Exoplanet Astronomers: 'Tilt-a-worlds' could harbour life

Tilted orbits such as those shown might make some planets wobble like a top that’s almost done spinning, an effect that could maintain liquid water on the surface, thus giving life a chance. 

Credit: NASA

A fluctuating tilt in a planet's orbit does not preclude the possibility of life, according to new research by astronomers at the University of Washington, Utah's Weber State University and NASA. In fact, sometimes it helps.

That's because such "tilt-a-worlds," as astronomers sometimes call them, turned from their orbital plane by the influence of companion planets, are less likely than fixed-spin planets to freeze over, as heat from their host star is more evenly distributed.

This happens only at the outer edge of a star's habitable zone, the swath of space around it where rocky worlds could maintain liquid water at their surface, a necessary condition for life.

Further out, a "snowball state" of global ice becomes inevitable, and life impossible.

The findings, which are published online and will appear in the April issue of Astrobiology, have the effect of expanding that perceived habitable zone by 10 to 20 percent.

And that in turn dramatically increases the number of worlds considered potentially right for life.

Such a tilt-a-world becomes potentially habitable because its spin would cause poles to occasionally point toward the host star, causing ice caps to quickly melt.

Rory Barnes

"Without this sort of 'home base' for ice, global glaciation is more difficult," said UW astronomer Rory Barnes.

"So the rapid tilting of an exoplanet actually increases the likelihood that there might be liquid water on a planet's surface."

John Armstrong

Barnes is second author on the paper. First author is John Armstrong of Weber State, who earned his doctorate at the UW.

Earth and its neighbour planets occupy roughly the same plane in space. But there is evidence, Barnes said, of systems whose planets ride along at angles to each other.

As such, "they can tug on each other from above or below, changing their poles' direction compared to the host star."

The team used computer simulations to reproduce such off-kilter planetary alignments, wondering, he said, "what an Earthlike planet might do if it had similar neighbours."

Their findings also argue against the long-held view among astronomers and astrobiologists that a planet needs the stabilizing influence of a large moon, as Earth has, to have a chance at hosting life.

"We're finding that planets don't have to have a stable tilt to be habitable," Barnes said. Minus the moon, he said, Earth's tilt, now at a fairly stable 23.5 degrees, might increase by 10 degrees or so. Climates might fluctuate, but life would still be possible.

"This study suggests the presence of a large moon might inhibit life, at least at the edge of the habitable zone."

The work was done through the UW's Virtual Planetary Laboratory, an interdisciplinary research group that studies how to determine if exoplanets—those outside the solar system—might have the potential for life.

"The research involved orbital dynamics, planetary dynamics and climate studies. It's bigger than any of those disciplines on their own," Barnes said.

Armstrong said that expanding the habitable zone might almost double the number of potentially habitable planets in the galaxy.

Applying the research and its expanded habitable zone to our own celestial neighborhood for context, he said, "It would give the ability to put Earth, say, past the orbit of Mars and still be habitable at least some of the time, and that's a lot of real estate."

More information: Paper: online.liebertpub.com/doi/abs/10.1089/ast.2013.1129

Hubble traces subtle signals of water on hazy worlds

To determine what's in the atmosphere of an exoplanet, astronomers watch the planet pass in front of its host star and look at which wavelengths of light are transmitted and which are partially absorbed. 

Credit: NASA's Goddard Space Flight Center

Using the powerful eye of NASA's Hubble Space Telescope, two teams of scientists have found faint signatures of water in the atmospheres of five distant planets.

The presence of atmospheric water was reported previously on a few exoplanets orbiting stars beyond our solar system, but this is the first study to conclusively measure and compare the profiles and intensities of these signatures on multiple worlds.

The five planets—WASP-17b, HD209458b, WASP-12b, WASP-19b and XO-1b—orbit nearby stars.

The strengths of their water signatures varied. WASP-17b, a planet with an especially puffed-up atmosphere, and HD209458b had the strongest signals.

The signatures for the other three planets, WASP-12b, WASP-19b and XO-1b, also are consistent with water.

"We're very confident that we see a water signature for multiple planets," said Avi Mandell, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., and lead author of an Astrophysical Journal paper, published today, describing the findings for WASP-12b, WASP-17b and WASP-19b.

"This work really opens the door for comparing how much water is present in atmospheres on different kinds of exoplanets, for example hotter versus cooler ones."

The studies were part of a census of exoplanet atmospheres led by L. Drake Deming of the University of Maryland in College Park.

Both teams used Hubble's Wide Field Camera 3 to explore the details of absorption of light through the planets' atmospheres.

The observations were made in a range of infrared wavelengths where the water signature, if present, would appear.

The teams compared the shapes and intensities of the absorption profiles, and the consistency of the signatures gave them confidence they saw water.

The observations demonstrate Hubble's continuing exemplary performance in exoplanet research.

"To actually detect the atmosphere of an exoplanet is extraordinarily difficult. But we were able to pull out a very clear signal, and it is water," said Deming, whose team reported results for HD209458b and XO-1b in a Sept. 10 paper in the same journal.

Deming's team employed a new technique with longer exposure times, which increased the sensitivity of their measurements.