Here’s GJ 367b, an iron planet smaller and denser than Earth

This is an article I wrote for The Conversation about a new exoplanet, for which I was a co-author on the discovery paper. One reason for reproducing it here is that I can reverse any edit that I didn’t like!

As our Solar System formed, 4.6 billion years ago, small grains of dust and ice swirled around, left over from the formation of our Sun. Through time they collided and stuck to each other. As they grew in size, gravity helped them clump together. One such rock grew into the Earth on which we live. We now think that most of the stars in the night sky are also orbited by their own rocky planets. And teams of astronomers worldwide are trying to find them.

The latest discovery, given the catalogue designation GJ 367b, has just been announced in the journal Science by a team led by Dr Kristine Lam of the Institute of Planetary Research at the German Aerospace Center.

The first signs of it were seen in data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Among the millions of stars being monitored by TESS, one showed a tiny but recurrent dip in its brightness. This is the tell-tale signature of a planet passing in front of its star every orbit (called a “transit”), blocking some of the light. The dip is only 0.03 percent deep, so shallow that it is near the limit of detection. That means that the planet must be small, comparable to Earth.

But Dr Lam also wanted to know the planet’s mass. To do that her team set about observing the host star at every opportunity with HARPS, an instrument attached to a 3.6-metre telescope at the European Southern Observatory in Chile, that was specially designed to find planets. It does this by detecting a slight shift in the wavelength of the host star’s light, caused by the gravitational pull of the planet. It took over 100 observations to detect that shift, meaning that the planet, in addition to being small, must also have a low mass.

Artist’s impression of GJ 367b (Credit: Patricia Klein)

Eventually, as observations accumulated, the numbers were tied down: GJ 367b has a radius of 72 percent of Earth’s radius (to a precision of 7 percent), and a mass of 55 percent of Earth’s mass (to a precision of 14 percent). That demonstrates that astronomers can both find Earth-sized planets around other stars, and then measure their properties. The measurements tell us that this planet is denser than Earth. Whereas Earth has a core of iron surrounded by a rocky mantle, this planet must be nearly all iron, making it similar to our Solar System’s Mercury.

Mercury orbits our Sun every 88 days. Blasted by fierce sunlight, the “daytime” side is bare rock heated to 430 degrees Celsius. GJ 367b is even more extreme. The recurrent transits tell is that it orbits its star in only 8 hours. Being so close, the daytime side will be a furnace heated to 1400 Celsius, such that even rock would be molten. Perhaps GJ 367b was once a giant planet with a vast gaseous envelope, like Neptune. Over time, that gaseous envelope would have boiled off, leaving only the bare core that we see today. Or perhaps, as it formed, collisions with other proto-planets stripped off a mantle of rock, leaving only the iron core.

GJ 367b is, of course, way too hot to be habitable. But it shows that we can find and characterise rocky, Earth-sized planets. The task now is to find them further from their star, in the “habitable zone”, where the surface temperature would allow water to exist as a liquid. That is harder. The further a planet is from its star, the less likely it is to transit, and the longer the recurrence time between transits, making them harder to detect. Further, orbiting further out, the gravitational tug on the host star is reduced, making the signal harder to detect.

But GJ 367b’s host star is a red dwarf, a star much dimmer than our Sun. And, with less heating from starlight, the habitable zone around red dwarfs is much closer in. NASA’s Kepler spacecraft has already found planets in the habitable zone of red-dwarf stars, and the TESS survey promises to find many more.

The next step is to ask whether such planets have atmospheres, what those atmospheres are made of, and whether they contain water vapour. Even there, answers may soon be forthcoming, given the imminent launch of the James Webb Space Telescope. If JWST is pointed at a star when a planet is in transit, it can detect the starlight shining through the thin smear of atmosphere surrounding the planet, and that might show subtle spectral features caused by molecules in the planetary atmosphere. We’ve already found water vapour in the atmospheres of gas-giant exoplanets. As planet discoveries continue apace, it is becoming feasible that we could, before long, prove the existence of a planet that has an atmosphere and a rocky surface, on which water is running freely.

