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The Weather on Kepler-7b

Dave Reneke’s wrap-up of space and astronomy news.

Astronomers using data from NASA’s Kepler and Spitzer space telescopes have created the first cloud map of a planet beyond our solar system, a sizzling Jupiter-like world known as Kepler-7b. The planet is marked by high clouds in the west and clear skies in the east. Previous studies from Spitzer have resulted in temperature maps of planets orbiting other stars, but this is the first look at cloud structures on a distant world.

“By observing this planet with Spitzer and Kepler for more than 3 years, we were able to produce a very low-resolution ‘map’ of this giant, gaseous planet,” said Brice-Olivier Demory of Massachusetts Institute of Technology in Cambridge. “We wouldn’t expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds.”

Kepler has discovered more than 150 exoplanets, and Kepler-7b was one of the first. The telescope’s problematic reaction wheels prevent it from hunting planets any more, but astronomers continue to pore over almost 4 years of collected data.

Kepler watches for a planet to cross in front of its star, blocking a tiny fraction of the star’s light. By measuring how much the star dims during such a transit, astronomers can calculate the planet’s size, and by observing the time between successive events they can derive how long it takes the planet to revolve around its parent star.

Observations of Kepler-7b’s moon-like phases led to a rough map of the planet that showed a bright spot on its western hemisphere, but the data was not strong enough to decipher whether the bright spot was coming from clouds or heat. The Spitzer Space Telescope played a crucial role in answering this question.

Like Kepler, Spitzer can fix its gaze at a star system as a planet orbits around the star, gathering clues about the planet’s atmosphere. Spitzer’s ability to detect infrared light means it was able to measure Kepler-7b’s temperature, estimating it at 1500–1800°F. This is relatively cool for a planet that orbits so close to its star, and too cool to be the source of light Kepler observed.

Astronomers then determined that light from the planet’s star is bouncing off cloud tops located on the west side of the planet. Unlike Earth, the cloud patterns on this planet don’t seem to change much over time – in fact it has a remarkably stable climate. The findings are an early step toward using similar techniques to study the atmospheres of planets more like Earth in composition and size, even worlds trillions of kilometres away.

Kepler identified planets by watching for dips in starlight as the planets transit in front of their stars. This technique and other observations of Kepler-7b previously revealed it is one of the puffiest planets known – if it could be placed in a tub of water it would float. The planet whips around its star in less than 5 days.

A Graveyard of Comets

A team of astronomers from the University of Anitoquia, Colombia, has discovered a graveyard of comets. The researchers describe how some of these objects, inactive for millions of years, have returned to life, leading them to name the group the “Lazarus comets”.

Comets are among the smallest objects in the Solar System, typically just a few kilometres across and composed of a mixture of rock and ice like dirty snowballs. If they come close to the Sun, some of the ice turns to gas before being swept back by the light of the Sun and the solar wind to form a characteristic tail of gas and dust.

Most observed comets have highly elliptical orbits, so they only rarely approach the Sun. Some of these so-called long period comets take thousands of years to complete each orbit. There is also a population of about 500 short period comets, deflected by Jupiter into orbits that last anything between 3 and 200 years. Comets also collide with the Earth and may have helped bring water to our planet.

The new work looked at a third and distinct region of the Solar System – the main asteroid belt between Mars and Jupiter. This volume of space contains more than a million objects ranging in size from 1 metre to 800 km. Traditionally, asteroids are thought of as the building blocks of a planet that never formed, their movement disrupted by the strong gravitational field of Jupiter.

In the past decade 12 active comets have been discovered in the main asteroid belt. This was something of a surprise, and the Columbian team set out to investigate their origin.

“We found a graveyard of comets”, exclaimed Prof Ignacio Ferrín. “We have found that some of these are not dead rocks after all, but are dormant comets that may yet come back to life if the energy that they receive from the Sun increases by a few per cent.”

This can happen fairly easily, as the orbits of many objects in the asteroid belt are nudged by the gravity of Jupiter, changing those orbits. According to this interpretation, millions of years ago the main belt was populated by thousands of active comets. This population aged and the activity subsided. What we see today is the residual activity of that glorious past.

Ferrín said the 12 active comets are the Lazarus comets returning to life after being dormant for thousands or even millions of years. Potentially any one of the many thousands of their quiet neighbours could do the same thing.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com