Astronomers intensify search for life beyond Earth

• Giant asteroid impact, volcanic eruptions accelerated mass extinctions’ Astronomers have created a way to compare and rank exoplanets to help prioritize which of the thousands discovered warrant close inspection in the search for life beyond Earth even as geologists have uncovered compelling evidence that an asteroid impact on Earth 66 million years ago accelerated the eruptions of volcanoes in India for hundreds of thousands of years, and that together these planet-wide catastrophes caused the extinction of many land and marine animals, including the dinosaurs.

Astronomers with the University of Washington’s (UV) Virtual Planetary Laboratory have created a way to compare and rank exoplanets to help prioritize which of the thousands discovered warrant close inspection in the search for life beyond Earth.

The new metric, called the “habitability index for transiting planets,” is introduced in a paper accepted for publication in the Astrophysical Journal by UW astronomy professors Rory Barnes and Victoria Meadows, with research assistant and co-author Nicole Evans.

Meanwhile, for 35 years, paleontologists and geologists have debated the role these two global events played in the last mass extinction, with one side claiming the eruptions were irrelevant, and the other side claiming the impact was a blip in a long-term die-off.

The new evidence, published October 2 issue of the journal Science, includes the most accurate dates yet for the volcanic eruptions before and after the impact. The new dates show that the Deccan Traps lava flows, which at the time were erupting at a slower pace, doubled in output within 50,000 years of the asteroid or comet impact that is thought to have initiated the last mass extinction on Earth.

Both the impact and the volcanism would have blanketed the planet with dust and noxious fumes, drastically changing the climate and sending many species to an early grave.
Barnes said: “Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme. So that as we move into a time when there are hundreds of targets available, we might be able to say, ‘OK, that’s the one we want to start with.'”

The Kepler Space Telescope has enabled astronomers to detect thousands of exoplanets, those beyond our solar system — far more than can be investigated one by one. The James Webb Space Telescope, set for launch in 2018, will be the first able to actually measure the atmospheric composition of a rocky, possibly Earthlike planet far off in space, and so vastly enhance the search for life.

Astronomers detect some planets when the worlds “transit” or pass in front of their host star, thus blocking some of the light. The Transiting Exoplanet Survey Satellite, or TESS, is scheduled to launch in 2017 and will find many more worlds in this way. But it’s the Webb telescope and its “transit transmission spectroscopy” that will really be able to study planets closely to hunt for life.

But access to such telescopes is expensive and the work is methodical and time-consuming. The Virtual Planetary Laboratory’s index is a tool to help fellow astronomers decide which worlds might have the better chance of hosting life, and so are worthy of focusing limited resources on.

Traditionally, astronomers have focused the search by looking for planets in their star’s “habitable zone” — more informally called the “Goldilocks zone” — which is the swath of space that’s “just right” to allow an orbiting Earth-like planet to have liquid water on its surface, perhaps giving life a chance. But so far that has been just a sort of binary designation, indicating only whether a planet is, or is not, within that area considered right for life.

The new index is more nuanced, producing a continuum of values that astronomers can punch into a Virtual Planetary Laboratory Web form to arrive at the single-number habitability index, representing the probability that a planet can maintain liquid water at its surface.

In creating the index, the researchers factored in estimates of a planet’s rockiness, rocky planets being the more Earth-like. They also accounted for a phenomenon called “eccentricity-albedo degeneracy,” which comments on a sort of balancing act between the a planet’s albedo — the energy reflected back to space from its surface — and the circularity of its orbit, which affects how much energy it receives from its host star.

The two counteract each other. The higher a planet’s albedo, the more light and energy are reflected off to space, leaving less at the surface to warm the world and aid possible life. But the more noncircular or eccentric a planet’s orbit, the more intense is the energy it gets when passing close to its star in its elliptic journey.

Meanwhile, lead researcher Paul Renne, a University of California (UC) Berkeley, United States, professor-in-residence of earth and planetary science and director of the Berkeley Geochronology Center, said: “Based on our dating of the lavas, we can be pretty certain that the volcanism and the impact occurred within 50,000 years of the extinction, so it becomes somewhat artificial to distinguish between them as killing mechanisms: both phenomena were clearly at work at the same time. It is going to be basically impossible to ascribe actual atmospheric effects to one or the other. They both happened at the same time.”

The geologists argue that the impact abruptly changed the volcanoes’ plumbing system, which produced major changes in the chemistry and frequency of the eruptions. After this change, long-term volcanic eruptions likely delayed recovery of life for 500,000 years after the KT boundary, the term for the end of the Cretaceous and the beginning of the Tertiary period when large land animals and many small sea creatures disappeared from the fossil record.

Renne said: “The biodiversity and chemical signature of the ocean took about half a million years to really recover after the KT boundary, which is about how long the accelerated volcanism lasted. We are proposing that the volcanism unleashed and accelerated right at the KT boundary suppressed the recovery until the volcanoes waned.”

Co-author Mark Richards, a UC Berkeley professor of earth and planetary science and the one who originally proposed that the comet or asteroid impact reignited the Deccan Traps lava flows, is agnostic about which event was the real death knell for much of life on Earth. But the link between the impact and the flood basalts is becoming harder to deny.

If our high-precision dates continue to pin these three events – the impact, the extinction and the major pulse of volcanism – closer and closer together, people are going to have to accept the likelihood of a connection among them. The scenario we are suggesting – that the impact triggered the volcanism – does in fact reconcile what had previously appeared to be an unimaginable coincidence,” he said.
Renne, Richards and their colleagues will publish the new dates for the Deccan Traps eruptions in the October 2 issue of the journal Science.

Meanwhile, a life-friendly energy equilibrium for a planet near the inner edge of the habitable zone — in danger of being too hot for life — Barnes said, would be a higher albedo, to cool the world by reflecting some of that heat into space. Conversely, a planet near the cool outer edge of the habitable zone would perhaps need a higher level of orbital eccentricity to provide the energy needed for life.

Barnes, Meadows and Evans ranked in this way planets so far found by the Kepler Space Telescope, in its original mission as well as its “K2” follow-up mission. They found that the best candidates for habitability and life are those planets that get about 60 percent to 90 percent of the solar radiation that the Earth receives from the sun, which is in keeping with current thinking about a star’s habitable zone.