making them the closest known potentially habitable exoplanets.

Astronomer Mikko Tuomi of the University of Hertfordshire and his colleagues analyzed more than 6000 prior spectroscopic observations of Tau Ceti. Delving deeply with powerful Bayesian analysis methods, they were looking for evidence that the star was being pulled from side to side by the gravitational influence of one or more orbiting planets.

They found it in spades. The best fit to the data suggests that Tau Ceti is orbited by five planets, all residing closer to Tau Ceti than Mars does to the Sun. The tally reads like this:

    Planet.....Orbit Radius (AU).....Orbital Period (days)......Mass (Earths)
  • TC b.................0.105........................14 days.........................2.0
  • TC c.................0.195........................35 days.........................3.1
  • TC d.................0.374.......................94 days........................3.6
  • TC e.................0.552......................168 days........................4.3
  • TC f..................1.35.........................642 days........................6.6
  • The planetary orbits are all very close to circular.

    Comparison of the smaller Tau Ceti to the Sun. It is a bit smaller, cooler, and more stable than the Sun (Photo: R.J. Hall)

    Tau Ceti is the twentieth closest star to our Sun, being a mere 11.9 light years distant. It closely resembles the Sun (unlike most stars) being a bright G-class star, with a mass of 0.78 solar masses, radius of 0.793 solar radii, and a luminosity of about half that of the Sun. It is older than the Sun by over a billion years, and is extremely stable, showing little variation or evidence of sunspots or flares.

    Aside this small difference in size, the main distinction is that Tau Ceti is lacking in metals compared to the Sun, a factor probably leading to formation of less rocky planets than in our solar system. Because of this, Dr. Tuomi says: "It is impossible to tell the composition (of Tau Ceti e), but I do not consider this particular planet to be very likely to have a rocky surface. It might be a 'water world,' but at the moment it's anybody's guess."

    Because Tau Ceti has only 55 percent of our Sun's brightness, the habitable zone lies closer than it does to our Sun. Despite this, the three innermost planets around Tau Ceti will be too hot to support life – Tau Ceti d's orbit is equivalent to an orbit around our Sun midway between Venus and Mercury.

    The outer two planets are potentially habitable. Tau Ceti e falls clearly within the Core Habitability Zone, that being the region within which a planet like Earth will have liquid water. The Planetary Habitability Laboratory (U Puerto Rico, Arecibo) notes that both Tau Ceti e and f fall within the Extended Habitability Zone where larger planets with thicker atmospheres might retain sufficient heat for water to be liquid.

    "They're pushing the envelope," says Gregory Laughlin, an astronomer at the University of California, Santa Cruz. "Some or even many of these planets could go away. But I think that they've done absolutely the best job that you can do, given the data. You have to get tons and tons and tons of velocity measurements over many years, and then you really, really have to take extreme care—as this Tuomi et al. paper does—to get rid of all the systematic noise."

    Source: University of Hertfordshire (PDF) via Space.com

    About the Author
    Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer. All articles by Brian Dodson

    If we're ever going to a place with 4 to 6 times our gravity we'd better be very, very fit! Better yet, wearing one of those powered exoskeletons. The native inhabitants will be VERY strong. Let's hope they're friendly.


    lol only 14 light years away?, you guys are funny. with our present technologies it would take thousands of years to get there. regards Freelance Eng.

    Dave Hargraves

    Dave, our current technology is growing exponentially. Hell, we may not even need to design the technology ourselves. Within 25 years, computers with higher-than-human intelligence may design the technology for us.

    Walter Costescu

    It's always amusing to see the varied conditions to which the characterization "Earth-like" are applied. A planet can have an atmosphere of sulfuric acid and tempeatures of 800 degrees F, and be considered "Earth-like" by the simple virtue of its size and the fact that it has an atmosphere. "Earth-like" can mean an atmosphere of nitrogen and methane, gasoline precipitate and temperatures of -275 degrees F... as long as it HAS an atmosphere. I expect that as more planets are discovered and studied the definition of "Earth-like" will be narrowed to become more "Earth-like".

    John Hagen-Brenner

    Walter may I respectfully reply, Yes advances are coming faster but please put your crystal ball away and stop definitively announcing break through(s) that appear to be just around the corner, like 25 years. No one can say when a 12 light-year destination will be a hop skip and jump away. No one can predict when the reality of that kind of distances can be possible. Not even those who are claimed to be masters in the field. By the way, at today's space shuttle speeds a 12 light-years trip would take 440,000 years, I look it up and that's without being able to fill up along the way. The news is interesting but for the immediate future not of any real value as far as going there.

    May I point out in the late 40's and fifties we were told that nuclear power, in the "future" would be so cheap there would be no need to meter it. It's now turning 2013 and nuclear generated electricity is not cheap. In fact nuclear power is so expensive today it's impossible to build one without federal government funding and guarantees. Also, for the past 40/50 years being able to have a sustainable fusion reaction is just 20 years away. That's still being said today.

    Remember reality demands they no one should count their chicks before they hatch. Dave did misspeak by saying "it would take thousands of years", As I stated it much much longer than that. Most people are vastly incapable of understanding just how far it REALLY is.


    re; Dave Hargraves

    Top speed of an interstellar sub-light ship is limited by energy availability, reaction mass, and particle deflection. Nuclear reactors can provide plenty of power and a century's worth of fuel is not an insurmountable problem.

    If you use a particle accelerator to accelerate your reaction mass to relativistic velocities you could in theory make an engine that provides thrust but does not throw mass overboard. Accelerate two particles in opposite directions 90degrees away from intended thrust until you have notably increased their mass and then accelerate it in the opposite direction than you want to go and then reduce both particle's lateral velocity until the mass has been greatly reduced and 'slowly' return them to the starting positions to be run through again. While this would not actually be a reactionless thruster it is very efficient in the use of reaction mass.

    With a very high output laser you could vaporize and push small objects out of the way and illuminate and detect large objects with time to avoid them. What you lose on the acceleration faze will be made up by the additional thrust in the breaking faze.


    I just love how apparently empty data, can hold so much information.

    what could one learn from data that is not apparently empty, and where could one obtain such data...

    Michiel Mitchell

    Wouldn't it be better to just keep cataloging all the Earthlike worlds with the realization that it's just advanced astronomy (and cool to do so!)? We will NEVER "need" to colonize. We may step foot for reasons of tourism, but our numbers will surely exceed that of the real estate that mere planets offer. Consider, the alien days, years, tilt of axis', and especially, gravities of what these "off the wall" worlds have to offer.

    It will be FAR better to wait till AI tells us how to build an Earth ring (with a cross section shaped like a parabola that forms walls to hold an atmosphere) from the easier to access materials in our own solar system. I believe it would have to be 2.2 million miles in diameter in order to give the inhabitants a 24 hour day!

    Robert Bernal

    Only twelve light years! Just thought I would remind you that is about eighty-five TRILLION miles. That's a million miles multiplied by 85 Million.

    If you could crank a space ship up to 1 million miles an hour it will cover about 8.7 billion miles in a year. At that rate you will reach this possibly lifeless rock in about one thousand years, give or take a few decades.


    This star being a billion years older than our Sun could mean that this solar system have seen a few iterations of life already. Pity they haven't managed to phone us before the plug was pulled. ;-]

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