Erie, Pa. — In B science fiction movies, a terrible force often pushes the
Earth off its axis and spells disaster for all life on Earth. In reality, life
would still be possible on Earth and any Earth-like planets if the axis tilt
were greater than it is now, according to Penn State researchers.

"We do not currently have observations of extrasolar planets, but I imagine
in the near future, we will uncover some of these small planets," says Dr.
Darren M. Williams, assistant professor of physics and astronomy, Penn State
Erie, the Behrend College. "The issue before us is what will they be like? Will
they have moons? What will their climates be like? Will they be teaming with
life or will life be rare?

"I suspect, based on simulations and our own solar system, that many Earth-like
planets will have spin axes that are tipped more severely than Earth’s axis."

Williams, working with David Pollard, research associate in geoscience at Penn
State, used general circulation climate models to simulate a variety of tilts,
carbon dioxide levels and planets. They reported on their findings in the
International Journal of Astrobiology.

The researchers first looked at present-day Earth with tilts of 23, 54, 70 and
85 degrees. Earth’s tilt today is about 23 degrees. The simulation that
today’s Earth and tilt closely matched today’s climate, including regional
precipitation patterns, snow and ice cover and drought.

"Tilts greater than the present produce global annual-mean temperatures higher
than Earth’s present mean temperature of about 57 degrees Fahrenheit," says
Williams. "Above 54 degrees of tilt, the trend is for the global annual-mean
temperature to decrease as tilt increases."

The Penn State scientist explains that this decrease occurs because more land
exists north of the equator in present-day Earth. Annual-mean temperatures,
however, are not the best way to determine if a planet might be habitable, as
seasonal temperature variations could be extreme.

The researchers also looked at these tilted Earths with ten times the carbon
dioxide in the atmosphere. Carbon dioxide as a greenhouse gas increases the
temperatures on a planet. These models produced Earths with 11 to 18 degrees
Fahrenheit higher annual-mean temperatures.

Because all planets will not have Earth’s geography, the researchers took a
from Earth’s history and modeled a 750-million-year-old Earth representing the
Sturtian glaciation and a 540-million-year-old Earth, the closest approximation
available for the Varanger glaciation.

"During the Sturtian, land masses were mainly equatorial and clumped mostly
within 30 degrees of the equator," says the Penn State Erie researcher. "In the
Varanger model, everything is close to the south pole."

While current day Earth is about 30 percent land to 70 percent water, these
ancient geographies are about 22 percent land and 78 percent water.

"The highest temperatures and seasonal variations happen with the largest land
areas at the mid to high latitudes," says Williams.

The researchers also ran some of the model Earths with zero tilt.

"Present Earth is one of the most uninhabitable planets that we have
says Williams. "Approximately 8.7 percent of the Earth’s surface is colder than
14 degrees Fahrenheit on average, and this percentage peaks at 13.2 percent in
February owing to the large landmasses at high latitude covered by snow."

The only planets colder than today’s Earth are those planets simulated with no

The Varanger simulation, with most land in the southern hemisphere, is the most
extreme with 15.6 percent of the surface below 14 degrees Fahrenheit in July
9.3 percent of the surface above 122 degrees Fahrenheit in January. On average,
nearly 28 percent of this planet’s land mass is uninhabitable by Earth

"This simulation suggests that planets with either large polar supercontinents
or small inventories of water will be the most problematic for life at high
obliquity," says Williams.

None of the planets with increased tilt had permanent ice sheets near the
equator. This, however, does not guarantee that a world is suitable for life,
the researchers note. The extremes of temperature on most of the simulated
earths would make it difficult for all but the simplest Earth life forms to
survive. Extremes caused because the tilt puts large portions of the planet in
24-hour darkness or 24-hour sunlight for long periods would also inhibit
photosynthetic organisms.

The researchers suggest that even with high tilt, life can exist on the planets
they modeled.

"Provided the life does not occupy continental surfaces plagued seasonally by
the highest temperature, these planets could support more advanced life," the
researchers say. "While such worlds exhibit climates that are very different
from Earth’s, many will still be suitable for both simple and advanced forms of
water-dependent life."

So there is no reason to eliminate Earth-like planets with more tilt than Earth
from future searches for life beyond the solar system.

"We have one planet and we have a lot of species on this planet, but it is only
one data point," says Williams. "Maybe one day we will figure out everything
about life on our own planet, but no where near what is possible elsewhere."

The National Science Foundation supported this work.

The International Journal of Astrobiology, founded in 2002, is published by
Cambridge University Press. The editors are Dr. Simonj Mitton (Cambridge),