Terrestrial Planets Could be More Common Than Gas Giants

by Andy Tomaswick on June 15, 2012

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Editor’s note: This guest post was written by Andy Tomaswick, an electrical engineer who follows space science and technology.

As acclaimed astronomer Carl Sagan once famously noted, “We are all made of star-stuff.” So are the multitudes of extra-solar planets that are currently being discovered at a breathtaking pace. What Sagan meant was that all of the elements heavier than hydrogen and helium, commonly known as “metals” to astrophysicists, must be created in the interior furnaces of stars. But it takes time for stars to create these heavier elements, and since they are needed to start planets those time spans could have a major impact on solar system formation.

New research led by the University of Copenhagen with help from the Harvard-Smithsonian Center for Astrophysics sheds some light on those time spans. In a paper recently presented at a meeting of the American Astronomical Society, Lars Buchhave and his team selected more than 150 stars with known planetary systems that were cataloged by NASA’s Kepler mission. They then studied these star’s metal content and the size of the planets in their solar systems. What they found was that gas giant planets were more likely to form around metal rich stars, whereas terrestrial planets were equally likely to form around metal rich or metal poor stars.

As the team explains, the reason for this fits neatly into the “core accretion” model of planetary formation. Each gas giant has a metal core which hydrogen and helium accumulate around. However, if there is no core to collect around, the lighter elements will be blown away by stellar winds while the star is still relatively young. If a star has a high enough metal content, its potential planets might be able to form a large metallic core quickly, before the winds do their work. The core will then gravitationally attract the remaining gas to itself and a new gas giant is born.

On the other hand, the formation of terrestrial planets is not dependent on helium and hydrogen and therefore not subject to the same time constraints. If a star has lower metal content it might take longer to form terrestrial planets, but all the ingredients are still there. Essentially, there is no upper time limit for a terrestrial planet to form whereas a gas giant must develop quickly to keep its hydrogen and helium trapped within the solar system.

Like all good research, these results open up many more questions. How quickly must a gas giant’s core form before its material is lost? Are terrestrial planets much more common given their greater creation timescales and more numerous potential parent stars? Future work on extra-solar planetary systems might help to provide more answers.

Lead image caption: This artist’s conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Credit: University of Copenhagen/Lars Buchhave

Source: Harvard-Smithsonian Center for Astrophysics

Aerandir90 June 15, 2012 at 2:36 PM

Does the rate at which solar wind is ejected from a star significantly decline as the star transitions from its formation to its steady-state phase? If not, then there needn’t be a time limit for gas giants to form. As long as cores form from the proto-planetary disk, they will eventually attract large volumes of gas over time. In fact, have we observed any ‘growth’ in our gas giants due to capture of solar wind? I guess our time scales of observation have been too small as of yet.

magnus.nyborg June 18, 2012 at 1:40 PM

The solarwind is hot and moves to fast to be effectively captured by a gas-giant, in fact it more likely slowly strips gas-giants from their upper atmosphere.
In order to capture (accrete) gas and dust, it needs to be relatively cold and dense, a condition that was true when the solarsystem was just forming, but not today.

Aerandir90 June 15, 2012 at 2:44 PM

Second comment:

In our solar system we know that the size of the gas giants decreases with distance from the Sun. Is this because in the early days of formation the solar wind was favorably swept up by Jupiter and Saturn first as they traverse their orbits and then by Uranus and Neptune?

It would be interesting to observe this in other systems, but I think this trend would only hold in the outer belts of the solar system. Within the inner parts, any gas giants would possibly suffer from star-wind blasting and subsequent gas-loss.

WaxyMary June 15, 2012 at 4:28 PM

Aerandir90,

In answer to your implied question. No, the stellar wind does not ADD to the content of any of the planets as far as I know. If it does add ANY mass that mass would be offset by the increased energy carried along with that wind and the stripping effects the wind has on the atmosphere or surface (for those objects void of a gaseous covering) .

The actual article states the stellar wind blows away the gases NEEDED to create the dense gaseous atmosphere around the solid metal cores.

The length of time needed to actually create the solid metal cores would vary of course, based on the available materials in the proto-disc. If there is not enough dusty metals left after the proto-star has formed and in advantageous orbital paths then there will not be enough left to form the high gravity cores needed to attract the two main gases currently assumed to be the primary constituents of a ‘gas giant’. If the gas giant can be formed and travels further from the source of energy (the stellar object) which will ‘fluff’ the materials composing the giant then we have the frozen giants such as Uranus and Neptune. These are thought to be gaseous but in a quieter state due to the lack of intense energy.

If you go to http://www.solstation.com/stars/jovians.htm there is abundant information.

Mary

Torbjörn Larsson June 15, 2012 at 11:39 PM

First it was water, water everywhere.

