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Star's Chemistry Key to Creating Habitable Zones



The search for potentially habitable planets involves discussion of what is sometimes referred to as the "Goldilocks Zone" – the relatively thin band in a solar system in which conditions on a planet can support life. Astrobiologists and planetary scientists agree that a planet's distance from its parent star is of paramount importance for creating those optimum conditions.

A new study by Arizona State University researchers suggests that the host star's chemical makeup also can impact conditions of habitability of planets that orbit them. The team's paper, published in the August issue of The Astrophysical Journal Letters, demonstrates that subtle differences in a star's internal chemistry can have huge effects on a planet's chances of long-term habitability.

"We have identified changes in the ratios of different elements as particularly important for a given solar system's habitability," says Patrick Young, an assistant professor in ASU's School of Earth and Space Exploration and lead author on the paper. "The more abundant elements carbon, oxygen, silicon, magnesium and sodium are particularly important. The greater the abundances of these four elements in a star, the slower it, and the location of its Goldilocks Zone, will evolve."

"Habitability is very difficult to quantify because it depends on a huge number of variables, some of which we have yet to identify," says Young. "It also depends on the definition of habitable that we choose to use. We chose to use a relatively simple model that predicts whether a planet can sustain liquid water on its surface with reasonable assumptions about planetary atmospheres."

As a star evolves, it becomes brighter, causing the habitable zone to move outwards through its solar system. The team's study indicates that a greater abundance of oxygen, carbon, sodium, magnesium and silicon should be a plus for an inner solar system's long-term habitability because the abundance of these elements make the star cooler and cause it to evolve more slowly, thereby giving planets in its habitable zone more time to develop life as we know it.

To explore whether stellar internal chemistry causes significant changes in the evolution of stars and therefore their habitable zones, Young and his colleagues, graduate students Mike Pagano and Kelley Liebst, did simulations of stars that are like our sun. "We used spectra from 145 broadly sun-like stars targeted by planet to estimate the amount of variation in the abundance ratios of elements," explains Pagano, who is a graduate student in the School of Earth and Space Exploration astrophysics program.

"For each model, we varied the amount of one element to the extremes of variation we estimated from our analysis of the observations. Oxygen is the most abundant element in the universe besides hydrogen and helium, so a change in the oxygen abundance results in a significant change in the total amount of heavy elements in the star," Pagano said. "Oxygen turns out to be highly variable in abundance. The effect of increased heavy element abundance on a star is to make it harder for the energy produced by nuclear fusion to escape the star. This means less energy needs to be produced to support the star, and it can live longer."

The stellar abundance of oxygen seems crucial in determining how long planets stay in the habitable zone around their host star. If there had been less oxygen in the Sun's chemical makeup, for example, Earth likely would have been pushed out of the Sun's habitable zone about a billion years ago, well before complex organisms evolved. Considering the first complex multicellular organisms only arose about 650 million years ago, such a move would have likely destroyed any chance of complex life taking hold on Earth. Planets being searched for signs of life may be about to leave their habitable zones or only have just entered them.

Journal reference: Astrophysical Journal Letters

The Daily galaxy via Arizona State University


I have real problems with the articles assumptions, not only because of the unknown variables but because we cannot possibly even begin to claim that we have enough information about the universe to even form a educated guess on this subject. The article makes no mention of binary star systems or how that would impact habitable zones. For all we know life could thrive far past or even closer to a parent star given certain strange and unique variables not currently known to us. Until we can confidently say we have surveyed at least 20% of the known universe, which i think will take decades at our current level of technology. How can we make such assumptions? Knowing they will likely be proven false as many other assumptions are daily, with regards to science in general let alone the greatest mysteries of the universe. The goldilocks zone crap is just another man made idea of how the universe should operate, and hasn't it already proven to us that we don't understand how it operates on both grand and small scales.

seems like every week we get a new crop of scientists talking about habitable zones.

Actually, I thought this was a rather well-done article, except for one detail: it discusses the "four" elements of carbon, oxygen, silicon, magnesium, and sodium.

(I'm not readily finding the article under discussion here, so I can't tell if this was an error at TDG, of Dr. Young's, or at some middle point. I more strongly suspect the last.)

But never mind that. This discovery should help our astronomers to more easily identify and locate the stars with the correct chemistry to have planets habitable by us. For that, I think this development is prize-worthy.

Goldilocks was not an alien, she was a high class hooker who liked expensive meals and slept a lot in hotel rooms.

“Goldilocks, Goldilocks, Goldilocks”! Are we really betting the universe is indeed trivial when it comes to conditions for life? Or, are we engaged in ‘human-speak’ and talking about conditions, the “G-zone’, that…might…just might lead to life 'FORMS’ that resemble us? I would be shocked if the answer was “Yes”, only the G-zone spawns life forms of any kind. But maybe we bow too deeply when we speak of the vastness, and vast probabilities, of the universe. Is the universe trivial when it comes to life forms?

Although a Daily Galaxy article in the last few weeks pointed out the possibility of alternate biochemistries, terrestrial aqueous hydrocarbon biochemistry is the only system that we know for sure works, so it's reasonable to consider and discuss the constraints of this hydrocarbon chemistry. The high abundance of carbon and oxygen relative to other "metals" may also produce a bias toward this biochemistry system and increase its importance.

I like the consideration of the evolution of the habitable zone. Planets like Gliese 581d may be becoming more habitable albeit very slowly.

Astrobiologists and planetary scientists agree that a planet's distance from its parent star is of paramount importance for creating those optimum conditions.

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