Odds against Earth-like planets

http://news.bbc.co.uk/2/hi/science/nature/2701977.stm ^ | January 28, 2003 | Dr David Whitehouse

[conservativecorner]

Earth-like worlds circling stars in orbital zones suitable for life may be few and far between in the cosmos, according to new research. In the first comprehensive study of extrasolar planetary systems, astronomers have shown that in most of them it would not be possible to keep an Earth-like world in orbit around a star so that it was neither too hot nor too cold for life.

In general, other planetary systems fall into two types: those with Jupiter-like worlds circling close to their parent star, and those with more distant Jupiters in elliptical orbits.

In both systems, maintaining an Earth-like world in a temperate orbit is difficult, although not in all cases impossible.

"This work shows us just how unusual our own Solar System is when compared with the other planetary systems," Dr Kristen Menou of Princeton University, US, told BBC News Online.

Habitable zone

Eighty-five planetary systems were studied, all that were known when the research was carried out.

Dr Menou said: "They fall into two categories: large planets circling very close to their sun - the so-called 'hot Jupiters', and systems with Jupiter-like planets in distant non-circular orbits."

Dr Menou, along with Dr Serge Tabachnik, created computer dynamical models of the known exoplanetary systems to see if it was possible for Earth-like worlds to exist for long periods in the so-called habitable zone.

This work shows us just how unusual our own Solar System is when compared to the other planetary systems

Dr Kristen Menou, Princeton University

This zone is the region around a star in which a planet would be able to sustain liquid water, being neither too close to the star for it all to be vaporised, nor too distant that it all freezes.

In our Solar System, the Earth is in the middle of the habitable zone. Astronomers believe such a position is essential for life to develop and thrive.

But it seems difficult for worlds to stay in the habitable zone in the majority of the extrasolar planetary systems found so far.

"We found that in the systems with the distant Jupiters, these worlds can disrupt the orbit of any Earth-like world in the habitable zone," says Dr Menou.

"Any Earth-like world in the temperate zone would either crash on to its parent star or be slung out into interstellar space," he added.

Over half of the planetary systems studied had distant Jupiters making them unlikely to contain habitable Earth-like worlds.

"We have identified some systems where distant Jupiters would pull Earth-like worlds into elliptical orbits that keep them inside the habitable zone. Such worlds would have dramatic and extreme seasons. We don't know how that would affect the development of life."

Cast asunder

The new analysis of the systems containing hot Jupiters shows that Earth-like worlds could remain orbiting in the temperate zone, seemingly an encouraging finding.

"The good news is that in about a quarter of the systems we studied, there could be habitable planets present."

But even in these systems, Earth-like worlds may have been cast asunder.

Current models of the evolution of planetary systems have hot-Jupiters reaching their tight orbits by migrating inwards from more distant ones.

This means that as they slowly travelled sunwards, they would have scattered any smaller worlds that got in their way, suggesting that there could be no Earth-like worlds in hot Jupiter systems at all.

"The way we are trying to get out of this pessimistic position," says Dr Menou, "is by seeing if Earth-like worlds could form in a planetary system after the inward migration of Jupiter worlds."

The research is to be published in a forthcoming edition of the Astrophysical Journal.

[Doctor Stochastic]

Techincally it's not the wobble, it's the change in tilt. (I hope the IWW doesn't sue me.)

[Incorrigible]

 

Listed below is information about the various planets that are encountered throughout space, their type, size and descriptions of each planet. With only a tiny fraction of the Galaxy actually being explored by Starfleet, the Klingon Defense Force, and the Romulan Star Republic, it is more than likely that other unknown classes of planets are yet to be encountered in the deepest of space. When new classes are discovered they will be added to this page.

 

 

 

