RSSAll Entries Tagged With: "Gliese 581"

Billions of super-Earths “out there”

Artist's impression of a planet circling a red dwarf star

Artist's impression of a planet circling the star Gliese 581. Astronomers estimate there could be tens of billions of "super-Earth" planets in our galaxy.

ROCKY PLANETS NOT MUCH BIGGER THAN EARTH are very common in the habitable zones around faint red stars, say astronomers.

The habitable zone is the distance from a star where it is neither too hot nor too cold for liquid water to exist on the surface of a rocky planet.

The international team used a “planet finder” instrument to estimate that there are tens of billions of such planets in the Milky Way galaxy alone, and probably about 100 in the Sun’s immediate neighbourhood.

This is the first direct measurement of the frequency of super-Earths around red dwarfs, which account for 80% of the stars in the Milky Way.

This first direct estimate of the number of light planets circling red dwarf stars used observations made with the HARPS spectrograph on the 3.6-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile.

Super-Earths abound

The HARPS team has been searching for exoplanets orbiting the most common kind of star in the Milky Way—red dwarf stars (also known as M dwarfs. These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in the Milky Way.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone…,” says Xavier Bonfils (IPAG, Observatoire des Sciences de l’Univers de Grenoble, France), the leader of the team.

Diagram showing the habitable zone for small, medium and large stars.

Diagram showing the habitable zone (green area) varies depending on the size and temperature of the star. Too close in (red area) and it's too hot; too far out (blue area) and it's too cold.

“Because red dwarfs are so common—there are about 160 billion of them in the Milky Way—this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) were found, including two inside the habitable zones of stars Gliese 581 and Gliese 667 C respectively.

The astronomers could estimate how heavy the planets were and how far from their stars they orbited.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team has been able to work out how common different sorts of planets are in red dwarf systems.

They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%.

On the other hand, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare in red dwarf systems. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1,000 times that of the Earth).

In the zone

As there are many red dwarf stars close to the Sun the new estimate means that there are probably about 100 super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years.

“The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to [a red dwarf] star than the Earth is to the Sun,” says Stéphane Udry (Geneva Observatory and member of the team).

“But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely.”

One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. This is the second planet in this triple-star system and it seems to be situated close to the centre of the habitable zone.

Artist’s impression of a sunset seen from the super-Earth Gliese 667 Cc

This artist’s impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Astronomers have estimated that there are tens of billions of such rocky worlds orbiting faint red dwarf stars in the Milky Way alone.

Although this planet is more than four times heavier than the Earth it is the closest twin to Earth found so far, and almost certainly has the right conditions for the existence of liquid water on its surface.

Gliese 667 Cc is the second super-Earth planet inside the habitable zone of a red dwarf discovered during this HARPS survey, after Gliese 581d was announced in 2007 and confirmed in 2009.

“Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments,” concludes Xavier Delfosse, another member of the team.

Some of these planets are expected to pass in front of, or transit, their parent star as they orbit, and astronomers can use these transits to learn more about the planets’ atmospheres and look for signs of life.

Adapted from information issued by ESO / L. Calçada.

Get SpaceInfo.com.au daily updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates. Or follow us on Twitter, @spaceinfo_oz

Like this story? Please share or recommend it…

“Goldilocks” planet discovered

Artist's impression of a planet orbiting Gliese 581

An artist's impression of a planet orbiting the red dwarf star Gliese 581. Astronomers have just discovered one in the star's "habitable zone", where temperatures could be right for liquid water to exist.

  • Gliese 581g orbits in its star’s “Goldilocks” zone
  • Temperature okay for liquid water – not too hot, not too cold
  • If confirmed, will be first potentially habitable planet yet found

A team of planet-hunting astronomers, utilising the HIRES spectrometer on the W.M. Keck Observatory’s Keck I Telescope, has announced the discovery of an Earth-sized planet orbiting a nearby red dwarf star.

The new planet, known as Gliese 581g, is at a distance that places it squarely in the middle of the star’s “habitable zone” where liquid water could exist on the planet’s surface.

If confirmed, this would be the most Earth-like exoplanet and the first bona fide potentially habitable one yet discovered.

To astronomers, a “potentially habitable” planet is one that could sustain life—even the simplest of life—and not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

The discovery by the team, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington DC, is based on 11 years of observations made at the Keck Observatory atop Mauna Kea mountain on the Big Island of Hawaii.

“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.”

Diagram showing a star's habitable zone

Earth is in our Solar System's habitable or "Goldilocks" zone (blue band) where the temperature is neither too hot nor too cold for liquid water to exist on a rocky planet's surface. For hotter (whiter) or cooler (red) stars (shown at left), the zone is at a different distance.

“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” added Paul Butler of the Carnegie Institution.

“Our ability to find potentially habitable worlds is now limited only by our telescope time.”

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team’s new findings are reported in a paper to be published in the Astrophysical Journal.

Planet of perpetual night and day

The astronomers have deduced that the planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness.

One effect of this is to stabilise the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light (known as the “terminator”), with surface temperatures decreasing toward the dark side and increasing toward the light side.

“Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said.

The researchers estimate that the average surface temperature of the planet is between -31 to -12 degrees Celsius. Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.

Artist's impression of planets orbiting Gliese 581

It is now thought there are six planets circling the star Gliese 581, making it the most Solar System-like place discovered so far in the cosmos.

If Gliese 581g has a rocky composition similar to the Earth’s, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright on the planet, Vogt said.

In fact, the scientists have reported the discovery of not one but two new planets circling Gliese 581. This brings to six the number of known planets around this star, the most yet discovered in a planetary system other than our own solar system.

Like our Solar System, the planets of Gliese 581 have nearly circular orbits. Gliese 581g has a mass 3 to 4 times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and that it has enough gravity to hold on to an atmosphere, according to Vogt.

A difficult discovery

Although the planets themselves can’t be seen, the effect of their gravitational pull on their parent star can be measured. It shows up as a slight movement, or radial velocity change, in the star.

Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to distinguish the effects of the planets and determine their orbits and masses.

“It’s really hard to detect a planet like this,” Vogt said. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet.”

W.M. Keck Observatory

Domes of the twin giant telescopes of the W.M. Keck Observatory on Mauna Kea mountain on the Big Island of Hawaii.

To get that many radial velocity measurements (238 in total), Vogt’s team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).

In addition to the radial velocity observations, co-authors Gregory Henry and Michael Williamson of Tennessee State University made precise night-to-night brightness measurements of the star with one of Tennessee State University’s robotic telescopes.

“Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself,” Henry said.

How many habitable planets are out there?

Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

“If these are rare, we shouldn’t have found one so quickly and so nearby,” Vogt said.

“The number of systems with potentially habitable planets is probably on the order of ten or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”

Gliese 581, located 20 light years away from Earth, has a somewhat chequered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone…one on the hot side (planet c) and one on the cold side (planet d).

While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are sceptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.

“It’s the Goldilocks planet,” Vogt said. “That’s a well-worn analogy, but in this case it fits. We had planets on both sides of the habitable zone—one too hot and one too cold—and now we have one in the middle that’s just right.”

Adapted from information issued by W.M. Keck Observatory / ESO / L. Calçada / NASA / ESA / G. Bacon (STScI).

Get SpaceInfo.com.au daily updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates. Or follow us on Twitter, @spaceinfo_oz