Small Planets’ Evolutionary Edge

The discovery that planets can form around a variety of stars — and not just specific types, as previously thought — might open the floodgates on the search for habitable worlds in the galaxy.

The hunt for life elsewhere in the galaxy starts with the search for habitable planets — which scientists may have just made a little easier. Astronomers have found that relatively small planets can form around a wide variety of stellar types, observations that imply there may be more stars hosting habitable worlds than we ever imagined.

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Kepler searches for habitable exoplanets in Cygnus and Lyra, which lie in the galactic plane. The mission has discovered more than 2,000 exoplanet candidates. New research suggests the hunt can be continued around stars previously thought unlikely to host planetary systems. Click for a larger view.
Carter Roberts / Eastbay Astronomical Society
Planets are born in disks of gas and dust that surround their parent stars, so both the planets and the star pull their raw material from the same source. For this reason, the earliest stars — those composed only of hydrogen and helium — probably didn’t host planets, as the heavier elements required to form planets didn’t yet exist. Instead, planets began to form as the star population matured and built up stores of carbon, silicon, and oxygen, elements now found in rocky planets.

Two decades of ground-based observations had suggested that metal-rich stars (“metal” is the astronomical term for elements other than hydrogen or helium) were more likely to harbor planets — particularly gas giants like Jupiter — than metal-poor ones. But today, a team led by Lars Buchhave (Niels Bohr Institute), working with results from the planet-hunting Kepler satellite, announced that smaller planets can form around stars with a wide range of metallicities, including those with as little as a quarter the heavy-element content of the Sun.

In a nutshell, this means that the stellar requirements for forming smallish planets are not particularly stringent. This implies two things: First, that planets may have started forming early in the evolution of the Milky Way, giving those worlds a long time to evolve life; and, second, that far more planets might be hiding out there than previously thought.

This illustration shows the Kepler spacecraft in its Earth-trailing, heliocentric orbit. NASA recently extended the telescope’s mission by four years, giving it more time to search for exoplanet candidates.
NASA / Kepler mission / Wendy Stenzel
The discovery comes at an opportune time. NASA’s Kepler mission, originally scheduled to end in November of this year, has been given a four-year extension.

Most of the 226 planet candidates in the new study orbit their stars closely. Such tight orbits are probably outside the “habitable zone” where liquid water, and therefore life, could potentially form on a planet’s surface. But with Kepler’s search extended, scientists will have the opportunity to examine planets with larger orbits and greater chances of having environments conducive to life.

“This really opens up the possibility for searches for habitable planets,” says Guillermo Torres (Harvard-Smithsonian Center for Astrophysics), a coauthor of the paper, which will appear in an upcoming issue of Nature. “We can now look at basically any star, and there’s a chance it will have a small planet … I am more optimistic about [the search for life] now, and I think many other people will be as well.”

8 thoughts on “Small Planets’ Evolutionary Edge

  1. Rod

    At, the Mar/2012 data for Kepler showed 2321 candidates listed. The average size was 3.787 earth radii and average semi-major axis = 0.134 AU. The average surface temperature for these exoplanet candidates is 803 K. Comparing the metrics to the 2011 list of 1235 candidates shows very little change. Clearly Kepler data favors planets that are in very close orbits to their host stars. The 2321 listed in 2012, 2177 have semi-major axis <=0.38 AU or closer than Mercury’s orbit to our Sun.

  2. Bruce

    If you’re an exoplanet enthusiast there’s much to like about this article. Giant Jupiter’s are starting to get boring. As the easiest planets to find, they crowd the early exoplanet lists, as Rod’s often sighted stats show. Naturally, being terra firma planetary beings, we humans want most of all to find planets like ours. Intuitively, earthlike planets ought to be out there, and the numbers of them could still be enormous, but, as Rod would say, “the proof remains elusive.” If rocky planets really do frequently form around even metal poor stars then the implications for habitability are huge. The average stellar metal content in a spiral galaxy drops off with increasing distance from the core, where Supernova enrichment is more common. This finding, if true, greatly expands Galactic Habitable Zones both temporally and spatially. (Yes, I used a spell checker, thank you Merriam-Webster.)

  3. Rod

    Bruce I am not quite seeing the logic in your post. Just because a rocky planet may form around stars with low metal content, this does not = Earth. Kepler 10b is a good example of an exoplanet that is rocky and orbits a host star with lower metals than our Sun (Fe/H = -0.15) yet is quite uninhabitable and much denser than Earth, see

    Q:Are you suggesting that rocky planets will naturally evolve into Earth planets that can support life after millions of years of random particle collisions or just perhaps based upon high stats, somehow an Earth will form all by accumulation of cosmic accidents?

