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I was expecting an ‘exomoon’…

Us and them: ‘e’ is the smallest exoplanet yet and ‘d’ is in the habitable zone

By Hamish Johnston

Imagine my excitement last week when I received an email press release proclaiming:

“At 11:00 BST/12:00 CEST [21 April], a press conference will highlight a major and truly unique discovery in the field of exoplanets, made possible with ESO telescopes.”

“That’s got to be the first sighting of an exomoon”, I thought — a satellite of planet orbiting a star other that the Sun.

I now know I was jumping the gun on exomoons (more on this later) — it turns out that astronomers have found the smallest exoplanet yet, which is a mere 1.9 times more massive than Earth.

Now, I’m not trying to demean this discovery, but hundreds of exoplanets have already been discovered and this one just happens to be the smallest yet. And I’m guessing that in a few month’s time, a smaller exoplanet will be found.

Perhaps the most interesting thing about this exoplanet is that astronomers know it orbits the star Gliese 581 in the company of at least three other exoplanets.

Three of these planets (including the smallest) are too close to the star to harbour life, but the team has also revealed today that the fourth is inside the “habitable zone”, where life could exist. Indeed, at the press conference the team speculated that the planet could be home to “a large and deep ocean” and described it as the first serious “water world candidate” to be found.

The paper was presented today at the The European Week of Astronomy & Space Science at the University of Hertfordshire.

I may have had exomoons on the brain last week after reading this paper on the arXiv preprint server about how an exomoon could be spotted.

Although the exomoon itself would be too small to see, David Kipping at University College London has worked out that its presence would affect the motion of its exoplanet — and this perturbation could be detected by astronomers. Well, at least in principle, as Kipping says “this determination will likely require photometry at the limit of planned missions”.

I suppose I was hoping he was being over cautious!

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  1. Isn’t an exoplanet whose size and location are consistent with the possibility of liquid water a lot more exciting than an exomoon?

  2. Good point!
    I suppose I was looking at it it terms of technological achievement rather than astrobiology. It’s not surprising that today’s telescopes have found a planet in the habitable zone…indeed it would be puzzling if they were not able to.
    On the other hand, finding an exomoon would be a significant technological leap (as Kipping points out).
    Of course, if our solar system is anything to go by, there are probably many more exomoons out there than habitable exoplanets!


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