When stellar metallicity sparks planet formation

Apr 10, 2012 By Ray Sanders
Artist’s concept showing a young Sun-like star surrounded by a planet-forming disk of gas and dust. Credit: NASA/JPL-Caltech/T. Pyle

New research predicts the criteria needed for Earth-like planets to form around a star that have one-tenth the metallicity of our Sun. If researchers find small, rocky planets orbiting stars with lower metallicity, it may challenge the presently accepted "core accretion" model of planetary formation.

In new research, scientists have attempted to determine the precise conditions necessary for planets to form in a . Jarrett Johnson and Hui Li of Los Alamos National Laboratory assert that observations increasingly suggest that takes place in star systems with higher metallicities.

Astronomers use the term “” in reference to elements heavier than hydrogen and helium, such as oxygen, silicon, and iron. In the “core ” model of , a rocky core gradually forms when that make up the disk of material that surrounds a young star bang into each other to create small rocks known as “planetesimals”. Citing this model, Johnson and Li stress that heavier elements are necessary to form the dust grains and planetesimals which build planetary cores.

Additionally, evidence suggests that the circumstellar disks of dust that surround young don’t survive as long when the stars have lower metallicities. The most likely reason for this shorter lifespan is that the light from the star causes clouds of dust to evaporate.

The Planet Epoch

Our cosmic history has several defining “epochs”, one of which is the point at which star systems began to form planets. Heavy elements such as carbon, silicon and oxygen first needed to be created from supernovae and the stellar cores of the first generations of stars before the first planets could form.

“Our calculation is an estimate of the minimum amount of heavy elements that must be present in circumstellar disks before planets can form,” says Johnson. “Because these heavy elements must be produced by the first stars in the universe, the first planets could only form around later generations of stars.”

Understanding how the first planets formed provides crucial information about the early universe. Additionally, a better understanding of early planetary formation impacts many facets of astronomy, including the search for life elsewhere.

HST image of a Dust Disk Around Star HD141569. Credit: NASA, M. 1Clampin (STScI), H. Ford (JHU), G. Illingworth (UCO/Lick), J. Krist (STScI), D. Ardila (JHU), D. Golimowski (JHU), the ACS Science Team and ESA

According to the team, a successful theory of planet formation should make predictions about the properties of the earliest planets and their host stars. Such a theory could be tested by studying very old planetary systems in our galaxy. The enrichment of gas with metals from supernovae is thought to not only affect planetary formation, but the formation of low-mass stars like our Sun as well.

“A planet as massive and dense as the could only form once stars and supernovae had enriched the gas with an abundance of heavy elements that is at least 10 percent that in the Sun,” adds Johnson. “This suggests that many generations of stars had to form and evolve before habitable planets could form.”

One important consideration for planetary formation is the dispersal rate of the circumstellar disk of gas and dust around a host star. Two of the more prominent mechanisms for dispersing a planetary disk are giant planet formation and photoevaporation by the host star. Photoevaporation appears to be the more dominant process which dictates the lifetime of a planetary disk around a star. Observations show that low-metallicity disks have shorter lifetimes, which is bolstered by data showing higher-metallicity disks are better “shielded” from evaporation by a host star’s radiation.

Johnson and Li further state that disks with higher metallicity tend to form a greater number of high mass giant planets.

This image shows the critical metallicity for Earth-like planet formation, expressed as iron abundance relative to that of the Sun, as a function of distance (r) from the host star. If systems with planets are discovered in the “forbidden zone”, it may pose a challenge to the “core accretion” model of planetary formation. Credit: Johnson & Li

The Lifetime of Dust

In order to obtain estimates of the critical metallicity necessary for planet formation, Johnson and Li compared the lifetime of the disk and the length of time required for dust grains in the disk to settle. Basically, for a star system to form planets, the time required for dust grains to settle cannot exceed the lifetime of the planetary disk.

The team explains that the dust-to-gas ratio that occurs when the timescales are equal gives an estimate of the critical metallicity, the point at which their model suggests planets can form. Since the settling time for dust grains depends on the density and temperature of the disk, which are related to the distance from the host star, the critical metallicity is also a function of distance from the host star.

“Our calculation is really fairly simple compared to many others, as we have focused only on what we believe are the key processes that set the timescale required for planetesimal formation at low metallicity,” Johnson says. “These are the growth of dust grains into planetesimals and the destruction of the disk by the high energy radiation from the host star. While the calculation is simple, it does show that current models of planet formation can in principle explain how the lowest metallicity planets form.”

