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Avenging Pluto: NASA finds smallest planet yet

A Moon-size Line Up: The line up compares artist's concepts of the planets in the Kepler-37 system to the moon and planets in the solar system.
A Moon-size Line Up: The line up compares artist's concepts of the planets in the Kepler-37 system to the moon and planets in the solar system.

The smallest planet ever found to be orbiting a star like our sun has been measured by NASA's Kepler telescope, the space agency announced this week. Lest you get carried away with a colonization fantasy, scientists say the orb — located in a different planetary system — likely lacks the criteria for sustaining life. 

The planets are located in a system called Kepler-37, about 210 light-years from Earth in the constellation Lyra. The smallest planet, Kepler-37b, is slightly larger than our moon, measuring about one-third the size of Earth. 

NASA estimates the surface temperature of Kepler-37b somewhere in the neighborhood of 800 degrees Fahrenheit (or, "hot enough to melt the zinc in a penny,") and it likely has no atmosphere or water, notes the article "A sub-Mercury-sized exoplanet," published Thursday in the journal Nature.

Hitherto, no planets have been found that are smaller than those we see in the Solar System. Here we report a planet significantly smaller than Mercury8...Owing to its extremely small size...and highly irradiated surface, the planet, Kepler-37b, is probably rocky with no atmosphere or water[...]

The Kepler mission — developed at NASA's Jet Propulsion Laboratory in Pasadena — is searching the "habitable zone" of planetary systems for orbiting, Earth-sized planets where water might exist on the surface.


Presuming the size of a star is known and accurate, here's how Kepler measures a planet's size:

  • The telescope simultaneously and continuously measures the brightness of more than 150,000 stars every 30 minutes. 
  • When a planet candidate transits, or passes, in front of the star from the spacecraft's vantage point, a percentage of light from the star is blocked.
  • This causes a dip in the brightness of the starlight that reveals the transiting planet's size relative to its star. 


Sound waves generated by a boiling motion beneath the surface of Kepler-37 were studied by asteroseismologists to learn about the star's interior structure (similar to geologists using seismic waves generated by earthquakes to probe Earth's interior):

  • Sound waves travel into a star and bring information to the surface.
  • The waves cause oscillations that are observed as a rapid flickering of the star's brightness.
  • Small stars ring at high tones; larger stars boom in lower tones.
  • Barely discernible, high-frequency oscillations in the brightness of small stars are the most difficult to measure.
  • This is why most objects previously subjected to asteroseismic analysis are larger than the sun. 

The radius of the star is known to three percent accuracy, say astronomers, thanks to the precision of the Kepler instrument. That measurement translates to "exceptional accuracy in the planet's size," says NASA