Alien earths could form... planet formation
 

Alien earths could form earlier than expected
June 13, 2012




Building a terrestrial planet requires raw materials that weren't available in the early history of the universe. The Big Bang filled space with hydrogen and helium. Chemical elements like silicon and oxygen - key components of rocks - had to be cooked up over time by stars. But how long did that take? How many of such heavy elements do you need to form planets?




Previous studies have shown that Jupiter-sized gas giants tend to form around stars containing more heavy elements than the Sun. However, new research by a team of astronomers found that planets smaller than Neptune are located around a wide variety of stars, including those with fewer heavy elements than the Sun. As a result, rocky worlds like Earth could have formed earlier than expected in the universe's history.

"This work suggests that terrestrial worlds could form at almost any time in our galaxy's history," said Smithsonian astronomer David Latham (Harvard-Smithsonian Center for Astrophysics). "You don't need many earlier generations of stars."

Latham played a lead role in the study, which was led by Lars A. Buchhave from the University of Copenhagen and will be published in the journal Nature. The work is being presented today in a press conference at the 220th meeting of the American Astronomical Society.

Astronomers call chemical elements heavier than hydrogen and helium "metals." They measure the metal content, or metallicities, of other stars using the Sun as a benchmark. Stars with more heavy elements are considered metal-rich while stars with fewer heavy elements are considered metal-poor.

Latham and his colleagues examined more than 150 stars known to have planets, based on data from NASA's Kepler spacecraft. They measured the stars' metallicities and correlated that with the sizes of the associated planets. Large planets tended to orbit stars with solar metallicities or higher. Smaller worlds, though, were found around metal-rich and metal-poor stars alike.

"Giant planets prefer metal-rich stars. Little ones don't," explained Latham.

They found that terrestrial planets form at a wide range of metallicities, including systems with only one-quarter of the Sun's metal content.

Their discovery supports the "core accretion" model of planet formation. In this model, primordial dust accumulates into mile-sized planetesimals that then coalesce into full-fledged planets. The largest, weighing 10 times Earth, can then gather surrounding hydrogen and become a gas giant.


A gas giant's core must form quickly since hydrogen in the protoplanetary disk dissipates rapidly, swept away by stellar winds in just a few million years. Higher metallicities might support the formation of large cores, explaining why we're more likely to find a gas giant orbiting a metal-rich star.

"This result fits with the core accretion model of planet formation in a natural way," said Latham.

Journal reference: Nature



Provided by Harvard-Smithsonian Center for Astrophysics





http://phys.org/news/2012-06-alien-earths-earlier.html

That's a good use of data from the Kepler spacecraft.

> Their discovery supports the "core accretion" model of planet formation. In this model, primordial dust accumulates into mile-sized planetesimals that then coalesce into full-fledged planets. The largest, weighing 10 times Earth, can then gather surrounding hydrogen and become a gas giant.

Yes. That's the standard model.

> A gas giant's core must form quickly since hydrogen in the protoplanetary disk dissipates rapidly, swept away by stellar winds in just a few million years.

Good point.

Where Are The Metal Worlds And Is The Answer Blowing In The Wind
by Andy Tomaswick for SpaceDaily
Los Angeles CA (SPX) Jun 22, 2012

The good news for extra-solar Earth seekers is that terrestrial planets don't seem to have any bias towards the "heavier" stars and can be found orbiting stars with all different levels of metal content. This means there are even more stars that might potentially harbor rocky Earth-sized planets than are likely to harbor Jupiter-sized gas giants.


The torrent of discoveries that has come out of the exo-planet research community lately has brought up more new questions than it has answered. Some of the more interesting ones have to deal with solar system formation. Up until the discovery of the first exo- planet in 1988, scientists had a sample size of one system to use in the development their models of how planets form around a star. Today they have a sample size of 726.

One hundred and fifty of those samples were used in a study on planetary system formation published by a team led by Lars Buchhave at the University of Copenhagen's Niels Bohr Institute.

The team was attempting to understand the distribution of different types of planets around different types of stars. It had previously been found that Jupiter- like gas giants form around metal-rich "heavy" stars but there was no such understanding of the types of stars that rocky, terrestrial planets form around.

The reason for the bias of the gas giants, the team explains, has to do with a time limit on their formation imposed by the solar wind. In the core-accretion model of solar system formation, a newly born star is surrounded by a disk of gas and dust that is gravitationally pulled together to form planets.

However, the stellar wind of the star is also trying to push the gas and dust out of the system and will eventually sweep almost all the remaining dust away.

To counteract that push, the solar system needs large, metallic planetary cores whose gravitational pull is stronger than that of the solar wind's push.

