Could Earth Survive Sun’s Demise?

by Paul Gilster | Sep 14, 2007 | Exoplanetary Science | 10 comments

I, for one, wouldn’t want to be around to witness what happens when the Earth is faced with an ever expanding Sun that has exhausted its hydrogen fuel. Conventional wisdom has it that the planet will likely be engulfed by what will then become a red giant. Certainly Mercury and Venus will, and the Earth’s orbit is close enough that it may meet the same fate. But it’s intriguing to learn that other outcomes are possible.

Thus news out of Iowa State that the planet known as V 391 Pegasi b has evidently survived just such an encounter with its own star. Larger than Jupiter, the distant world in the constellation Pegasus was once situated at roughly the same distance from its parent that the Earth is from the Sun. That distance has changed over time as the star lost its outer regions in the helium flash, the onset of helium fusion that is produced as hydrogen is exhausted and contraction heats the stellar core.

Image: An artist’s conception of V 391 Pegasi b as it survives the red giant expansion of its dying sun. Credit: HELAS, the European Helio- and Asteroseismology Network.

Now located at some 1.7 AU from V 391 Pegasi, the doughty planet is still there, testimony to the durability of planetary systems, and perhaps an indication that the Earth of the distant future might survive such a catastrophe. Steve Kawaler (Iowa State), a member of the research team working on this project, puts it this way:

“The exciting thing about finding a planet around this star is that it indicates that planetary systems can survive the giant phase and the helium flash of their parent star. It bodes well for the survival of our own Earth in the distant future. Before V 391 Pegasi lost its outer regions at the helium flash, the planet orbited the star at about the same distance that the Earth orbits our sun.”

Of course, what would be left on the surface of an Earth-class survivor scarcely bears contemplating. In any case, no one can say for sure whether the Earth will escape engulfment like the inner planets. Its orbit should widen as the Sun loses mass even as tidal forces drag the planet inward. We seem to be in an ambiguous zone about which too little is known to feel confidence in the outcome. Mario Livio (Space Telescope Science Institute) is quoted in this New York Times story: “Earth’s fate is actually the most uncertain because it is at the border line between being engulfed and surviving.”

But the new work does show that planets in orbits closer than 2 AU can survive the red giant phase. V 391 Pegasi’s maximum radius is thought to have reached 0.7 AU, a close brush indeed with the planet in question. Earth’s fate won’t be decided for five billion years. The paper is Silvotti et al., “A giant planet orbiting the ‘extreme horizontal branch’ star V 391 Pegasi,” Nature 449 (13 September 2007), pp. 189-191 (abstract).

Habitable Zones Around Gliese 581

by Paul Gilster | Sep 13, 2007 | Exoplanetary Science | 11 comments

Figuring out planetary habitable zones gets a little less theoretical when we start talking about known systems. And when that system is Gliese 581, the interest level rises considerably. After the initial announcements of a possibly habitable planet around that star, Gliese 581c was later analyzed (in a paper by Werner von Bloh and team) as being too close to its star for liquid water to exist. But another planet, the more distant GL 581d, seemed to hold distinct promise of being in the habitable zone.

Now a new paper tackles the question with intriguing results. Petr Chylek and Mario Pérez (Los Alamos National Laboratory) find some reason to think that both inner planets in this system may, under special but feasible conditions, have become suitable for life. The thinking here depends upon analyzing planetary environments as they evolve, with reference to our own Solar System in terms of that evolution.

Start with this: Early on, Venus, Earth and Mars lost their original, hydrogen-rich atmospheres, to produce secondary atmospheres that resulted from the release of water vapor and carbon dioxide from within the planets themselves. The widely divergent atmospheres we see today are, Chylek and Perez argue, the result of surface temperature, gravity, planetary motion and geological composition. Planetary systems that evolve like this may not support a simplified notion of habitable zones. In fact, we may find that habitable planets can exist well outside the zones we theorize.

With that in mind, the researchers look at the planets of GL 581 to determine their effective surface temperatures during the time after they lost their primitive atmospheres and before formation of their secondary atmospheres via outgassing from the planets’ interiors. Under conditions like those of our own Solar System, neither planet develops an atmosphere allowing liquid water on the surface. One is too hot, the other too cold, assuming processes like those on Venus, Earth and Mars.

But if we look at a different set of conditions, things change. Examining the greenhouse effect and cloud formation in the context of planetary albedo, the team concludes that if Gliese 581d developed an atmosphere producing a larger greenhouse effect than found on Earth, it could fit well within the habitable zone. Note this from the paper:

We can assume that a medium strength (comparable to Earth) atmospheric greenhouse effect could provide about 35K warming. The accompanying cloudiness would increase the planetary albedo to about 0.30, which would produce a cooling of about 11K. Thus the result of Earth-like atmosphere would produce a net surface warming of about 24K, which is not sufficient to bring the planet into the range of habitability. However, a considerably denser atmosphere that would lead to a planet completely covered by clouds, producing a planetary albedo of about 0.6, and warming of around 100K (about three times the greenhouse effect of the Earth’s atmosphere) would be sufficient to bring the planet d into a HZ. No objectionable mechanism is apparent that would prohibit the formation of such an atmosphere.

