Posted by: mattburleigh | December 6, 2011

Not another “Earth-like planet”

Oh dear, here we go again. The world’s media are falling over themselves to report, for what seems like the two millionth time, the discovery of the “first Earth-like planet around another star”. This one is called Kepler-22b, has been discovered by NASA’s Kepler mission, and lies in the so-called “habitable zone” of its parent star, ie the region where conditions are just right for life as we know it to exist. See the
NASA press release and an example of hyperbolic media nonsense on the BBC website here .

This isn’t the first time we’ve heard exaggerated claims of the discovery of the “first Earth-like planet in habitable zone around another star”. Check out this story from 2010 for example.

So what’s wrong with this announcement?

Well, first of all, Kepler-22b is significantly larger than Earth. Indeed, its radius is 2.4x bigger than ours. And we don’t know its mass. Unfortunately, as with a lot of Kepler’s candidate planets, it orbits a star that is too faint for us to measure the planet’s mass by the radial velocity method. If we assume it has the same density as Earth, then it has a mass around 13x that of Earth.

For comparison, Uranus has a mass 14.5x Earth’s, and a radius about 4x Earth’s.

So in truth, we have no idea whether Kepler-22b is rocky, or a gas planet like Uranus.

Then there’s the claim that the planet’s temperature is 22 degrees centigrade. That’s right, as precise a measurement as 22 degrees. Pleeeeeze. Meteorologists often cannot agree on the temperature in their own back gardens. It depends on whether you place the thermometer in the shade or in the Sun, for a start.

Kepler-22b is clearly an interesting discovery, and I don’t have any quibble with the claim that it likely lies in its host star’s “habitable zone”. But it is tiresome, and in the long-term potentially damaging to astronomy’s credibility, to exaggerate the claim that it is “Earth-like”, when that can simply be shown not to be true.

One day, someone (quite possibly Kepler) will find a bona fide one Earth mass, one Earth radius rocky planet orbiting a Sun-like star at just about the same distance we do. Let’s hope we haven’t Cried Wolf once too often before then…..



  1. Look on the bright side, at least this one actually exists. I also feel the coverage is missing the significance of what Kepler has done so far which is provide excellent statistics on the diversity of systems with hot Neptunes.

  2. Bingo!

  3. kepler is a fantastic experiment that has and will continue to make fantastic discoveries. This is not one of them, I’m afraid. To sit hear and say nothing means that experiments that will actually look at the HZ planet content will find funding hard to get as Kepler has “done it all”.
    From the Kepler conference I hear the mass limit is 38 Earth masses so it can be any time of planet you want! Fair enough the NASA press release points this out but then why call it “Earth 2” – surely this gives the impression of an earth analog….

  4. excuse my spelling mistakes – too much rushing!

  5. Plus there is all the fun of:

    1) it isn’t a planet as per the IAU definition (which seriously guys, I thought the point was to help the extrasolar planet people out, not wank about demoting Pluto and defining planets as only things in orbit about Sol).

    2) none of us can read the paper. As per my Facebook feed, the paper has been accepted but it isn’t on the arXiv server yet, which is a major NASA press release foul and you’d think they’d be a little more cautious given that whole arsenic life snafu.

    3) it’s only a transit measurement, there is no radial velocity data yet so it could just become another Gliese 581g

    and 4) yeah the only mass estimate they have for a potential planet is 35 Earth mass upper limit via Arecibo.

    and let’s not talk about the fact that the host star is metal poor, so that’d affect the ability to have a rocky planet (Al-26 and Fe-60 decay arguments go here for differentiation) and that also could affect the geometric albedo since the rocky bodies in our own solar system that have reached higher temperatures (presumably through Al-26) have higher albedos and thus lower temps.

  6. In fairness, the paper does say that the temperature is the blackbody temperature for an assumed albedo (I haven’t read the press release, that detail might have been glossed over). We’ve made no attempt to estimate the uncertainty on that value because we have no idea what the atmosphere is, but the true value is probably warmer.

    • Accurate to 1 degree? I’d give my students 0/10 for that!
      But the main point is that the world’s press seems to think this is another Earth, which is hugely misleading. Dreadful reporting is going on. Today’s Independent newspaper has the headline “Kepler 22b Could Be Inhabited” and says “Scientists believe Kepler 22b may not only be inhabitable but may be inhabited”. Today’s Daily Telegraph states “The planet contains both land and water and has a similar temperature to that of the Earth”.
      This is COMPLETE BULLSHIT. Where are these journalists getting this utter crap from? Are there members of the Kepler team putting these ideas out and deliberately failing to correct journalists misinterpretations?
      As you can see from the quotes I’ve lifted from two of Britain’s leading heavyweight papers, this is getting very serious and once people realise it is nonsense, it risks seriously damaging the credibility of astronomy (and science) in the way say the MMR scandle did. We don’t know the mass, nor whether it is rocky or gaseous (I’d wager it’s probably similar to Uranus). Hopefully Kepler will one day find a one Earth mass rocky planet at one AU around a G2V star. THEN we can get excited. If it is indeed the case that the world’s press are going mental because they are hopelessly over-interpreting Kepler’s press release, then could I suggest that Kepler puts out another release clarifying the situation? After all, Kepler doesn’t want to diminish the impact of the first bona fide discovery of “another Earth” if/when that happen…..does it?

  7. One of my twitter followers has made a good point in contrasting the reporting of this discovery with the much more measured and sensible reporting of the FLT neutrinos story.

  8. […] Not another “Earth-like planet” – Matt Burleigh […]

  9. Hilarious twitter account has appeared: @kepler22b_REAL. Sample tweets: “Earth is the smallest and “simplest” of all known planets here on Kep22.” and “God is real. We would know, he vacations here. Thinks earth is “pointless wad of loam”, has nice hair”

  10. […] This throttled-back candour seems to be reflected in all of NASA’s recent decisions.  With budgets in seemingly interminable flux, and each administration dreaming of unrealistic resurrection of the manned space program, the agency has been forced to box clever as of late.  It is still licking its wounds from the JWST budget scandal, as well as the less public academic backlash from the media circus surrounding missions such as Kepler (see e.g. here). […]

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