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“Earth-like” planets out there even more Earth-like than thought, simulation finds

March 30, 2005
Courtesy 
and World Science staff

Nearly all of the rocky, or Earth-like, plan­ets through­out our gal­axy have si­m­i­lar min­er­al con­tent to Earth, re­sults from a new sim­ula­t­ion sug­gest.

That’s a sur­prise, as­tro­no­mers say, be­cause cal­cula­t­ions had pre­vi­ously in­di­cat­ed that only about one in three Earth-like plan­ets would be such a close match. The great­er si­m­i­lar­ity could en­hance the like­li­hood of life on some of these worlds, they add.

In this artist's con­cep­tion, gas and dust-the raw ma­te­ri­als for mak­ing plan­ets-swirl around a young star. The plan­ets in our so­lar sys­tem formed from a si­m­i­lar disk of gas and dust cap­tured by our sun. (Cred­it: NA­SA/JPL-Cal­tech)


Con­di­tions for mak­ing the build­ing blocks of Earth-like rocks are in­deed com­mon through­out the Milky Way gal­axy, said the as­tro­no­mers who pre­sented the find­ings on July 8 at the U.K.’s Na­t­ional As­tron­o­my Meet­ing in Llan­dud­no, Wales.

Min­er­als made from car­bon, ox­y­gen, mag­ne­si­um, and sil­i­con are thought to con­trol the land­scape of rocky plan­ets that form in so­lar sys­tems around Sun-like stars. 

Slight varia­t­ions in min­er­al con­tent can greatly af­fect hab­it­abil­ity, be­cause they in­flu­ence im­por­tant pro­cesses such as plate tec­ton­ics and heat­ing and cool­ing of a plan­et’s sur­face.

As­tro­no­mers had thought rocky plan­ets fell in­to three dis­tinct groups: those with a si­m­i­lar set of build­ing blocks to Earth, those that had a much richer con­centra­t­ion of car­bon, and those that had sig­nif­i­cantly more sil­i­con than mag­ne­si­um atoms, the re­verse of Earth’s situa­t­ion.

“The ra­tio of el­e­ments on Earth has led to the chem­i­cal con­di­tions ‘just right’ for life. Too much mag­ne­si­um or too lit­tle sil­i­con and your plan­et ends up hav­ing the wrong bal­ance be­tween min­er­als to form the type of rocks that make up the Earth’s crust,” said Brad Gib­son of the Uni­vers­ity of Hull in the U.K., who pre­sented the find­ings.

More­o­ver, “too much car­bon, and your rocky plan­et might turn out to be more like the graph­ite in your pen­cil than the sur­face of a plan­et like the Earth.”

Gib­son and col­leagues built a sim­ula­t­ion of the Milky Way’s chem­i­cal ev­o­lu­tion, which they said re­sults in an ac­cu­rate recrea­t­ion of what we see to­day. This let them zoom in for de­tailed ex­amina­t­ion of the chem­is­try of var­i­ous pro­cesses, such as plan­etary forma­t­ion. 

The re­sults were a sur­prise, they said.

“At first, I thought we’d got the mod­el wrong!” said Gib­son. “As an over­all rep­re­senta­t­ion of the Milky Way, eve­ry­thing was pret­ty much pe­r­fect. Eve­ry­thing was in the right place… But when we looked at plan­etary forma­t­ion, eve­ry so­lar sys­tem we looked at had the same el­e­ment­al build­ing blocks as Earth, and not just one in three. 

“We could­n’t find a fault with the mod­el, so we went back and checked the ob­serva­t­ions. There we found some un­cer­tain­ties that were caus­ing the one-in-three re­sult. Re­mov­ing these, ob­serva­t­ions agreed with our pre­dic­tions that the same el­e­ment­al build­ing blocks are found in eve­ry exo­plan­et sys­tem, wher­ev­er it is in the gal­axy.” An exo­plan­et is a plan­et or­bit­ing a star oth­er than our Sun.

