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Kepler 10-b: the tiniest planet there ever was


As a child, did you lie in your bed at night unable to sleep for wondering how many planets like Earth existed? Does your curiosity still render you insomniac?


Well, have hope for your future REM prospects: NASA’s Kepler Mission, which boasts a dedicated planet-finding telescope, is taking a census of planets, from which we will be able to know, statistically, how common Earth-like planets are.



Artist's rendition of Kepler (JPL)
Recently, Kepler discovered the smallest planet ever. It, endearingly, is called Kepler 10-b and was discovered by Natalie Batalha, et al, “et al.” being fifty-one other people. Kepler 10-b has a radius 1.4 times the size of Earth’s and a mass 4.56 times the mass of Earth. The definition of “density” will tell you that it is more dense than our home planet, meaning that it cannot be a literal twin, but it is the closest we’ve come.

Information about Kepler 10-b comes from the draft paper Kepler’s First Rocky Planet: Kepler-10b by Natalie Batalha, et al. [2010 Jan 10].

For a long time, people did not know a lot of things.
  1. That there was a such thing as a planet.
  2. That there were other planets in our solar system.
  3. That the Sun is a star, meaning that all the stars are like the Sun, but farther away.
Until recently, we could not contemplate the existence of planets outside the solar system. And without the technology to observe tiny, tiny hunks of rock and gas and dirt from light-years away, we could never go farther than contemplation.
Only in the past 18 years have we known for sure that exoplanets exist. The first confirmed planets were found in 1992, orbiting, of all things, a pulsar (Wolszczan & Frail, D. A., 1992). No planet around a pulsar would be habitable. It would be freezing and subject to the kind of General Relativistic effects that would make your face look like those kids in The Ring.



However, in 1995, Mayor & Queloz found an exoplanet orbiting a main-sequence (not big, not small, just right) star called, affectionately, 51 Pegasi. Newsflash: Other stars like ours could have planets. The existence of our solar system might not be remarkable, which is what astronomers like. Ever since those Earth-is-the-center-of-the-universe-no-wait-the-Sun-is-no-wait-the-galaxy-is-no-wait worldview shifts, they have been loathe to think that we are special.



Since 1995, we have found more than 500 planets, and astronomers are beginning to feel more comfortable. These 500ish planets (I say “ish” because more planets are discovered all the time, and if I were precise today, I’d be wrong tomorrow) have been found using indirect detection. Planets are usually 1/1000000 times as bright as the star they orbit, making it virtually impossible to directly “see” the planet. So how do we know the planets are there if we can’t see them?



The most common way is "transit timing." When a planet goes in front of a star during its orbit, the star’s light will dim slightly, periodically. Example below courtesy of NASA. [To see exoplanet methods discussed in detail, check out “The Detection and Characterization of Exoplanets” (Lunine, et al., 2009)].
This and other popular methods, however, are most sensitive to large planets in tiny orbits, a.k.a. “hot Jupiters” (Cumming, et al., 2008). We’ve had a hard time finding planets too much smaller than Jupiter, until recently. Until Kepler.



Kepler uses the transit method, but it is ultraefficient. Its mission goal is to find “Earth-sized planets around Sun-like stars.” No more of these “hot” “Jupiters.” Kepler, in space, will spend its entire life staring at the same piece of sky. Kepler simultaneously monitors the brightness of all ~100,000 stars that are in its field-of-view--24/7. It lies in wait for periodic variations until BAM it finds one, the astronomers get excited, and someone at a telescope on solid ground confirms the detection.



Most information about planets comes from information about their parent stars, things like asteroseismology, or how the star oscillates, and thus how big it is and what its interior is like, and thus what kinds of stars correspond to which kinds of planets.



Animation of stellar oscillations (NASA).

Stars and their planets formed in the same place out of the same stuff, so properties of a star are likely to be relevant to what’s inside the planet. For example, we know that stars with higher metallicity (and when astronomers use the word “metal,” they mean anything heavier than Helium) are more likely to have more massive planets (Marcy, et al., 2005).



So Kepler was staring at its spot, bored, and then it found something new: something not too big and not too small. Something called 10-b.



10-b’s host star is a main-sequence star that is 11.9 billion years old and 5627 Kelvin. It is 0.895 times the mass of the sun and 1.056 times the radius of the sun. In other words, pretty damn Sun-like.



10-b is 4.56 times more massive than the sun, but only 1.4 times as wide, giving it a density of 8.8 grams per centimeter-cubed (Earth’s is 5.52). That means that you won’t be moving to 10-b any time soon, as it's made mostly of rock. Astronomers create plots of what a planet made of, say, water-and-rock would look like on a mass-radius diagram, and then they compare that to exoplanet data. In this case, the planet looks more like something that has a ton of iron than a ton of irises.



But its uninhabitability does not make 10-b less interesting. Given the way exoplanet searches have exploded, and the number of earth-sized planets Kepler is likely to discover before it closes its eyes, we are well on our way to understanding how our solar system fits in.



Astronomers estimate that about 40% of stars have planets. Given the 100 billion stars in the Milky Way, there are 40 billion stars with planets. Given the 100 billion other galaxies in the universe, there are 4,000,000,000,000,000,000,000 stars with planets in the universe. Our solar system probably represents only 0.000000000000000000025% of all solar systems.



If that doesn’t put you in your place, I don’t know what will.


Andrew Cumming, R. Paul Butler, Geoffrey W. Marcy, et al. (2008). "The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets". Publications of the Astronomical Society of the Pacific 120: 531–554.
Lunine, J;
Macintosh, B; Peale, S. (2009). "The Detection and Characterization of Exoplanets". Physics Today 62 (46): 47-51.
G. Marcy et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement 158: 24–42.

Mayor, D. Queloz (1995). "A Jupiter-mass companion to a solar-type star". Nature 378: 355–359.
Wolszczan, A.; Frail, D. A. (1992). "A planetary system around the millisecond pulsar PSR1257+12". Nature 355 (6356): 145–147.


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Reader Comments (2)

Thanks for all the zeros. Sometimes scientific notation just doesn't do a number justice.

January 19, 2011 | Unregistered CommenterJayme

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