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The first rocky worlds astronomers detectcircling other stars could resemble In-ferno more than Earth. The existence of

such lava-coated planets, which may provecommonplace, will force a reconsideration oftheories about planetary formation.

Since 1991 observers have discovered some120 exoplanets—worlds outside our solar sys-tem. All but three appear, by their great sizeand low density, to be gas giants. Roughly asixth are “hot Jupiters” surprisingly near theirstars, all closer than Mercury is to our sun.

Some hot Jupiters live just too close totheir stars for comfort. Last year the HubbleSpace Telescope provided the first evidence ofan evaporating atmosphere, from an exo-planet, HD 209458b, that circles its star at adistance of less than 1⁄ 20 the distance betweenthe sun and Earth. The star roasts the exo-planet and rips at it with its gravity. The re-sult: the exoplanet blows away at least 10,000tons of gas a second, which streaks off in avast plume 200,000 kilometers long. As-tronomer Alfred Vidal-Madjar of the Institute

More progress has been made by seedingstem cells onto a variety of simple scaffoldsimpregnated with growth-promoting chem-icals. Last fall, for example, researchers fromthe Massachusetts Institute of Technologyand the Technion-Israel Institute of Technol-

ogy reported generating tissues of neural, liv-er and cartilage cells, as well as formation ofa “3D vessel-like network” on a biodegrad-able polymer scaffold seeded with humanembryonic stem cells. When transplantedinto a mouse, the constructs remained intactand appeared to connect with the animal’sblood supply.

Still, scientists working with stem cells,embryonic or otherwise, admit that they arejust beginning to learn tricks for controllingthe kind of tissue the cells become and juststarting to discern the cues cells give to one an-

other as well as take from their natural envi-ronment during the course of organ develop-ment. “We don’t have anything like [nature’s]exquisite repertoire of tools,” Sefton says.

And so most models for growing entire or-gans involve using some kind of living “biore-actor.” In some cases, it could be the same pa-tient in need of the organ. Anthony Atala ofWake Forest University, who once grew asimple bladder in a beaker and transplanted itinto a dog, teamed up more recently withRobert P. Lanza, also now with Wake Forest,and others to grow a mini kidney inside acow. Kidney progenitor cells were taken froma fetal clone of the cow in question, then im-planted into the cow’s body, where they de-veloped into proto-organs with all the celltypes of a normal kidney. These “renal units”even produced a urinelike liquid.

The idea of seeding an organ and lettingthe body do the rest of the construction mightwork for a kidney, because the patient couldbe treated with dialysis while the new organwas being generated, according to Jeffrey L.Platt, director of transplantation biology at theMayo Clinic. For a patient suffering from lungor heart failure, however, growing a new or-gan would put too much strain on an alreadyweak body. But every advance toward creat-ing ever more complex tissues might yield alifesaving patch for a moderately damagedheart or liver, Platt says, along with fresh in-sight into how nature builds bigger body parts.

Burning Down to RockGAS GIANTS MIGHT GET COOKED CLEAN TO THEIR SOLID CORES BY CHARLES CHOI

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Custom-grown spare parts fromstem cells are years away. That

means animal organs may be theonly realistic alternative for

patients awaiting transplants. Butxenotransplantation took a serious

blow in January, when Jeffrey L.Platt of the Mayo Clinic and his

colleagues confirmed that a viruspresent in most pigs, porcine

endogenous retrovirus (PERV),could infect human cells in vivo.

PERVs are harmless to pigs, but noone knows how they might reactwhen transplanted into humans.

The Mayo team injected humanstem cells into fetal swine; after

the pigs were born, the researchersfound that PERV infected the host

cells as well as the human cells.What is more, they detected

chimeric cells containing fused pig and human DNA that

were positive for PERV, too.

WHEN HUMANSMEET PIGS

“RENAL UNIT”—a proto-kidney—produced urinelikeliquid after 12 weeks of growth.

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COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.

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of Astrophysics in Paris and his teamdubbed the world “Osiris,” after theEgyptian god torn to pieces by his evilbrother Set.

In contemplating the fate of Osiris,Vidal-Madjar and his team calculatedhow long it and other giants might live.At roughly 220 times Earth’s mass, Osirisboasts a gravitational pull strong enoughto hold its atmosphere until its star dies.But the researchers speculate the hellishrate of evaporation might completelyscour all gas off smaller hot Jupiters orthose closer to their stars than Osiris.

This could lead to a new class of plan-ets—a dead giant’s hard, bare heart. Theastronomers named such worlds “chtho-nians,” after primeval Greek deities ofthe underworld. In findings to appear inAstronomy and Astrophysics, astronomerAlain Lecavelier des Etangs of the Insti-tute of Astrophysics and his co-workersfigure that the four exoplanets discoveredso far may one day become chthonians.

Though remnants of far largerworlds, chthonians would still weigh inat roughly 10 to 15 times Earth’s massand six to eight times Earth’s diameter.With searing temperatures of roughly1,000 degrees Celsius at their surfaces,they would look “like lava planets,” Le-cavelier des Etangs imagines. If chthonianexoplanets exist, “it is probable that theywill be the first rocky planets to be de-tected around other stars,” Vidal-Mad-jar remarks. (Three planets, two aboutthree to four times Earth’s mass and thethird twice the mass of the moon, werediscovered in the 1990s and most likelyare solid, but they all orbit a pulsar.)

Spotting chthonians would help an-swer questions regarding planetary for-mation, explains astronomer Adam Bur-rows of the University of Arizona. Re-searchers think that worlds are bornfrom disks of gas and dust encirclingstars. The most popular idea proposesthat solid cores amass from protoplane-tary disks and behave like seeds, attract-ing gas to grow into giant planets.

The alternative theory suggests thatgiant planets may not possess hard cores.Instead they may have fluid centers, af-ter having condensed directly from pro-toplanetary disks without forming solid

hearts. Scientists have not conclusivelyidentified whether the centers of giants inour own solar system are solid. Detect-ing chthonians could prove one scenarioof planetary formation right.

The European Southern Observato-ry telescope in Chile has an outsidechance of finding them next year: a newinstrument there could detect planets aslow as about 15 times Earth’s mass bylooking for the gravitational tugs eachhas on its star. The best chance to spotchthonians will come from the first spaceprobes sensitive enough to see Earth-sizeplanets: the French satellite COROT,scheduled for launch in 2006, and NASA’sKepler, around 2007. These missionsmight uncover several tens of chthonians,probably by spotting them when theypass in front of their stars, dimming them.

Burrows thinks that chthonian exo-planets may not turn out to be all rock.If a chthonian’s star does not strip off itsatmosphere, ices found in a giant’s coremight survive underneath. Lecavelier desEtangs says that chthonians might evensupport life, although it would almostcertainly be “very different from whatwe know on Earth.”

Charles Choi, a frequent contributor, is based in New York City.

GAS GIANTS may lose their atmospheres to theirstars, resulting in rocky worlds called chthonians.

COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.