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Small Planets: No Need for High Metallicity?

In astronomy, the word ‘metals’ refers to anything heavier than hydrogen and helium. Stars fuse hydrogen into helium and from there work their way into the higher elements until hitting iron, at which point the end quickly comes, with ‘star stuff,’ as Carl Sagan liked to put it, being flung out into the universe. Through stellar generations we can trace a higher concentration of the heavier elements as stars are born from the materials of their predecessors. And we’ve learned that those metal-rich stars are the most likely to produce gas giants like Jupiter and Saturn.

What’s intriguing is the issue of smaller planets and the conditions for their formation. After all, the content of the disk from which planets are formed parallels the metallicity of the host star. I’m looking at new research from Lars A. Buchhave (Niels Bohr Institute/University of Copenhagen) into planet formation, using data from the Kepler telescope. In Buchhave’s words:

“We have analysed the spectroscopic elemental composition of the stars for 226 exoplanets. Most of the planets are small, i.e. planets corresponding to the solid planets in our solar system or up to four times the Earth’s radius. What we have discovered is that, unlike the gas giants, the occurrence of smaller planets is not strongly dependent on stars with a high content of heavy elements. Planets that are up to four times the size of Earth can form around very different stars – also stars that are poorer in heavy elements.”

Buchhave and team focused on whether small, Earth-like planets needed the same kind of metal-rich environment demanded by the gas giants, at least those with short orbital periods. Given that planets like the Earth are made up of heavier elements — iron, silicon, oxygen, magnesium — you would assume that small planet formation would be much more efficient around metal-rich stars. The new paper, which has been published in Nature, argues that the idea is wrong, and that opens up a lot of territory. Without special requirements for heavy elements in their stars, Earth-like planets could indeed be widespread in the galaxy.

Image: This artist’s conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Debris coalesces to create rocky ‘planetesimals’ that collide and grow to eventually form planets. The results of this study show that small planets form around stars with a wide range of heavy element content, suggesting that their existence might be widespread in the galaxy. Credit: University of Copenhagen/Lars Buchhave.

The work also implies that small planets could form earlier in galactic history than has previously been thought. Buchhave’s work examines this through the study of the spectroscopic metallicities of the host stars of the 226 Kepler candidates chosen. The average metallicity for planets smaller than four Earth radii turns out to be close to that of the Sun, but Buchhave says in this NASA news release that stars with just 25 percent of the Sun’s metallicity can also form small planets. Meanwhile the data continue to support the preferential formation of gas giants around higher metallicity stars.

From Natalie Batalha (NASA Ames), a member of the Kepler science team:

“Kepler has identified thousands of planet candidates, making it possible to study big-picture questions like the one posed by Lars. Does nature require special environments to form Earth-size planets? The data suggest that small planets may form around stars with a wide range of metallicities — that nature is opportunistic and prolific, finding pathways we might otherwise have thought difficult.”

Indeed. We are learning that it doesn’t take that many generations of stars to start producing rocky worlds. The work was presented yesterday at the 220th meeting of the American Astronomical Society. “Giant planets prefer metal-rich stars. Little ones don’t,” says David Latham (Harvard-Smithsonian Center for Astrophysics). The CfA’s own news release says the work supports the core accretion view of planet formation, in which steadily accumulating planetesimals combine to form planets, with the largest quickly gathering hydrogen. Higher metallicities make quick formation of large cores more likely, which explains the connection between heavier metals and gas giants.

Are there SETI implications here as well? This from Jill Tarter (SETI Institute):

“The idea that very old stars could also sport habitable planets is encouraging for our searches. In particular, intelligent life has taken a long time to evolve here on Earth. Consequently, it’s reasonable to suppose that older planetary systems are more likely to have technological societies – the kind we might detect with our radio telescopes.”

And that reminds me to note that the SETI Institute is hosting SETIcon II in Mountain View, California from June 22-24, where those in attendance can rub elbows with the likes of Geoff Marcy and Debra Fischer. I see that tickets are still available to the public.

The paper is Buchhave et al., “An abundance of small exoplanets around stars with a wide range of metallicities,” published online in Nature 13 June 2012 (abstract).


Comments on this entry are closed.

