Tiny Planet Kepler-37b Small Enough To Be Moon’s Twin Sister

Kepler-37b is a little more than 10 percent larger than Earth’s moon. Illustration: Bob King with NASA image

Another day, another planet, right? Astronomers have discovered 861 new planets beyond the solar system. Most are the large, Jupiter-sized variety because they’re easiest to detect. A few are Earth-sized and considerably harder to see.

Get ready for a new milestone. In an article published in the Journal Nature, a team of astronomers describes their discovery of a planet only 2,400 miles in diameter. That’s hardly bigger than the moon (2,160 miles) and smaller than the planet Mercury.

Named Kepler-37b, it orbits a star in the constellation Lyra the Harp some 210 light years from Earth. The moon-sized planet and two larger companion planets were found by NASA’s Kepler space telescope. With an unblinking eye, Kepler stares at more than 150,000 stars making measurements of their brightness every 30 minutes.

Stars ring like bells as convective gases churn beneath their surfaces. Small stars ring at high tones; large stars at low tones. Credit: NASA

When a potential planet orbits in front of its host star, it blocks a small percentage of the star’s light. Astronomers measure a dip in brightness which tells them the planet’s size relative to the star.

In the case of Kepler 37-b, to pin down the planet’s diameter, they accurately measured the size of the host star using a relatively new technique called asteroseismology.

The same way geologists use seismic waves to study the interior of the Earth, astronomers use sound waves generated by the boiling motions of hot gases in the star’s interior. Sound waves create “starquakes” that make the surface oscillate up and down and ring like a bell. Kepler observes the vibrations as a rapid flickering of the star’s light.

Using high-precision instruments, the scope determined the host star’s size to an accuracy of 3%. With that number in hand, calculating the planet’s diameter was a straightforward exercise.

While Kepler’s primary mission is to search for planets in a star’s “habitable zone” where liquid water is stable on a planet’s surface, it turns up all kinds of wild worlds. Kepler-37b orbits Kepler-37 every 13 days at a distance of only about 12 million miles or three times closer than Mercury is to the sun. It’s a hot, rocky world with a furnace-like surface temperature of over 800 degrees F. Not a place where life might easily likely a foothold.

An artist concepts of the planets in the Kepler-37 system to the moon and planets in the solar system. Kepler-37c, the second planet, is slightly smaller than Venus, measuring almost three-quarters the size of Earth. Kepler-37d, the third planet, is twice the size of Earth. Credit: NASA/Ames/JPL-Caltech

Kepler-37’s solar system is quite a bit different from the one we know best. Its other two planets, Kepler 37-c and 37-d, also orbit closer in than Mercury. Despite being within our planet’s size range, being so close to their host sun, they’re continuously baked. No Earth-like planets here.

Pluto animation based on Hubble Space Telescope images. Credit: NASA

So you might naturally ask, how does this moon-sized object rate as a planet? What about Pluto? Well, it does orbit a star, and its size tells us it’s spherical in shape, but whether it “clears it orbit” of other smaller bodies, as the current definition of planet requires, we may never know. My gut feeling tells me we’re in a state of transition about what defines a planet. Sooner or later, astronomers will have to take a look at the big picture that now includes hundreds of new planets and 128 multiple-planet solar systems and decide on something more comprehensive. Perhaps little Kepler-37b will nudge us in that direction.

13 Responses

  1. Giorgio Rizzarelli

    Exoplanets to help define better a planet.. Well put Bob. I recently learned that from 2008 IAU doesn’t consider Pluto anymore a dwarf planet, but a plutoid, after that Alan Stern of New Horizons observed that even Earth, Mars, Jupiter and Neptune haven’t really cleaned up their orbits from other bodies. They have of course quite cleaner orbits than Pluto or Ceres, but the point is if we can set a definite boundary for the definition of (dominant) planet. We’re really in a state of transition about, and good idea, exosystems could help.
    Thanx for sharing in previous blogs the great sun pillar and sundog photos, and the DA14 radar movie.

    1. That “IAU doesn’t consider Pluto anymore a dwarf planet, but a plutoid” is plain wrong: plutoids are dwarf planets with semimajor axes larger than Neptune’s; Ceres is the only dwarf planet that’s not in this category, a situation very unlikely to ever change. For practical astronomy all those terms are largely irrelevant anyway as in the Kuiper Belt there’s a gradual size spectrum without a clear gap between the dwarf planets Pluto, Eris, Makemake and Haumea and all the other bodies, so one usually talks about large and not so large Kuiper Belt Objects or Trans-Neptunian Objects.

