Goldilocks Planet HD 40307g Finds The Porridge “just Right”

There are currently 7 potentially inhabitable planets known. HD 40307 is the latest to make the list. Its average temperature could possible be as high as 48 degrees F. with large seasonal shifts as it orbits its host star. Credit: PHL@ UPR Areceibo (

A newly discovered planet with 7 times the mass of Earth orbits in the habitable or “Goldilocks” zone around a 7th magnitude star HD 40307 in the southern constellation Pictor the Easel. Habitable doesn’t necessarily imply life could take spring forth on this weightier version of Earth, but it does mean the planet orbits at just the right distance for liquid water to be stable on its surface. Goldilocks refers to conditions being neither too hot nor too cold but “just right”.

HD 40307 is an orange-colored star a little smaller, less massive and cooler than the sun located 42 light years from Earth. Unremarkable perhaps except for one little detail: this sun is orbited by six planets. Five of them are close in and hotter than Hades, but the sixth orbits at about the same distance Earth does from the sun. While you’d weigh considerably more there due its greater mass, floating in an ocean to relieve the inevitable back pain is theoretically possible.

Video with further description of the HD 40307 solar system

HD 40307 is one of three new super-Earths (rocky extrasolar planets at least several times more massive than Earth) discovered around the star but the only potentially hospitable one. An international team, including Carnegie Institution for Science co-author Paul Butler, was led by Mikko Tuomi of the University of Hertfordshire and Guillem Anglada-Escudé of the University of Göttingen. The researchers re-analyzed the spectra or fingerprints of the star’s light made with the European Southern Observatory’s HARPS spectrograph using new software. Hidden in the data were signatures of the three additional planets.

HD 40307g belongs to a small but growing list of Earth-like planets orbiting a comfortable distance from their host stars. Most of the confirmed 849 extrasolar planets currently known are nicknamed “hot Jupiters” for good reason. They’re massive and orbit close to their suns, exerting measurable gravitational wobbles in their host stars that our equipment can detect. Smaller worlds orbiting at more habitable distances tug but a little and are much more difficult to identify.

Gamma Cephei is an easy naked eye star in the constellation Cepheus the King, now high in the northern sky. Created with Stellarium

In 1988 Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang reported gravitational wobbles of the star Gamma in the constellation Cepheus and cautiously attributed it to a planet. Their observations weren’t confirmed until 2003, making Gamma Cephei Ab the first extrasolar planet discovered. The first definitive discovery of a pair of planets found orbiting a dense, rapidly-spinning star called a pulsar on April 21, 1992.

We’ve come far quickly. I suspect we’ll find a planet with signatures of life’s best indicators – water, methane and oxygen – in our lifetimes. Will it be HD 40307g?

6 Responses

  1. Giorgio Rizzarelli

    Great news, thanx! Another success by HARPS, after the recently discovered Alpha Cen Bb, and the famous Gliese 581 planets.
    I read on Wiki that the composition of this new planet is unsettled, in particular I guess it’s not sure if it’s rocky. But even if it’s a mini-Neptune, as usually we can hope it has moons.
    BTW (reprising the topic of my reply to your recent post about Mars seasons) a moon is usually tidally locked to its planet, so in particular I guess the planet and moon stabilize one each other’s axis, lending a stable climate also on long terms. Unless, of course, the moon is too close to the planet to be heated too much by tidal forces, like infernal Io…

    1. astrobob

      Yes, HARPS is justly famous for its precision. About Io – the other Galilean moons work in tandem with Jupiter to cause its interior to heat up and melt. By the way – great discussion on the topic!

      1. Giorgio Rizzarelli

        You’re right about Io, I forgot about the other galilean moons. Anyway I read about some recent studies about the exomoons habitability, which estabilish an “habitable edge”: a min distance from the planet, so that the moon, though having more illumination from the planet, doesn’t experience excessive tidal heating and eclipses (if interested, see Wikipedia under exomoon in the last paragraph).
        I thought I was wrong also about thinking that the planet may be gaseous, since planets with <10 Earth masses are usually called terrestrial. However I just read the abstract of an article which questioned the assumed rocky nature of M<10 planets, right in this system.
        Anyway, if the planet is rocky, the real question is, as usual for planets with unknown atmosphere, that the real temperature on the planet's surface can be very different if we take into account its albedo and greenhouse effect. Until then, I'll dream it's really habitable, and that the planet has the orbital plane just right so that Kepler can see it in transit, in order to get more info about its physical properties.

        1. astrobob

          I agree, it would be great if Kepler could point an eye in its direction, but as far I know, Kepler’s focused on the Lyra-Cygnus region, so we’ll need other large scopes – or perhaps even the upcoming space-based Webb telescope – to give us a better idea of the new planet’s physical characteristics.

          1. Giorgio Rizzarelli

            Thanx for the correction Bob.
            Or maybe the European Extremely Large Telescope (and a pity the Overwhelmingly Large Telescope is cancelled).

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