  • Rob Henry June 29, 2012, 0:07

    I feel that I should register my agreement that today there seems powerful evidence of ribosomal generated protein utilisers displacing much other life, and subsequently, DNA utilising forms displacing all but our shadow biosphere (if it exists). However I suggest that (IF their turns out to be no shadow biosphere) those earlier forms would have to have very similar levels of complexity to LUCA, and their biochemistries be surprisingly similar (else they could not be displaced completely).

    An example that could possibly bear me out is if nonribosomal peptide synthesis turns out to be primordial. To me it would not make any sense for this less efficient method of peptide production to have originated after better mechanisms were already in place. Thus I think that earlier forms would have made similarly large L-proteins, just made them in a worse way.

    I also should admit that my current thinking is coloured by my past stupidity. I had long known that only around 1% of the level of respiratory biological activity in nutrient-poor soil could be accounted for by bacteria that could be cultivated. And I had accepted without question (very unusual for me) that this could be explained in terms of an assumed high senescence rate of the few thousand species of easily cultural bacteria. When I was (slowly) availed of the truth (that its largely due to millions of other bacterial species that are hard to culture) by a string of discoveries I wondered how I could ever have been that blind. Thus my problem with the inexplicably high levels of organic material in these same soils that is now confidently assumed to represent dead bacteria.

  • Rob Henry June 29, 2012, 19:48

    I now see that my above version off pre-ribosomal biology is so radically different from the conventional RNA World, that I am in need of explaining how it conforms to the same evidence, but first I will continue where I left off…

    Nonribosomal peptide synthesis is a very common process and very similar to lipid synthesis. The need for a cell membrane predates the cell, and so this indicates that analogous kinds of synthetic processes are at least as old as the cell.

    The nature of nonribosomal peptide synthesis has a lower fidelity, but it would be very good at producing structural proteins with a repetitive primary sequence. It is, though, capable of producing some enzymes.

    Actually, if proteins were only structural at the beginning it would greatly alleviate the problem of how chaperone proteins ever originated (it would be fatal if they appeared after a great number of proteins already had enzymatic function).

    So imagine a primordial cell fill of structural proteins and RNA sequences that are meant to somehow do most of the enzymatic work. Considering how much RNA you would need just to wrap themselves in a large enough stable 3D ball to from active sites, it would take a massive amount of RNA – but wait. They can attach to those structural proteins, and then we would only need much shorter sequences.

    We have good evidence that RNA performs such highly evolved enzymatic action in ribosomes that it could not be displaced by better proteins in the modern cell, yet we are asked to accept that their (entire?) stabilising scaffold was replaced by proteins. I put it to you that, a better fit with the facts, is that such stabilising proteins were there from the beginning.

    And that is how I see it possibly that immediately pre-ribosomal biology had comparable complexity and similar composition to LUCA.

  • Eniac July 5, 2012, 23:39

    LUCA had RNA, proteins, DNA, and lipids. A complete set, if you will. However, I consider it extremely unlikely that abiogenesis involved all of these, as getting all of them together at once would have an enormous cost in terms of probability.

    This is what the RNA hypothesis is about: In the beginning, there was only RNA, likely in an environment where membranes are not necessary, such as porous rock, or clay, or what have you. All others where added later for the evolutionary advantages they confer: Peptides provide greater chemical and structural variety and stability, DNA greater replication fidelity and thus larger genomes, and lipid membranes greater physical independence from particular environmental conditions.

    I would not feel safe proposing the order in which these innovations happened, but for probabilistic reasons I feel sure they happened one at a time, in sequence. Each arose at some single point in space and time, then quickly covered the globe due to their new advantage, and ultimately rendered pre-innovation lifeforms extinct. LUCA is just one stage somewhere in the middle of this path, the earliest we can reliably describe. Intelligence is just the latest of many more of these transforming innovations. Abiogenesis is the first. It is special, because it did not have the benefit of evolution driving it.

    The progressive nature of this process does not imply any teleological sort of destiny, it is a simple consequence of the irreversible nature of evolutionary breakthroughs.

  • Rob Henry July 6, 2012, 17:37

    I agree, with almost everything you stated, but the few differences we have are of pivotal importance.

    You continue to assume that the chemistry of abiogenesis MUST be similar to that within the first freeliving organism, rather than this just being our default reconstruction.

    Our differences over the application of the reconstruction of LUCA to this matter are understandable, since each seems only able to supply subjective reasons for our stances. But this leads me to emphasise that I suspect our opinions would merge on finding a shadow biosphere.