But what about Gliese 581c, the planet that kicked off all the fuss in the first place? Even a moderate greenhouse effect there should create temperatures too high for liquid water on the surface. Even here, though, interesting mechanisms may be at work to offset this apparent problem.

For we know little about the atmospheric effects of tidal lock, through which both these planets presumably keep one faced turned forever toward their star. In what may be their most controversial contribution, the authors argue that atmospheric circulation could transport excess heat from the illuminated to the dark side of the planet, forming a region where liquid water could exist. “Perhaps tropical forests on the hot side can provide a sufficient amount of oxygen for the whole planet and near the illuminated/shadowed boundary a habitable region may occur.”

Clearly, we need to know more about atmospheric movement in a tidally-locked context. Its effect on our notion of a habitable zone could be profound:

We found that a gravitationally locked-in planet can have liquid water on its surface even if it is outside a HZ, defined traditionally by the parameters and distance to the sustaining star… Since half of the planet is permanently in “sunshine” an atmospheric circulation can develop between the illuminated and shadow parts and create a HZ with a possibility of surface liquid water in a planet that, otherwise, would be unsuitable. Thus the auxiliary definition of a HZ should be extended to include the rotational and atmospheric characteristics of planets…

The paper is Chylek and Perez, “Considerations for the habitable zone of super-Earth planets in Gliese 581,” available online.

Building the Celestial Bestiary

by Paul Gilster | Sep 12, 2007 | Exoplanetary Science | 2 comments

We’ve found our share of unusual planets in the short time since actual observations could be made. A decade ago, it would have been hard to come up with anything more unexpected that a ‘hot Jupiter,’ orbiting so close to its parent star that its orbital period is measured in scant days. Add in ‘super Earths’ around dim red dwarfs and pulsar planets (actually the first type of exoplanets to be discovered) like those around the pulsar PSR 1257+12, and you have a bestiary of odd objects in the making.

And now an outburst of gamma and X rays from the direction of the galactic center, one first detected with the Swift satellite’s Burst Alert Telescope, gives promise of yet another kind of object. Pulsing in X rays 182.07 times per second, the source is clearly a ‘millisecond’ pulsar, a neutron star spinning at fantastic rates. Precise studies of the X-ray timing data have revealed the existence of a low-mass companion with a minimum mass of seven Jupiters, but there is wide play in that estimate because of our lack of information about the system’s orbital inclination.

A pulsar planet? Evidently not. The formation scenario goes like this: The original system would have consisted of a massive star and a smaller one not much larger than our own Sun. When the larger star exploded as a supernova, a neutron star was left behind. The second star, moving toward red giant stage, caused the two objects to become embedded in what was now an extended stellar envelope. That would have ejected the envelope itself while drawing the two stars nearer to each other.

Image: In this artist depiction of the SWIFT J1756.9-2508 system, the foreground object is the planet-mass object. The pulsar, located at the upper right, is tidally distorting the companion into a teardrop-shaped object, and ripping gas from it. This material flows in a stream toward the pulsar and forms a disk around it. Eventually, enough gas builds up in the disk to produce an outburst bright enough to make the system visible from Earth. Credit: Aurore Simonnet/Sonoma State University.

So we’re looking not at a planet, despite its size, but the remains of a companion star, says Christopher Deloye (Northwestern University):

“Despite its extremely low mass, the companion isn’t considered a planet because of its formation. It’s essentially a white dwarf that has been whittled down to a planetary mass.”

And one that’s been through a hell of a beating. The cause of the eruption that called this system to our attention may be siphoning of gas from the white dwarf survivor. Flowing into a disk around the neutron star, the gas could trigger the occasional outburst. Because the object, known as SWIFT J1756.9, has never been known to erupt before, all bets are off as to how frequent an occurrence such events may be.

Centauri Dreams‘ note: Those bestiaries I mentioned earlier were wildly popular in the European Middle Ages, presenting (often in beautifully illuminated manuscripts) stories about various plants and animals, some of them mythic, as seen through the lens of current science and religion. Today we assemble collections of astronomical information that put the wildest imaginings of medieval travelers to shame. Each new planetary discovery (and, as we’ll see tomorrow, each new analysis of existing planetary data) sketches places that jog the imagination, and may serve, as did their medieval counterparts, to invigorate not only science but literature and art.

Image: The fabulous, mythical griffin seizing another animal for its dinner. Griffins, unicorns and other such creatures fired the medieval imagination. Will our growing catalog of extrasolar systems have the same effect upon a jaded public? Source: The Harley Bestiary (Harley MS 4751), an English manuscript ca. 1230-40 now preserved in the British Museum.

The paper on this new pulsar work is Krimm et al., “Discovery of the accretion-powered millisecond pulsar SWIFT J1756.9-2508 with a low-mass companion,” accepted for publication in Astrophysical Journal Letters (abstract).