[meteorite-list] Theory Proposes New View of Sun and Earth's Creation

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu May 20 20:15:14 2004
Message-ID: <200405210015.RAA22630_at_zagami.jpl.nasa.gov>

Contact: James Hathaway
hathaway_at_asu.edu
480-965-6375
Arizona State University
May 20, 2004

Theory proposes new view of sun and Earth's creation

Like most creation stories, this one is dramatic: we began, not as a mere
glimmer buried in an obscure cloud, but instead amidst the glare and turmoil
of restless giants.

Or so says a new theory, supported by stunning astronomical images and hard
chemical analysis. For years most astronomers have imagined that the Sun and
Solar System formed in relative isolation, buried in a quiet, dark corner of
a less-than-imposing interstellar cloud. The new theory challenges this
conventional wisdom, arguing instead that the Sun formed in a violent
nebular environment - a byproduct of the chaos wrought by intense
ultraviolet radiation and powerful explosions that accompany the short but
spectacular lives of massive, luminous stars.

The new theory is described in a "Perspectives" article appearing in the May
21 issue of Science. The article was written by a group of Arizona State
University astronomers and meteorite researchers who cite recently
discovered isotopic evidence and accumulated astronomical observations to
argue for a history of development of the Sun, the Earth and our Solar
System that is significantly different from the traditionally accepted
scenario.

If borne out by future work, this vision of our cosmic birth could have
profound implications for understanding everything from the size and shape
of our solar system to the physical makeup of the Earth and the development
of the chemistry of life.

"There are two different sorts of environment where low-mass stars like the
Sun form," explained ASU astronomer Jeff Hester, the essay's lead author.
"In one kind of star-forming environment, you have a fairly quiescent
process in which an undisturbed molecular cloud slowly collapses, forming a
star here? a star there. The other type of environment in which Sun-like
stars form is radically different. These are more massive regions that form
not only low-mass stars, but luminous high-mass stars, as well."

More massive regions are very different because once a high-mass star forms,
it begins pumping out huge amounts of energy that in turn completely changes
the way Sun-like stars form in the surrounding environment. "People have
long imagined that the Sun formed in the first, more quiescent type of
environment," Hester noted, "but we believe that we have compelling evidence
that this is not the case."

Critical to the team's argument is the recent discovery in meteorites of
patterns of isotopes that can only have been caused by the radioactive decay
of iron-60, an unstable isotope that has a half life of only a million and a
half years. Iron-60 can only be formed in the heart of a massive star and
thus the presence of live iron-60 in the young Solar System provides strong
evidence that when the Sun formed (4.5 billion years ago) a massive star was
nearby.

Hester's coauthors on the Science essay include Steve Desch, Kevin Healy,
and Laurie Leshin. Leshin is a cosmochemist and director of Arizona State
University's Center for Meteorite Studies. "One of the exciting things about
the research is that it is truly transdisciplinary, drawing from both
astrophysics and the study of meteorites - rocks that you can pick up and
hold in your hand - to arrive at a new understanding of our origins," noted
Leshin.

When a massive star is born, its intense ultraviolet radiation forms an "HII
region" - a region of hot, ionized gas that pushes outward through
interstellar space. The Eagle Nebula, the Orion Nebula, and the Trifid
Nebula are all well-known examples of HII regions. A shock wave is driven in
advance of the expanding HII region, compressing surrounding gas and
triggering the formation of new low-mass stars. "We see triggered low-mass
star formation going on in HII regions today," said Healy, who recently
completed a study of radio observations of this process at work.

The star does not have much time to get its act together, though. Within
100,000 years or so, the star and what is left of its small natal cloud will
be uncovered by the advancing boundary of the HII region and exposed
directly to the harsh ultraviolet radiation from the massive star. "We see
such objects emerging from the boundaries of HII regions,'' Hester said.
"These are the 'evaporating gaseous globules' or 'EGGs' seen in the famous
Hubble image of the Eagle Nebula."

EGGs do not live forever either. Within about ten thousand years an EGG
evaporates, leaving behind only the low-mass star and its now-unprotected
protoplanetary disk to face the brunt of the massive star's wrath. Like a
chip of dry ice on a hot day, the disk itself now begins to evaporate,
forming a characteristic tear-drop-shaped structure like the "proplyds" seen
in Hubble images of the Orion Nebula. "Once we understood what we were
looking at, we realized that we had a number of images of EGGs caught just
as they were turning into proplyds," said Hester. "The evolutionary tie
between these two classes of objects is clear."

Within another ten thousand years or so the proplyd, too, is eroded away.
All that remains is the star itself, surrounded by the inner part of the
disk (comparable in size to our Solar System), which is able to withstand
the continuing onslaught of radiation. It is from this disk and in this
environment that planets may form.

The process leaves a Sun-like star and its surrounding disk sitting in the
interior of a low density cavity with a massive star close at hand. Massive
stars die young, exploding in violent events called "supernovas." When a
supernova explodes it peppers surrounding infant planetary systems with
newly synthesized chemical elements - including short-lived radioactive
isotopes such as iron-60.

"This is where the meteorite data come in," said Hester. "When we look at
HII regions we see that they are filled with young, Sun-like stars, many of
which are known to be surrounded by protoplanetary disks. Once you ask the
question, 'what is going to happen when those massive stars go supernova?',
the answer is pretty obvious. Those young disks are going to get enriched
with a lot of freshly-made elements."

"When you then pick up a meteorite and find a mix of materials that can only
be easily explained by a nearby supernova, you realize that you are looking
at the answer to a very longstanding question in astronomy and planetary
science," Desch added.

"So from this we now know that if you could go back 4.5 billion years and
watch the Sun and Solar System forming, you would see the kind of
environment that you see today in the Eagle or Trifid nebulas," said Hester.

"There are many aspects of our Solar System that seem to make sense in light
of the new scenario," notes Leshin. "For example, this might be why the
outer part of the Solar System - the Kuiper Belt - seems to end abruptly.
Ultraviolet radiation would also have played a role in the organic chemistry
of the young solar system, and could explain other peculiar effects such as
anomalies in the abundances of isotopes of oxygen in meteorites."

One of the most intriguing speculations is that the amount of radioactive
material injected into the young solar system by a supernova might have
profoundly influenced the habitability of Earth itself. Heat released by the
decay of this material may have been responsible for "baking out" the
planetesimals from which the earth formed, and in the process determining
how much water is on Earth today.

"It is kind of exciting to think that life on Earth may owe its existence to
exactly what sort of massive star triggered the formation of the Sun in the
first place, and exactly how close we happened to be to that star when it
went supernova," mused Hester. "One thing that is clear is that the
traditional boundaries between fields such as astrophysics, meteoritics,
planetary science, and astrobiology just got less clear-cut. This new
scenario has a lot of implications, and makes a lot of new predictions that
we can test."

If it is accepted, the new theory may also be of use in looking for life in
the universe beyond. "We want to know how common Earth-like planets are. The
problem with answering that question is that if you don't know how
Earth-like planets are formed - if you don't understand their connection
with astrophysical environments - then all you can do is speculate," Hester
said.

"We think that we're starting to see a very specific causal connection
between astrophysical environments and the things that have to be in place
to make a planet like ours."

                                     ###

Sources: Jeff Hester, 480-965-0741, jhester_at_asu.edu
Laurie Leshin, 480-965-0796, laurie.leshin_at_asu.edu

Images: http://clas.asu.edu/newsevents/pressreleases/photos/HII/
Received on Thu 20 May 2004 08:15:03 PM PDT