  4. Bruce

    Hi Rod. I welcome your question. No, I don’t mean to suggest that life bearing, earthlike planets are being formed accidentally. I’m convinced completely and I wholeheartedly believe that Genesis 1:1 is true, that “God created the heavens and the earth.” As the “heavens” referred to here means the physical universe, all the stars, planets, etc. contained therein are products of this creation. We now know from astronomy that this is a continuing process, new stars and stellar systems including planets are coming into existence all the time. This happens because matter and energy conform to the well ordered universal laws of physics. As far as we now know, at least once, God gave more attention to one of the planets at his disposal and took actions resulting in the life bearing planet we now call home. If there is now life on other worlds, I believe that it would have been put there by our Creator as well. Apparently, judging from our home, God loves life, and in fantastic diversity at that. As Psalms 104:24 says, “How many your works are, O Jehovah! All of them in wisdom you have made. The earth is full of your productions.” As the earth is full of God’s living productions, what about other planets as well?

  5. Rod

    Bruce, read your post and thanks for clearing it up. In the case of Kepler-10 b there are some areas I will point out following the Galileo method of astronomy. 1. No one observed the parent gas cloud collapse and form the parent star as ZAMS on H-R diagram that Kepler-10 b orbits. 2. There is no dust disk or accretion disk at the parent star observable today where Kepler-10 b orbits. 3. No one observed Kepler-10 b rocky exoplanet form from tiny dust grains and evolve into the present exoplanet and its orbit. There are many assumptions involved in the naturalistic answers concerning the origin of exoplanets and exolife in astronomy and circular reasoning as well.

  6. Bruce

    The Kepler-10 system is an historic and an interesting case Rod. The b planet is the first rocky planet discovered by the Kepler team, while Kepler 10c may have been the second. This system illustrates one of the main points of Craft’s article; rocky planets can indeed be found around low metal stars. Your above post made me curious about the star’s age. I looked it up at It’s age listed as 11.9 +or- 4.5 Gyr, yielding a range of 7.4 to an impossibly old 16.4 billion years. Despite this large uncertainty, this system clearly is old enough for any disk dust or gas to have been accreted or blown away long before man developed the means to observe it. Are you suggesting that since such accretion remnants are now missing that planetary accretion never occurred in this system?

  7. Rod

    Bruce you asked about planetary accretion in this system because we do not see a dust disk or accretion disk today. This question assumes that it is *missing* because the disk dissipated over millions and billions of years which is the standard answer in astronomy thus in the remote past the disk existed. However such explanations are a good example of circular reasoning in astronomy. Applying the Galileo method, we do not factually know that the accretion disk ever existed and most exoplanets documented today do not show dust disks/accretion disks just like most of the stars in the Milky Way do not have such disks. The evolutionary origin explanations are model dependent and based upon a number of assumed, initial conditions to explain the present. I prefer to know all the assumptions to see what is really going on in the reports published.

  8. Bruce

    Speaking strictly scientifically Rod, ‘we didn’t see it happen, therefore it didn’t happen’ seems like a pretty weak argument. Granted though, actual observation IS the gold standard in judging reality. So then, what do we actually observe today? As even your last post acknowledged, we do actually see some stars with accretion disks. These stars are observed to be young, often just emerging from dust clouds. True, the stellar models are as yet imperfect, but even so they do a fairly good job explaining many, many actual observations. These observations include (but I’m sure this list is incomplete as I’m merely an amateur) stellar population abundances on the H-R charts, stellar mass, temperature, radii and their various ratios, variable star fluctuations, occurrence rates and distributions of planetary nebulae, nova and supernova explosions, elemental and isotopic abundance ratios in stars, interstellar gas, dust and meteorites, and the spectra of distant galaxies. The stellar models used to come up with predictions closely matching what we actually observe are based upon the laws of physics applied to predictable conditions inside stars. Laboratory work at particle accelerator facilities provides the reaction rates for the fusion reactions taking place inside stars. All of this scientific work is subject to peer review and confirmation by other researchers. Therefore, in view the foregoing and more, I do believe that the stellar models, including proto-stellar disk planetary accretion, are basically an accurate description about how stars and planets come into existence and likely even when they came into existance. God is still in the equation, Rod. He’s the One who made the rules (the laws of physics), provided the raw material (energy), and keeps it all moving (universal acceleration epochs).

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