A rocky, Mercury-like exoplanet.

17 thoughts on “Here’s GJ 367b, an iron planet smaller and denser than Earth

  1. Rob

    Fascinating! Thanks for posting this.

    Thousands of extra-solar planets have now been discovered. Coel, as an astronomer, can you tell us what proportion of these are rocky planets that have atmospheres and water and are in a habitable orbit? And do you think we will ever discover life on an extra-solar planet?

    Reply
    1. Coel Post author

      Hi Rob, currently all such questions are hugely driven by what sort of planets we can detect: smaller, rocky planets are way harder to detect than gas-giant planets (so we’ve found many more gas-giant planets than rocky planets, even though it’s likely that in fact rocky planets are more common).

      Detecting atmospheres on a rocky planet is then harder still, and is currently right at the limit of what can be done. There are claims of such detections (e.g. on GJ 1132b, https://arxiv.org/abs/2103.05657) but I wouldn’t say that any of these are yet secure and confirmed.

      Putting such a planet further out, in the habitable zone, makes things harder again. So, currently, we don’t have any known rocky planet in the habitable zone where there is a secure detection of an atmosphere. It may be that JWST (set to launch this month) could make such a detection on, for example, the TRAPPIST-1 planets, provided the launch goes fine and provided it works as planned.

      As for proving the existence of life on such a planet. Well, the plausible way of doing that would be to detect “biomarker” molecules in an exoplanet atmosphere (that is, molecules such as free oxygen that are hard to explain except by life; though this also requires a good understanding of non-biotic mechanisms for producing such a molecule). In principle, JWST could do that, if we found the right planet to point it at (the TESS survey could in principle do that).

      So it’s plausible that in coming decades we might prove the existence of biomarker molecules in the atmosphere of habitable-zone rocky planets.

  2. Rob

    Thanks, Coel.

    Given the of billions galaxies and trillions of stars, it’s hard to not think that there must be life out there. Do you think life must be out there somewhere? Surely we can’t be the only place.

    Reply
    1. Coel Post author

      Yes, looking at the numbers, I’d say that there must be thousands of planets with life on them, in our galaxy alone. The biggest unknown is the fraction of stars that could develop life that actually do develop life. The only data point we have on that is that Earth seemed to develop life about as early as it could in its history, suggesting that abiogenesis is not wildly improbable. And there are likely at least 10 billion habitable-zone rocky planets in our galaxy (even ignoring other galaxies, each with billions more).

  3. Rob

    Thanks, Coel.

    “…suggesting that abiogenesis is not wildly improbable.”

    Yes, even though we have a sample of only one so far, it’s hard to see why, given the gazillions of planets, conditions an some of them would not be conducive to abiogenesis. I find it hard to imagine this is the only place it’s happened. Just as a matter of statistics, it would seem very strange if earth was the only place in the universe with life. It would make it very special indeed. But ever since the theory that the earth was the center of the universe bit the dust, the more we find out the less special we seem to be.

    Reply
    1. Coel Post author

      Though if abiogenesis were indeed wildly improbable, then you could explain the fact that our planet is highly unusual by the anthropic principle, that any observer asking the question would necessarily be on a highly unusual planet.

  4. Rob

    That’s true, Coel. But the anthropic principle seems not to tell us much other than we don’t yet know how likely or unlikely abiogenesis is. We know it happened on this planet and it seems likely that there are countless planets not too different from earth and so it’s hard not to think that the odds are that, if it can happen here, it can happen there. I’m not a scientist but I’ve read that life is a very efficient way of dissipating free energy so as to arrive at thermodynamic equilibrium. (Jeremy England) Do you think there is anything in this idea? If it is true, then maybe thermodynamics makes it likely that life is ubiquitous. Life may be just one of the universe’s ways of winding down.

