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
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

Extrasolar Planets, Kepler Mission

Terrestrial Planets Could be More Common Than Gas Giants

15 Jun , 2012 by

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

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aerandir
Member
June 15, 2012 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
Guest
magnus.nyborg
June 18, 2012 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.

aerandir
Member
June 15, 2012 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
Member
WaxyMary
June 15, 2012 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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
June 15, 2012 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… Read more »
SJStar
Guest
SJStar
June 16, 2012 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
Guest
magnus.nyborg
June 16, 2012 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.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
June 16, 2012 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… Read more »
Ray Fowler
Guest
Ray Fowler
June 17, 2012 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
Guest
Ray Fowler
June 17, 2012 12:38 AM

The small rocks always outnumber the large rocks

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