Now it is terrestrials, terrestrials everywhere.

Soon it will be habitables, habitables everywhere…

@ Aerandir90:

To answer some of your questions, not all which I fully understand (giant formation is concluded in our system, so why would today’s processes bear on that), I think it easiest to C&P from the current Nice model of our system formation:

“the four giant planets (Jupiter, Saturn, Uranus and Neptune) were originally found on near-circular orbits between ~5.5 and ~17 astronomical units (AU), much more closely spaced and more compact than in the present. A large, dense disk of small, rock and ice planetesimals, their total about 35 Earth masses, extended from the orbit of the outermost giant planet to some 35 AU.”

“Despite the minute movement each exchange of momentum can produce, cumulatively these planetesimal encounters shift (migrate) the orbits of the planets by significant amounts. This process continues until the planetesimals interact with the inmost and most massive giant planet, Jupiter, whose immense gravity sends them into highly elliptical orbits or even ejects them outright from the Solar System. This, in contrast, causes Jupiter to move slightly inward.”

“After several hundreds of millions of years of slow, gradual migration, Jupiter and Saturn, the two inmost giant planets, cross their mutual 1:2 mean-motion resonance. This resonance increases their orbital eccentricities, destabilizing the entire planetary system. The arrangement of the giant planets alters quickly and dramatically.[7] Jupiter shifts Saturn out towards its present position, and this relocation causes mutual gravitational encounters between Saturn and the two ice giants, which propel Neptune and Uranus onto much more eccentric orbits.”

“Eventually, the giant planets reach their current orbital semi-major axes, and dynamical friction with the remaining planetesimal disc damps their eccentricities and makes the orbits of Uranus and Neptune circular again.”

“In some 50% of the initial models of Tsiganis et al., Neptune and Uranus also exchange places about a billion years (20%) into the life of the Solar System.”

In an improved model covering the gas giant formation, a 5th giant is ejected during the relocation. This makes for a thicker disk which predicts Uranus and Neptune better, since otherwhise a thinner disk scatters too rapidly.

So now we get to the time scales of giant formation. They are set by core accretion, which the results described in the above post tests more than ever before: core accretion is faster than other modeled mechanisms:

“Once the cores are of sufficient mass (5–10 Earth masses), they begin to gather gas from the surrounding disk.[3] Initially it is a slow process, increasing the core masses up to 30 Earth masses in a few million years.[19][43] After that, the accretion rates increase dramatically and the remaining 90% of the mass is accumulated in approximately 10,000 years.[43] The accretion of gas stops when it is exhausted. This happens when a gap opens in the protoplanetary disk.[45] In this model ice giants—Uranus and Neptune—are failed cores that began gas accretion too late, when almost all gas had already disappeared. The post-runaway-gas-accretion stage is characterized by migration of the newly formed giant planets and continued slow gas accretion.

ADDED AFTER POSTING:

The editor ate my links. Let me try this way:

Nice model: http://en.wikipedia.org/wiki/Nice_model
5th giant: http://en.wikipedia.org/wiki/Hypothetical_fifth_gas_giant
Core accretion: http://en.wikipedia.org/wiki/Nebular_theory#Giant_planets

SJStar June 16, 2012 at 6:54 AM

“Terrestrial Planets Could be More Common Than Gas Giants” ..and yet there is more hydrogen and helium than all the other chemical elements combined. It logically doesn’t make any sense at all…

magnus.nyborg June 16, 2012 at 12:50 PM

Planets large enough to hold on to a Hydrogen/Helium atmodsphere would still provide more total mass.
But a 1000 Mars-sized rocky bodies would still weigh less than 1 Jupiter.

Torbjörn Larsson June 16, 2012 at 2:02 PM

I think it makes eminent sense.

The average degree of metallicity of stars is ~ 1 % of mass, I believe. This is also the mass proportion of dust in a protoplanetary disk (proplyd).

And the Sun contains 99.8 % of the system mass. While the initial proplyd formation contains several times the central star mass.

Hence all we need is a process that kicks out the superfluous volatiles while retaining most of the dust, and we will have enough dust for forming planets.

That is what the light and solar wind and initially heavy CMEs do when the young star has ignited. There is also almost certainly an intermediate stage of a magnetically constrained X-wind, that partly recirculates and aggregates dust onto the disk while emitting huge proplyd jets that blows away volatiles. The jets are definitely observed, but I am not sure if they have pinned down the dust refining.

Ray Fowler June 17, 2012 at 12:40 AM

Hydrogen & Helium are gaseous in a vacuum and have a far lower escape velocity. Small planets can form from silicates and iron far, far more easily.

Ray Fowler June 17, 2012 at 12:38 AM

The small rocks always outnumber the large rocks

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