Research finds planets can form around different types of stars

June 13, 2012
New research shows that planets up to four times the size of the Earth can form around very different stars -- including stars that are poorer in heavy elements. Credit: MediaFarm / Niels Bohr Institute

It had previously been thought that planets were more likely to form around a star if the star had a high content of heavier elements. But new research from the University of Copenhagen, among others, shows that small planets can form around very different types of stars – also stars that are relatively poor in heavy elements. This significantly increases the likelihood that Earth-like planets are widespread in the universe. The results have been published in the prestigious scientific journal, Nature.

3,000 exoplanets, i.e. planets orbiting a star other than the Sun, have now been discovered. 2,300 of these potential planets are being observed with the Kepler Satellite by measuring the brightness of the host . If a planet moves in front of its star, there is a small decrease in the brightness and if this happens repeatedly, it could be a planet orbiting the star and dimming its light.

A multitude of planets have been discovered so far and by measuring their size it is possible to distinguish between gas giants like Saturn and Jupiter, or whether they are smaller, terrestrial planets like Earth and Mars.

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Astrophysicist Lars Buchhave, University of Copenhagen explains about his new research showing, that planets up to four times the size of the Earth can form around very different stars -- including stars that are poorer in heavy elements. The conclusion, says Lars A. Buchhave, is that these observations mean that Earth-like planets could be widespread throughout our galaxy, as they have no special requirements for an elevated content of heavy elements in stars in order to be formed. Credit: Niels Bohr Institute, University of Copenhagen

Requirements for planet formation?

But it is not only the planets that are interesting. It is also the stars that they are orbiting. Because what are the requirements for planet formation?

"I wanted to investigate whether planets only form around certain types of stars and whether there is a correlation between the size of the planets and the type of host star it is orbiting," explains Lars A. Buchhave, astrophysicist at the Niels Bohr Institute and the interdisciplinary research centre, StarPlan at the University of Copenhagen.

Lars A. Buchhave therefore developed a method to 'wring' more information from the stellar spectra. Up until now, we have seen that most of the gas giants were associated with stars with a high content of heavy . For a star to have a high content of heavy elements it has to have gone through a series of rebirths.

Cosmic cycle

A star is a large ball of glowing gas that produces energy by fusing hydrogen and helium into heavier and heavier elements. When the entire core has been converted into iron, no more energy can be extracted and the star dies flinging massive clouds of dust and gas out into space. These large clouds of gas and dust condense and are recycled into new stars and planets in a gigantic cosmic cycle. The new stars that are formed will have a higher content of than the previous and for each generation of star formation there are more and more of the heavy elements and metals.

Remnants from the stars

The planets are formed from the remnants of the clouds of gas and dust that rotate in disc around the newly formed star. In this protoplanetary disc, the elements begin to accumulate and clump together and slowly the planets are formed.

In the later generations of stars with a high content of heavy elements, the rotating disc of dust and gas particles has an elemental composition that is most likely to promote the formation of gas giants like Saturn and Jupiter.

Recent research shows a different picture for the smaller planets.

Fewer requirements for small planets

"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 , 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," Lars A. Buchhave.

The conclusion, says Lars A. Buchhave, is that these observations mean that Earth-like planets could be widespread throughout our galaxy, as they have no special requirements for an elevated content of heavy elements in stars in order to be formed. This conclusion resonates well with the picture that is emerging of the distribution of small planets in our galaxy, namely that it seems more the rule than the exception that a star has small planets orbiting them.

Because small Earth-like are not dependent upon a high content of in their host star, they could be both widespread and could have been formed earlier in our galaxy.

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Jun 13, 2012
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5 / 5 (7) Jun 13, 2012
jesus christ(taken in vain, fuck your god) kevin, you sure are trolling hard today.
5 / 5 (7) Jun 13, 2012
Don't bother. Notice how he's always the first to post on anything sciency (especially stellar/big-bang/evolution related)...and then never posts again opn the same thread? He never comes back to check.

I guess he has himself notfiied as soon as a new article pops up in the astronomy and biology sections. Then he heads over here to post some crap, then goes back to collect the fifty cents from his church group he gets for stifling/derailing scientific debate.

Just click on "report" and ignore it.
5 / 5 (2) Jun 13, 2012
It is interesting that older stars with low metal content might have Earth-style planets. Alas, the absence of Jovian planets will permit a lot of orbital "junk" to remain in the system, making asteroid impacts more common than in our system.
Also, if an Earth-size planet orbits a yellow dwarf like our sun, there is no guarantee that an ecosphere can compensate for the slow increase of stellar luminosity (Earth is expected to succumb to a Venus-style runaway warming in ca. 500 million years).
If some of those planets are significantly older than our system they will be on the edge of roasting, even if they once had a pleasant climate.
5 / 5 (2) Jun 13, 2012
This was expected from earlier Kepler, and perhaps Corot, data, nice to see it confirmed!

@ Birger: Most stars are M stars, and they form the most terrestrials as well. M stars, being small, may live on the main sequence for over 100 billion years. Then it is the atmospheric lifetime that sets the limit, so you want super-Earths to have atmospheres that last for ~ 20 billion years or more. That is older than the universe is now, so most of the inhabited planets are still around.

The evolution of the biosphere is argued. The first threat is claimed to be that in some 200-500 million years the CO2 content of the atmosphere will be reduced due to increased geological cycling to a level that can't support C4 plants. Most herbivores lives on C4, so bye bye complex life as we know it.
4 / 5 (4) Jun 13, 2012
I should add that there isn't an absence of giants around low metallicity star, just a slight trend towards lesser frequency (as per Kepler).

But as for them protecting inner terrestrials, are you sure this old idea is still viable? I seem to remember a recent paper where they had actually modeled it, perhaps for the first time, and Jupiter _increased_ the number of impactors hitting us.

Can't say if that was a generic result, because I haven't kept up ever since Mojzic (sp? sorry, its late and I haven't the ref handy) et al modeled how even the Late Heavy Bombardment impactors, even crust busters, won't suffice to sterilize crustal cellular life once it takes.
5 / 5 (1) Jun 14, 2012
Torbjorn, I stand corrected regarding the impact frequency, but what about the amount of isotopes in the planet, and the decay-induced warming keeping tectonic processes going?

If a terrestrial planet (in a very early stellar system) has less of the isotopes needed to heat the interior, it might soon become tectonically dead, or at least unable to maintain plate tectonics. Some volcsnism might remain rather like Mars, but even in the presence of oceans the necessary carbon cycle could become ineffective. After a couple of billion years you could end up with a senescent world whose carbon dioxide has been tied up in carbonates.
5 / 5 (1) Jun 14, 2012
Addendum: I do not know anything about the relative abundances of the long-lived isotopes in the core and mantle, and the abundances of elements in rather old stellar systems.
If those isotopes are created at the same frequency as other heavier elements, planets in such systems may indeed have long-lived plate tectonics. Wether such planets remain habitable long enough to still support life after being around twice as long as the Earth depend on several variables.

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