The cloud that gave rise to our own so­lar sys­tem has around twice as many ox­y­gen as car­bon atoms, Gib­son said, and about five sil­i­cons for eve­ry six mag­ne­si­um. Ob­servers try­ing to as­cer­tain the chem­i­cal make-up of plan­etary sys­tems have tended to look at large plan­ets or­bit­ing very bright stars, which can lead to un­cer­tain­ties of 10 or 20 per cent, he ex­plained. In ad­di­tion, his­tor­ic­ally the sig­na­tures of ox­y­gen and nick­el have been hard to tell apart in the da­ta that in­stru­ments pick up.

Of course, “even with the right chem­i­cal build­ing blocks, not eve­ry plan­et will be just like Earth,” Gib­son added. 

“Con­di­tions al­low­ing for liq­uid wa­ter to ex­ist on the sur­face are needed for hab­it­abil­ity,” he said. “We only need to look to Mars and Ve­nus to see how dif­fer­ently ter­res­tri­al plan­ets can evolve. How­ev­er, if the build­ing blocks are there, then it’s more likely that you will get Earth-like plan­ets – and three times more likely than we’d pre­vi­ously thought.”

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Nearly all of the rocky, or Earth-like, planets throughout our galaxy have similar mineral content to Earth, results from a new simulation suggest. That’s a surprise, astronomers say, because calculations had previously indicated that only about one in three Earth-like planets would be such a close match. The greater similarity could enhance the likelihood of life on some of these worlds, they add. Conditions for making the building blocks of Earth-like rocks are indeed common throughout the Milky Way galaxy, said the astronomers who presented the findings on July 8 at the U.K.’s National Astronomy Meeting in Llandudno, Wales. Minerals made from carbon, oxygen, magnesium, and silicon are thought to control the landscape of rocky planets that form in solar systems around Sun-like stars. Slight variations in mineral content can greatly affect habitability, because they influence important processes such as plate tectonics and heating and cooling of a planet’s surface. Astronomers had thought rocky planets fell into three distinct groups: those with a similar set of building blocks to Earth, those that had a much richer concentration of carbon, and those that had significantly more silicon than magnesium atoms, the reverse of Earth’s situation. “The ratio of elements on Earth has led to the chemical conditions ‘just right’ for life. Too much magnesium or too little silicon and your planet ends up having the wrong balance between minerals to form the type of rocks that make up the Earth’s crust,” said Brad Gibson of the University of Hull in the U.K., who presented the findings. Moreover, “too much carbon, and your rocky planet might turn out to be more like the graphite in your pencil than the surface of a planet like the Earth.” Gibson and colleagues built a simulation of the Milky Way’s chemical evolution, which they said results in an accurate recreation of what we see today. This let them zoom in for detailed examination of the chemistry of various processes, such as planetary formation. The results were a surprise, they said. “At first, I thought we’d got the model wrong!” said Gibson. “As an overall representation of the Milky Way, everything was pretty much perfect. Everything was in the right place… But when we looked at planetary formation, every solar system we looked at had the same elemental building blocks as Earth, and not just one in three. “We couldn’t find a fault with the model, so we went back and checked the observations. There we found some uncertainties that were causing the one-in-three result. Removing these, observations agreed with our predictions that the same elemental building blocks are found in every exoplanet system, wherever it is in the galaxy.” An exoplanet is a planet orbiting a star other than our Sun. The cloud that gave rise to our own solar system has around twice as many oxygen as carbon atoms, Gibson said, and about five silicons for every six magnesium. Observers trying to ascertain the chemical make-up of planetary systems have tended to look at large planets orbiting very bright stars, which can lead to uncertainties of 10 or 20 per cent, he explained. In addition, historically the signatures of oxygen and nickel have been hard to tell apart in the data that instruments pick up. Of course, “even with the right chemical building blocks, not every planet will be just like Earth,” Gibson added. “Conditions allowing for liquid water to exist on the surface are needed for habitability,” he said. “We only need to look to Mars and Venus to see how differently terrestrial planets can evolve. However, if the building blocks are there, then it’s more likely that you will get Earth-like planets – and three times more likely than we’d previously thought.”