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EXO WORLDS
Seven Earth-like planets discovered around single star
By Marlowe HOOD
Paris (AFP) Feb 22, 2017


Exoplanets 101: Looking for life beyond our Solar System
Paris (AFP) Feb 22, 2017 - Seven Earth-like planets orbiting a small star in our Galaxy called Trappist-1, revealed Wednesday, are the most recent -- and arguably the most spectacular -- in a string of exoplanet discoveries going back 20 years.

Herewith a backgrounder on the search for life beyond the horizon of our Solar System.

- What is an exoplanet? -

Simple: any planet outside our Solar System.

The first exoplanet was detected in 1995, but the number has exploded in the last few years. A recent statistical study estimated that there are a trillion in our galaxy alone.

Today, according to a tally by NASA, there are 3,449 known exoplanets.

Of those, 1,264 are so-called ice giants, 1,043 are gas giants, and 781 are "super Earths" with masses many times higher than the rock we call home.

Before Wednesday's announcement, astronomers had spotted only 348 smaller terrestrial planets with Earth-like mass, and of those only a handful in a "temperate" zone that would allow for the presence of liquid water -- a key ingredient for life (as we know it).

- How are exoplanets detected? -

There are several ways to find planets that cannot be directly observed, according to NASA.

WOBBLE WATCHING - This involves looking for changes in the colour spectrum emitted by a star due to the gravitational pull of one or more invisible planets.

If these patterns are regular and cyclical, corresponding to a tiny wobble in the star, chances are they are caused by a planet. Also called radial velocity, this is how another exoplanet, Proxima b, was discovered last year. Exoplanets found this way: 17.6 percent.

SHADOW SEARCHING - When a planet passes directly between its star and an observer -- an astronomer peering through a telescope, or a satellite in space -- it dims the star's light by a tiny but measurable amount. This so-called "transit" method has been the most successful so far -- NASA's Kepler spacecraft used it to find thousands of candidate planets from 2009 to 2013. Obviously, if a planet doesn't happen to be on the same plane as the star AND the observer, it doesn't work. Exoplanets -- including the seven orbiting Trappist-1 -- found this way: 79 percent.

PICTURE PRODUCING - Snapping a picture of an exoplanet in front of its star is something like trying to photograph a microscopic speck of dust on a glowing lightbulb. But by removing the blinding glare of the star, astronomers can capture an image, a method called direct imaging. Only a tiny fraction of distant planets have been detected this way: 1.2 percent.

BEAM BENDING - In another technique, light from a distant star is bent and focused by gravity as an orbiting planet passes between the star an Earth.

Called gravitational micro-lensing, the gravity of the planet and star focus light rays of the distant planet on an observer in the same way that a magnifying glass focuses the Sun's light onto a tiny, bright spot. Only a handful of exoplanets have been found using this method.

- What conditions support life? -

That depends on what the meaning of "life" is!

For life as we know it, liquid water is an essential ingredient. Of the exoplanets found to date, however, only a handful are in a "temperate" zone in relation to their star: not so hot that water evaporates, not so cold that it freezes rock solid.

Life on Earth is also unimaginable without an atmosphere, containing in our case the oxygen we need to survive. An atmosphere also protects animal species in particular from damaging high-energy radiation from the Sun -- ultraviolet and X-rays.

But as we do not even know yet how life emerged on Earth, it is also possible that living creatures elsewhere in the Universe could survive and thrive in conditions that would be lethal for our tender-footed species.

Source: NASA, ESA

Researchers announced Wednesday the stunning discovery of seven Earth-like planets orbiting a small star in our galaxy, opening up the most promising hunting ground so far for life beyond the Solar System.

All seven roughly match the size and mass of our own planet and are almost certainly rocky, and three are perfectly perched to harbour life-nurturing oceans of water, they reported in the journal Nature.

Most critically, their proximity to Earth and the dimness of their red dwarf star, called Trappist-1, will allow astronomers to parse each one's atmosphere in search of chemical signatures of biological activity.

"We have made a crucial step towards finding life out there," said co-author Amaury Triaud, a scientist at the University of Cambridge.

"Up to now, I don't think we have had the right planets to find out," he said in a press briefing.

"Now we have the right target."

The Trappist-1 system, a mere 39 light years distant, has the largest number of Earth-sized planets known to orbit a single star.

It also has the most within the so-called "temperate zone" -- not so hot that water evaporates, nor so cold that it freezes rock-solid.

The discovery adds to growing evidence that our home galaxy, the Milky Way, may be populated with tens of billions of worlds not unlike our own -- far more than previously suspected.

Remarkably, professional stargazers may simply have been looking in the wrong place.

"The great idea of this approach was to study planets around the smallest stars of the galaxy, and close to us," said lead author Michael Gillon, a professor at the University of Liege in Belgium.

- 'Ultracool' dwarf star -

"That is something nobody did before us -- most astronomers were focused on stars like our Sun," he told journalists ahead of publication.

Gillon and his team began to track Trappist-1 -- a so-called "ultracool" dwarf star with less than 10 percent the mass of the Sun -- with a dedicated telescope in 2010, and reported last year on three planets in its orbit.

They detected the invisible exoplanets using the so-called "transit" method: when an orbiting world passes between a star and an astronomer peering through a telescope, it dims the starlight by a tiny but measurable amount.

But when subsequent calculations didn't quite tally, Gillon realised that there might be other stars that had escaped Earth-bound observation.

"So we requested time with NASA's Spitzer Space Telescope," said co-author Emmanuel Jehin, also at the University of Liege.

"This allowed us to get 20 consecutive 24-hour periods of observation, which was crucial to discovering that we had seven transiting planets."

Looking from Earth, the astronomers could only track activity around the star at night.

"From space, we observed continually and matched all the transits," 34 in all.

Compared to the distance between our Sun and its planets, the Trappist-1 family is very tightly bunched.

Indeed, the dwarf star and its seven satellites -- with orbits ranging from 1.5 to 12 days -- would all fit comfortably in the distance between the Sun and its closest planet, Mercury.

- Like a sunset -

If Earth were that close to the Sun, it would be a hellish ball of fire.

But because Trappist-1 emits far less radiation, temperatures on its planets -- depending on the atmosphere -- could be between zero and 100 degrees Celsius (32 and 212 degrees Fahrenheit), the scientists said.

Gillon and his team have started to analyse the chemical make-up of the atmospheres.

"There is at least one combination of molecules, if present with relative abundance, that would tell us there is life, with 99 percent confidence," said Gillon.

A certain mix of methane, oxygen or ozone, and carbon dioxide, for example, could almost certainly come only from biological sources.

"But except for detecting a message from beyond our solar system from intelligence out there, we will never be 100 percent sure," he added.

Someone standing on, say, Trappist-1 D, E or F -- the three middle planets -- would have a breathtaking panorama of the star and its system, Triaud said.

The red dwarf -- which would loom 10 times larger than the Sun in our sky -- would be a "deep crimson" shading into a salmon-like colour, he said.

"The view would be beautiful -- you would have about 200 times less light that from the Sun on Earth at midday," he added.

"It would be like the end of a sunset."

EXO WORLDS
Excited reports of 'habitable planets' need to come back down to Earth
Santiago, Chile (SPX) Feb 22, 2017
In 1950, physicist Enrico Fermi famously asked, "Where are they?" as a kind of lament about the lack of observational evidence for alien intelligence in our universe. Today, the question is still asked in the context of the always-hoped-for discovery of other worlds like our own, with the thought that maybe, just maybe, we will finally find those aliens. Against this backdrop, advances tha ... read more

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