Observations Upset Models of Stellar Evolution
Stellar evolution models go back decades. Ever since the Hertzsprung-Russell diagram came out in 1910 (graphing temperature against luminosity), showing most stars fell on a line called the “main sequence”, astrophysicists have sought to understand the life cycle of stars from birth to death. In general, the story goes, collapsing clouds of gas and dust produce main-sequence stars that burn nuclear fuel till they run out. Depending on their masses, they end up as supernovae, red giants or slowly-cooling cinders. While red dwarfs cool down slowly into the darkness, supernovae and red giants eject mass outward into space . Two new planets found close to a red giant are among new headaches for theorists.
Hot survivors: The paper by Charpinet et al. in Nature1 was discussed by Eliza M. R. Kempton in the same issue,2 who said, “The prospect of planets being detected in close proximity to an evolved star is certainly of great interest, because it was previously assumed that such objects would be destroyed during the stars evolution.” While other sites like the BBC News and Live Science focused on how scorching hot these planets must be, having once been inside the envelope of the red giant star, Science Daily put the deeper issue into the headline: “Discovery of Two Earth-Size Planets Raises Questions About the Evolution of Stars.”
In the paper, Charpinet et al. discuss two possibilities. One is that the planets are cores of gas giant planets that lost their gaseous envelopes. The authors believe this implies the startling idea that planets can affect the evolution of their parent stars. If they are indeed survivors, the rocky core remnants must have endured hellish conditions that astronomers believed would have vaporized any planet. Another problem is how they migrated inward so far when the red giant phase involves mass movement outward.
Alternative scenarios may also be considered. Another way to form single sdB stars is through the merger of two helium white dwarfs, and planet formation following this event may be possible. We could speculate that http://www.nature.com/nature/journal/v480/n7377/full/480328a.htmlthe collapse of the extended envelope resulting from this merger could produce a circumstellar disk, where second generation planets may form. However, it seems unlikely that new, sufficiently dense, planets could have formed within a rather short period of time (less than ~18 Myr) in an environment that close to this hot star.
Neither possibility seems likely. Since the masses of these planets have not been constrained with sufficient detail, more observations may be needed.
All bets are off: Meanwhile, PhysOrg was reporting that “Some nearby stars may be much older than previously thought,” over twice the assumed age (11 million years, not five million). “The findings are surprising given Upper Scorpius's status as one of the best-studied samples of young stars in the sky.” The reasoning may be circular, though, since the conclusions are theory-laden: “we used state-of-the-art stellar evolution models to determine the ages.” If the same stellar evolution models led to a 220% revision of the age, it calls into question the usefulness of the models. The researchers at the University of Rochester believe their estimates are based on firmer measurements of distance and mass, but theory still drives the conclusions about how stars age. For now, though, they are asking other astronomers to reassess their assumptions about the ages of other clusters. “If a stellar group as well-studied as Upper Scorpius can be twice as old as previously believed, then all bets are off on the accuracy of the previously published ages for other similar groups of young stars,” the article ended.
Heavy hearts: Last month Space.com reported, “Unexpectedly Heavy Stars from Long Ago Puzzle Astronomers.” The article about unpredicted amounts of heavy elements in stars on the outer fringes of our galaxy contained various alternatives to explain the anomalies. Bottom line, though: astronomers did not predict this, either.
Dash away all: Last week, Nature revealed that astronomers still do not understand Type 1a supernova. These are critically important stellar events used to calculate the large-scale structure of the universe. “Not knowing the exact nature of type Ia supernovae, which have such a crucial place in cosmology, is an embarrassing situation,” Mario Hamuy remarked.3
Privileged Planet: Meanwhile, count your lucky stars. There are more indications that Earth is uniquely suited for life. While NASA was trumpeting Kepler’s discovery of two Earth-size (but uninhabitable) planets (Astrobiology Magazine), PhysOrg had reported (again) that tidal locking around many stars would render many planets uninhabitable. Even those diamond planets may not be man’s best friend, remarked New Scientist. Valerie Jamison on New Scientist gave a surprisingly favorable review to John Gribbin’s pessimistic new book, Alone in the Universe: Why Our Planet Is Unique. “Gribbin's argument proves compelling as he ranges over issues of astrophysics, geology, atmospheric chemistry and evolution,” she wrote of this “thought-provoking and sobering” book. “Although he cannot quantify exactly how the potential for life around those trillion trillion other suns whittles down to zero, you can feel any optimism that ET is out there ebb away with each turn of the page.” Another sobering conclusion is that there is nowhere else out there for us to go. For more reasons to consider the rarity of Earth, watch the documentary, The Privileged Planet.
1. Charpinet et al., “A compact system of small planets around a former red-giant star,” Nature 480 (22 December 2011), pp. 496–499, doi:10.1038/nature10631.
2. Eliza M. R. Kempton, “Planetary science: The ultimate fate of planets,” Nature 480 (22 December 2011), pp. 460–461, doi:10.1038/480460a.
3. Mario Hamuy, “Astrophysics: Cosmic explosions under scrutiny,” Nature 480 (15 December 2011), pp. 328–329, doi:10.1038/480328a.
Astronomers can be forgiven for puzzlement over occasional anomalies. When venturing out to explore any unknown realm, there will be surprises. We must keep distinct in our minds, however, the difference between scientific exploration and scientific explanation. Cataloging and describing phenomena is non-controversial. Astronomers sometimes show unwarranted hubris in proclaiming they’ve mapped out the territory enough to explain how it got that way and where it’s going. The anomalies described above should at least give pause to anyone tempted to trust scientists as the most reliable humans on the planet. When reading reports like this, enjoy the show (the discoveries) but beware the commercial (scientists have it all figured out).
Research Project: Determine whether the expected outward momentum of a red giant’s expanding envelope poses a problem for the location of the two planets.