	CLASS A
	Type:	Gas Supergiant
	Size:	140,000km - 10,000,000km
	Description:	Usually found in a star's outer or "cold zone." They have high core temperatures causing them to radiate heat. Low stellar radiation and high planet gravity enables them to keep a tenuous surface comprised of gaseous hydrogen and hydrogen compounds.
	CLASS B
	Type:	Gas Giant
	Size:	50,000km - 140,000km
	Description:	Usually found in a star's outer or "cold zone." They have high core temperatures, but do not radiate much heat. Low stellar radiation and high planet gravity enables them to keep a tenuous surface comprised of gaseous hydrogen and hydrogen compounds.
	CLASS C
	Type:	Reducing
	Size:	10,000km - 15,000km
	Description:	Usually found in a star's "habitable zone." They have high surface temperatures due to the "greenhouse effect" caused by their dense atmospheres. The only water found is in vapor form.
	CLASS D
	Type:	Geo-Plastic
	Size:	10,000km - 15,000km
	Description:	Usually have a molten surface because they have been recently formed. The atmosphere contains many hydrogen compounds and reactive gases. Class D planets eventually cool, becoming Class E.
	CLASS E
	Type:	Geo-Metallic
	Size:	10,000km - 15,000km
	Description:	Usually found in a star's "habitable zone." Their atmospheres still contain hydrogen compounds. They will cool further eventually becoming Class F.
	CLASS F
	Type:	Geo-Crystaline
	Size:	10,000km - 15,000km
	Description:	Usually found in a star's "habitable zone." They have surfaces that are still crystalizing. Their atmospheres still contain some toxic gases. They will cool eventually becoming Class C, M or N.
	CLASS G
	Type:	Desert
	Size:	8,000km - 15,000km
	Description:	Can be found in any of a star's zones. Their surfaces are usually hot and their atmospheres may contain heavy gases and metal vapors.
	CLASS H
	Type:	Geo-Thermal
	Size:	1,000km - 10,000km
	Description:	Usually found in a star's "habitable zone" or "cold zone." They have partially molten surfaces and atmospheres that contain many hydrogen compounds. They cool becoming Class L.
	CLASS I
	Type:	Asteroid
	Size:	100km - 1,000km
	Description:	Planetary bodies of this class can be found in any of a star's zones. They are usually found in orbit of larger planets or in asteriod fields. They have no atmospheres. Their surfaces are barren and cratered.
	CLASS J
	Type:	Geo-Morteus
	Size:	100km - 1,000km
	Description:	Usually found in a star's "hot zone." They have high surface temperatures due to the proximty to the star. Their atmospheres are extremely tenuous with few chemically active gases.
	CLASS K
	Type:	Adaptable
	Size:	5,000km - 10,000km
	Description:	Usually found in a star's "habitable zone." They are adaptable for humanoid colonization through the use of pressure domes and other life support devices. They have thin atmospheres. Small amounts of water are present.
	CLASS L
	Type:	Geo-Inactive
	Size:	1,000km - 10,000km
	Description:	Usually found in a star's "habitable zone" or "cold zone." Low solar radiation and minimal internal heat usually result in a frozen atmosphere.
	CLASS M
	Type:	Terrestrial
	Size:	10,000km - 15,000km
	Description:	Found in a star's "habitable zone." They have atmospheres that contain oxygen and nitrogen. Water and life-forms are typically abundant. If water covers more than 97% of the surface, then they are considered class N.
	CLASS N
	Type:	Pelagic
	Size:	10,000km - 15,000km
	Description:	Usually found in a star's "habitable zone." They have atmospheres that contain oxygen and nitrogen. Water and life-forms are typically abundant. If water covers less than 97% of the surface, then they are considered class M.
	CLASS S
	Type:	Near Star
	Size:	50,000,000km - 120,000,000km
	Description:	Usually found in a star's "cold zone." They have high core temperatures causing them to radiate heat and light. These are the largest possible planets, because most planetary bodies that reach this size do become stars.
	CLASS T
	Type:	Ultra Giant
	Size:	10,000,000km - 50,000,000km
	Description:	Usually found in a star's "cold zone." They have high core temperatures causing them to radiate enough heat to keep water in a liquid state.
	CLASS Y
	Type:	Demon
	Size:	10,000km - 15,000km
	Description:	Demon Planets and planetoids of this class can be found in any of a star's zones. Atmospheric conditions are often turbulent and saturated with poisonous chemicals and thermionic radiation. Surface temperatures can reach in excess of 500 K. Communication is frequently impossible, and transport may be difficult. Simply entering orbit is a dangerous prospect. No known environment is less hospitable to humanoid life than a Class Y planetary body.

 

[RadioAstronomer]

Yup, I think its close to 21 thousand years. This is off the top of my head so I may be wrong.

[RadioAstronomer]

Which Milankovitch worked out without the aid of a computer

Compared to most of those guys, I am a complete dunce! LOL!

[Mulder]

All I want to know is the location of the planet that is populated solely by tall blonde women with big hooters.

[RadioAstronomer]

That is a secret known only to us radio astronomers! :-)

[meema]

God Bess You. My thoughts exactly.