      1. astrobob

        Thanks Daniel for dropping by. I was coincidentally re-replying to Giorgio to tell him that couldn’t be correct. Last summer I heard an enjoyable back-and-forth exchange on the topic by Alan Stern himself, who feels the term ‘dwarf planet’ is not accurate, and another planetary scientist, who was comfortable with the new definition. Neither used the term ‘plutoid’ to describe Pluto.

    1. astrobob

      There are so many terms for some many different solar system objects it can be a little confusing at times.

  2. This may be an appropriate time to put a plug in for an upcoming 2013 summer astronomy conference in Thunder Bay, Ontario. Dr. Sara Seager, exoplanet expert extraordinaire from MIT, will be one of our keynote speakers at the General Assembly of the Royal Astronomical Society of Canada, June 27 to July 1. I have a few more words about this Bob if you can contact me.

  3. RC

    As I’ve seen the list of Kepler planets come out, it seems that they all have very short orbital periods with orbits similar to Mercury and are deemed “too hot”. I looked at the list of planets and most of them orbit their star in less than 25 days! Very few have orbits of more than 250 days. If we’re looking for an earth-like planet around an sun-like star, don’t we need to find a planet with an orbital period of around 300-400 days?

    Kepler has been watching the stars for almost 4 years now, and we’re still only finding these planets will small orbital periods. Obviously, there’s a TON of information to sort through in order to find these planets… but… Kepler was originally supposed to be a 3 1/2 year project, before it was extended to 7. Is it realistic to find a planet with a 350-400 day orbit when you’re only looking for 3 1/2 (now 7) years? That planet would only be spotted by Kepler maybe 1-3 (now 6-9) times.

    1. astrobob

      I have not looked at the list of Kepler planets but I do know that the longer the orbital period the greater the chance the planet will not be seen to transit its host star. Assuming stars’ equators all have random inclinations – in other words, we could be viewing stars from north or south pole on or some other inclination – coupled with any inclination of the planet’s orbit itself, it would be easy for stars with more distant planets (greater orbital periods) to completely miss transiting their host stars.

      1. RC

        Right, I was thinking about that too. Really, when you consider how rare it is for us to see one of the planets in our OWN solar system transit across the sun (Venus) it seems kind of amazing that we’re finding any planets at all!

  4. Bob Crozier

    So, since you posted this, I have been trying to understand this concept of asteroseismology. Am I to understand that the /entire/ star is resonating at a particular set of 3 frequencies and that the measurement of those frequencies can tell us (fairly) precisely the size _and_ the mass of that star? If that is correct, that could change a lot of things that we have thus far only estimated about stars, couldn’t it? Isn’t it true that if we knew with some precision the size and mass of a star, that we could then also know its actual brightness? And if we were to knew with some precision its actual brightness, couldn’t we then calculate its actual distance fairly precisely from its /apparent/ brightness? Here’s my real question: how well can we really measure that resonance, especially for more distant stars?

    1. astrobob

      I can tell you this – I’m no asteroseismology expert! Astronomers use parallax, Cepheid variables and other methods to arrive at a star’s distance, which gives us a handle on its true brightness or absolute magnitude. To measure a star’s diameter, you use the star’s absolute magnitude combined with its color to determine how much energy it emits, which in turn depends on its surface area. This is how most star diameters are measured. The seismology method is relatively new. Measuring star oscillations takes very sensitive equipment. I don’t know how effective it is at great distances.

      1. Bob Crozier

        Thanks. It is going to be interesting to watch over the next few years to see what comes of this.

        Almost everything I found about asteroseismology, starting with the link you provided, seemed (to me at least) to be pretty technical stuff and hard for me to stick into my little brain.

        Again, I am pretty new to all things astronomical, so perhaps my following questions will only show my ignorance, but… that’s how I learn. Parallax is only accurate for things relatively close (astronomically speaking… a few hundred light years or something?), isn’t it, and the further away an object is the less accurate this method becomes? And the use of Cepheid variable stars is really only good for measuring the distance to that particular star, isn’t it?

        If this asteroseismology method turns out to be as accurate a measurement as it sounds like it might be, _and_ if the things that we can infer from those measurements are as accurate as they seem to be suggesting, then it seems to me that this could fine-tune our view of the bigger world around us by several orders of magnitude!

        Thanks again for all the time and effort you obviously put into this blog, and for putting up with the never-ending questions from people like me!

        1. astrobob

          You’re welcome Bob. You ask good questions. Cepheids also are found in clusters with other stars and are bright enough to observe in nearby galaxies. Measuring their distance gives us the distance to the cluster (and all those stars) as well as distances to galaxies.

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