The team notes several assumptions made in their comparisons with the data. The first assumption is that surface metallicity of the host star is the same as that of the protostellar disk from which it and its planets formed. Secondly, the team assumes circular planetary orbits. When orbits are highly eccentric, comparing the data to the theoretical predictions is more difficult. Lastly, the team assumes planets have not migrated inward toward their star from their initial place of birth in the disk.

Mercury transit of the Sun on November 8, 2006. A sunspot located just below the equator at the left-hand side is much bigger than Mercury, which looks like a small black dot in the lower middle of the solar disk.

The team found that the formation of planetesimals can only take place once a minimum metallicity is reached in a protostellar disk. Since the earliest stars that formed in the universe (Population III stars) do not have the required metallicity to host planets, it is believed that the supernova explosions from such stars helped enrich subsequent (Population II) stars, some of which may still be in existence and could host planets.

The Earliest Planets

Based on their equations, the team finds that some of the earliest planets may have formed at a distance of 0.03 AU from their parent star (for comparison, Mercury orbits at just under 0.4 AU). Given the high temperatures at such compact orbits (estimated at roughly 1600 K or 1,300 C), planet formation is likely to have resulted in planets too hot to host life as we know it.

“Interestingly, our results also suggest that the first Earth-like planets may have formed in the habitable zones of stars slightly more massive than the Sun, Johnson adds. “Because more massive stars burn out faster, it is possible that any life that evolved on these planets may have already perished with the death of its , which may have lived only 4 billion years compared to the 10 billion year lifetime expected for the .”

Johnson and Li also note that the formation of Earth-like is not itself a sufficient prerequisite for life to take hold, stating that early galaxies contained numerous supernovae and black holes - both strong sources of radiation that would threaten life. Given the hostile conditions in the early universe, it is expected that conditions suitable for life were only present after early galaxy formation.

“However, with the wealth of new exoplanets being discovered and characterized, our theory of the minimum metallicity for planet formation may yet be challenged,” Johnson concludes. “It will be exciting to see how [our model] holds up.”

Johnson and Li’s research is scheduled to appear in the Astrophysical Journal.

Explore further: Mixing in star-forming clouds explains why sibling stars look alike

Related Stories

Heavy metal stars produce Earth-Like planets

Sep 30, 2011

New research reveals that, like their giant cousins, rocky planets are more likely to be found orbiting high metallicity stars. Furthermore, these planets are more plentiful around low mass stars. This could ...

A planetary system from the early Universe

Mar 27, 2012

A group of European astronomers has discovered an ancient planetary system that is likely to be a survivor from one of the earliest cosmic eras, 13 billion years ago. The system consists of the star HIP 11952 ...

Planetary systems can form around binary stars

Jan 10, 2006

New theoretical work shows that gas-giant planet formation can occur around binary stars in much the same way that it occurs around single stars like the Sun. The work is presented today by Dr. Alan Boss of ...

Do planets rob their stars of metals?

Aug 02, 2011

t has been known for several years that stars hosting planets are generally more rich in elements heavier than hydrogen and helium, known in astronomy as “metals”. These heavy elements help to form ...

Rocky planets could have been born as gas giants

Sep 16, 2011

When NASA announced the discovery of over 1,200 new potential planets spotted by the Kepler Space Telescope, almost a quarter of them were thought to be Super-Earths. Now, new research suggests that these ...

Cosmic births revealed by disks of dust

Nov 15, 2010

By carving 'gaps' in the disks of dust that create and enshroud them, newborn planets are giving astronomers clues to locating possible new worlds.

Recommended for you

How can we find tiny particles in exoplanet atmospheres?

Aug 29, 2014

It may seem like magic, but astronomers have worked out a scheme that will allow them to detect and measure particles ten times smaller than the width of a human hair, even at many light-years distance.  ...

Spitzer telescope witnesses asteroid smashup

Aug 28, 2014

(Phys.org) —NASA's Spitzer Space Telescope has spotted an eruption of dust around a young star, possibly the result of a smashup between large asteroids. This type of collision can eventually lead to the ...

User comments : 9

Adjust slider to filter visible comments by rank

Display comments: newest first

kevinrtrs
1 / 5 (10) Apr 10, 2012
a rocky core gradually forms when dust grains that make up the disk of material that surrounds a young star bang into each other to create small rocks known as planetesimals.