Stars with high metal concentrations are more likely to be able to form those types of cores early in their lives, when more dust and gas is present in the system. Those metal cores are then able to collect the gas and dust remaining in the system, and thus a gas giant is born.

If the star does not have enough metal content, the rocky cores don't form early enough and the gas is gone with the wind forever. The team also pointed out that eventually even low metallicity stars will form metal cores, which answers the question of what kind of stars the terrestrial planets prefer.

The good news for extra-solar Earth seekers is that terrestrial planets don't seem to have any bias towards the "heavier" stars and can be found orbiting stars with all different levels of metal content. This means there are even more stars that might potentially harbor rocky Earth-sized planets than are likely to harbor Jupiter-sized gas giants.

Additionally, terrestrial extra-solar planets might have formed earlier in the progression of the universe than their gas giant cousins. The metal concentrations in stars needed to form gas giants were not present at the beginning of the universe and took multiple iterations of stellar formation and death to accumulate.

Since terrestrial planets don't have the same metal content limitations, they were freer to form much earlier in the history of the universe without multiple solar life cycles.

Much work has to be done to prove these theories. With current technology it is much easier to find gas giants orbiting stars than the normally smaller terrestrial worlds, so there are still many questions about whether rocky planets do in fact significantly outnumber their larger gas counterparts.

With more research and better technology, scientists might be able to answer one of the questions extra-solar planetary research has opened up.





http://www.spacedaily.com/reports/Wh..._Wind_999.html

The good news for extra-solar Earth seekers is that terrestrial planets don't seem to have any bias towards the "heavier" stars and can be found orbiting stars with all different levels of metal content. This means there are even more stars that might potentially harbor rocky Earth-sized planets than are likely to harbor Jupiter-sized gas giants.
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Only a matter of time of course before we find an Earth like planet, situated in the comfy zone of a particular star...

Yes then we're one step of the way there.

A bit unexpected. First thing I did was to check if this result was purely theoretical based on a mathematical model of planet formation, in which case it wouldn't be worth much, or if its predominantly experimental. It's experimental, excellent, the following quote is from http://www.nasa.gov/centers/jpl/news...r20120613.html

"new ground-based observations, combined with data collected by NASA's Kepler space telescope, show small planets form around stars with a wide range of heavy element content and suggest they may be widespread in our galaxy. A research team led by Lars A. Buchhave, an astrophysicist at the Niels Bohr Institute and the Centre for Star and Planet Formation at the University of Copenhagen, studied the elemental composition of more than 150 stars harboring 226 planet candidates smaller than Neptune."

Exoplanet hosting stars give further insights on planet formation
August 16, 2012



An international team, led by EXOEarths researchers (Centro de Astrofísica da Universidade do Porto), proposes that metals like magnesium might have an important role in the formation of low mass planets.




The team, lead by CAUP researcher Vardan Zh. Adibekyan, analyzed high resolution spectra of 1111 sun-like stars, obtained by the HARPS spectrograph (ESO). Of these stars, 109 are known to harbor high mass (Jupiter-like) planets, and 26 have Neptune-like planetary companions.


The team focused especially on studying the abundance of Alpha Elements in these stars, like Magnesium (Mg), Silicon (Si) or Titanium (Ti). The research found that the ratio of these, compared with the amount of Iron (Fe), was consistently higher in stars with planets, with the greatest discrepancy observed for Mg.


The lead author of the paper, CAUP Astronomer Vardan Zh. Adibekyan commented “These findings indicate that some metals other than iron are involved in the process of planet formation, especially when the amount of iron is lower than solar. These results may provide strong constraints for the models of planet formation, especially for planets with low mass.”

The leading theories of planet formation suggest that planets form by clumping smaller particles of heavy elements (metals), into larger and larger bodies. The results put forward by the present study suggest that planets need a minimum amount of “metals” to be formed. The formation of planets, even the lowest mass ones, is dependent on the dust content of the cloud that gave origin to the star and planetary system.


More information: "Overabundance of alpha-elements in exoplanet-hosting", Vardan Zh. Adibekyan, Nuno Cardoso Santos, Sérgio Sousa, Garik Israelian, Elisa Delgado Mena, Jonay González Hernández, Michel Mayor, Christophe Lovis, Stephan Udry. Astronomy & Astrophysics Volume 543, July 2012, Article Number A89 (DOI: 10.1051/0004-6361/201219564 ).

Journal reference: Astronomy & Astrophysics


Provided by Centro de Astrofísica da Universidade do Porto





http://phys.org/news/2012-08-exoplan...ts-planet.html

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