    I think the next few decades in astronomy promise to be vey interesting.

    Good luck in your search for planets. I hope you’ll post updates.

    Reply
    1. Coel Post author

      Hi Rob,
      I think the best (and only!) indicator we have is timescale. Let’s suppose that abiogenesis were very unlikely, such that most planets that could harbour life just sit there for billions of years in a sterile state. And bear in mind that, while Earth would have “window” of about 10 billion years (before the sun turning into a red giant makes life here impossible), for lower-mass stars the “window” could last 100 billion years or more.

      Thus, given a wildly improbable abiogenesis, coupled with the anthropic principle, the most likely scenario is that an observer finds themselves on a planet that is much older than Earth (and with a Big Bang much further in the past) in which life arose relatively recently, *and* where there was a prior stretch of tens of billions of years when life could have arisen but didn’t.

      The evidence we have for Earth is, instead, that primitive life had arisen at least by 3.4 billion years ago, with some evidence pointing to 3.7 billion years ago or even earlier. Then one can model how early life would have spread, and conclude that, for there to be signs of life that early, the first actual “living replicator” must have been 4.0 billion or even earlier.

      To compare, Earth was in a cool enough state of have oceans only 4.5 billion years ago. So, abiogenesis on Earth appears to have taken substantially less than a billion years, and maybe much less. By the above arguments, and while this is only one data point, this points to abiogenesis being fairly probable.

  5. Rob

    Thanks, Coel. Yes, there’s the time it takes for abiogenesis to happen. It looks like life got going here within a short time after the planet formed and settled down, so if conditions are right elsewhere (and they must be so somewhere, given the gazillions of planets out there) then it’s likely life is out there. Let’s hope the JWST will quickly give us further insight into how common earth-like planets in the habitable zone are.

    Reply
  6. Rob

    I’ve just been reading about the JWST on the link you posted, Coel. If all goes to plan it will be amazing! So much bigger than Hubble. At a cost of $15 billion in todays money I’m hoping (if I were religious I’d be praying) that the launch goes without a hitch and that when it gets to it’s destination all runs smoothly. I’d have my fingers crossed if I thought it would do any good.

    Reply
    1. Coel Post author

      Yes, a lot of money all in one basket! There’s not only the obvious dangers of launch, but it then has to go out to L2 and unfold its mirrors. That’s not been done before with an astrophysics satellite (though there are on-the-quiet reassurances that it’s been done with hush-hush military satellites and so is tested technology).

  7. Rob

    Watching the launch live live tonight through NASA TV. Fingers crossed. I’m really looking forward to when the JWST starts to look at exoplanets such as those in the TRAPPIST-1 system to see if atmospheres can be detected and hopefully give us an indication of what gasses are present in those atmospheres. It’s an exciting time in astronomy.

    The TRAPPIST-1 system is less than 40 light years away. Coel, do you think there will ever be a way for us to get a spaceship up to a sizable fraction of the speed of light so that it would be possible to send probes to such planets and get data back with a single human lifespan? Signals from such probes would take about 40 years to reach us from TRAPPIST-1 so we would have to get the ships to travel at a really large fraction of the speed of light to get data back within a human lifespan. Do you think it will ever be possible in the foreseeable future, Coel?

    Reply
    1. Coel Post author

      I doubt that it’ll be possible in the foreseeable future. Currently our fastest probe to the outer solar system goes at about 0.01 percent of the speed of light. So we need to do 100 times better to even do 1 percent of light speed (which is a 4000-yr travel time). And as you get anywhere near the speed of light, relativistic effects mean that the energy requirements go up hugely.

  8. Rob

    Thanks, Coel.

    So, it’s unlikely that it will be possible anytime soon. No doubt scientists have thought about it, but it’s hard for me to imagine what kind of propulsion system could provide enough power to get a probe up to a sizeable fraction of light speed within a reasonable time.

    Oh, well, at least we can look forward to what the JWST will tell us. We’ll still be around for that.

    Reply

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