This is the theory but it hasn't panned out in real life. Rocks banging into each other tend to NOT stick! Except for highly dubious special conditions as used in the simulations. If you disagree, please bring supporting evidence to show us.

Let's be generous and say that planets actually DO form like this. Once you a planet in the gloriously magical habitable zone, how does life get started? Even the simplest self-sustaining life defies even man's most ardent attempts to emulate. Life just does not start by accident. Look at the internal complexities of the simplest cell and your hair will go gray just thinking about possible ways in which that state could have arisen by accident.

Such an event would have to defy all currently known scientific principles. You might as well have to call it a miracle.
Torbjorn_Larsson_OM
5 / 5 (5) Apr 10, 2012
These would just squeeze in with a bit of migration:

"With an estimated age of 12.8 billion years, the host starand thus the planetsmost likely formed at the dawn of the universe, ... During a recent survey, Setiawan and colleagues found the signatures of the two planets orbiting the star, dubbed HIP 11952.

Based on the team's calculations, one world is almost as massive as Jupiter and completes an orbit in roughly seven days. The other planet is nearly three times Jupiter's mass and has an orbital period of nine and a half months. ...

In the case of HIP 11952, "its iron abundance is only about one percent that of our sun," Setiawan said."

So one of its planets is situated in the safe zone.

@ kevintrs:

Evidently you are a creationist and already know it is impossible, this is presumably why you post religious inanities on a science site, about your emotional need for "miracles" and magical poofs from nothing.
barakn
5 / 5 (3) Apr 10, 2012
Once you a planet in the gloriously magical habitable zone, how does life get started? Even the simplest self-sustaining life defies even man's most ardent attempts to emulate.

So you're saying that it's too complicated to be created?
kevinrtrs
1 / 5 (3) Apr 11, 2012
So you're saying that it's too complicated to be created?

Assuming you're referring to the creation of life - the answer is "No, it's not too complicated". The intimation is that it's impossible to happen by chance.
kevinrtrs
1 / 5 (3) Apr 11, 2012
Evidently you are a creationist and already know it is impossible, this is presumably why you post religious inanities on a science site, about your emotional need for "miracles" and magical poofs from nothing.

Nothing religious about my statements.

On the contrary, if you can show that your rocks do indeed stick together in space you'll have a strong scientific case for the accretion theory. Up to now even the best brains have not been able to show how that is possible, except in the most twisted computer simulations.

The question then remains here: Who is the religious one? You who keep on insisting that planets form by somehow defying the most basic principles of physics or me who is pointing out that the theory has no basis in reality? Get Real, friend.

If you were to ask the best known and or qualified astronomers in private they'll confide to you that no one currently understands how stars can form in space. This conundrum therefore also excludes planet formation.
kevinrtrs
1 / 5 (4) Apr 11, 2012
Evidently you are a creationist and already know it is impossible, this is presumably why you post religious inanities on a science site, about your emotional need for "miracles" and magical poofs from nothing.

By the way the "miracle" refers to the creation of life by chance.
I see you're trying to say that the positioning of a planet in the habitable zone is not a miracle. So what? Once it's in there, how does life get started? This was referred to by the researcher, in case you missed it.
jsdarkdestruction
5 / 5 (4) Apr 11, 2012
Shootist
5 / 5 (1) Apr 11, 2012
a rocky core gradually forms when dust grains that make up the disk of material that surrounds a young star bang into each other to create small rocks known as planetesimals.

This is the theory but it hasn't panned out in real life. Rocks banging into each other tend to NOT stick!


(you) "Don't know much about history, don't know much biology, don't know much about a science book . . ."

The relative velocities of these bodies are similar. Van der Waals forces, used as a synonym for the totality of intermolecular force, are sufficient.
Pyle
not rated yet Apr 12, 2012
kev isn't worth the time, but for everyone else:
If you were to ask the best known and or qualified astronomers in private they'll confide to you that no one currently understands how stars can form in space.

Then again, if you ask those same astronomers to point out star systems in any of innumerable stages of star formation they will be able to show you. So although we don't know every mechanism that is at play, we are able to see systems at many different stages in the process to know that there is a process by which they develop.

Scientists develop hypotheses, test their theories and gather more data. Our observations suggest there is an underlying set of rules governing the evolution of the cosmos and we are working to understand them.

But I could be wrong and we really might be a sky fairy fart like kev says.