[Fiddlstix]

Bump

[Mulder]

That is a secret known only to us radio astronomers! :-)

LOL! Those must be some very interesting "radio transmissions" that y'all are detecting ;-)

[RadioAstronomer]

ROFL!!!

[CurlyDave]

so far, all the life forms we have found still require water to thrive. Sure, they can aestivate for a while in a dry condition, like a virus, but still need waterborne environment or host to propagate and develop. Water is the key to life.

So far, the only place we have seriously looked for life is on a planet which has lots of liquid water.

[pjd]

They use wobbles to detect planets, they see only the wobbliest systems, and they conclude that planetary systems are too wobbly to support Earth-like worlds.

The part about wobbles to detect planets is correct, but the the 'too wobbly to support Earth-like worlds' is not.

The wobble works like this: Of the star and planet system, only the star is visible. The star and planet rotate about their mutual center of gravity which is usually close to the center of the star but not exactly at the center. So the star will show a sight motion as it orbits the center of mass.

This wobble is not the same as the eccentricity of the planetary orbit. It is the eccentricity of the orbit which is the killer. What the article seems to be saying is that, if a Jupiter-like planet resides in the system, it is very likely to perturb the orbit of the lighter planet so that the lighter planet will likely have a large eccentricity. But this is completely separate from the so-called wobble. Earth's orbit wobbles tremensously as it circles the sun, but the center of mass of the Earth-moon system is smooth.

[RadioAstronomer]

Earth's orbit wobbles tremensously as it circles the sun, but the center of mass of the Earth-moon system is smooth.

I am not sure what you are driving at here. Almost all of the planets (except two) have nearly circular orbits. Indeed there is not only a precession of the Earth itself, but also a precession of its perihelion. Also there are a number of nutations that the Earth experiences due to influences of other solar system bodies such as the Sun, Moon and Jupiter.

[RadioAstronomer]

The part about wobbles to detect planets is correct, but the the 'too wobbly to support Earth-like worlds' is not.

Correct me if I am wrong, but I think what was intended here was that the wobble of the star is so tiny, only the extreme cases of a highly elliptical orbit or a large mass orbiting near the star is detectible. Neither of these types of systems is conducive to a stable earthlike planet.

[PatrickHenry]

Placemarker.

[ASA Vet]

RA, does such a planet classification system exist yet?

Thank you Incorrigible, but did you get permission from the Federation,
to provide that information to the 21st century?

[bimbo]

Seems to me that the Astrophysicists are trying to prove or disprove something – the possibility of existing planets orbiting in a “habitable zone” – that has empirically been proven already! After all, the earth exists in the “habitable zone” proving the possibility. True, it may be extremely rare, but there exists at least one example – the earth.

Are these College Campus “geniuses” trying prove that the earth is the only example, or are they trying to find another example? Either way, they have A LOT of searching to do ( at least until the Government funding runs out).

[Physicist]

What the article seems to be saying is that, if a Jupiter-like planet resides in the system, it is very likely to perturb the orbit of the lighter planet so that the lighter planet will likely have a large eccentricity.

That's exactly what I meant by "too wobbly to support Earth-like worlds". I was talking about the system, not just the star.

[pjd]

I was talking about the system, not just the star.

I'm not sure if I understand you correctly. The point I'm trying to make is that a significant wobble in the star's position does not necessarily mean that the orbits around that star are very elliptical.

For example, consider two equal mass stars, one bright, one dark. These two stars can orbit around each other in perfectly circular orbits. However, a distant observer can only see the bright star which will appear to be non-stationary and 'wobbling' with respect to the background stars. Its 'wobble' will be as big as its orbit, but the two objects will still have perfectly circular orbits.

However, if you mean that such orbits are only detectable if the mass of one of the components is large enough to rival the mass of the sun, still, the sun-planet orbits can still be circular. But it is very likely that any smaller planet in the system will have a fairly elliptical orbit.

So if you mean that we can only detect systems which have a large planet component, and in those systems it is unlikely to find an Earth-like planet in a non-eccentric system, then I agree.

The real question is what is the relative abundance of systems with small planets and no large planets? Of that, we essentially have no data.

[steve-b]

The Rare Earth hypothesis agrees that archaea-like organisms are fairly common, since the conditions where they can exist (geothermal heat and very basic forms of nutrient) appear likely on just about any large rockball (as long as the crust isn't too hot to cook any form of large organic molecules). The question of traditional "life as we know it" (multicellular surface critters) is more complicated.