Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way

28 March 2012

A new result from ESO’s HARPS planet finder shows that rocky planets not much bigger than Earth are very common in the habitable zones around faint red stars. The international team estimates that there are tens of billions of such planets in the Milky Way galaxy alone, and probably about one hundred 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 around red dwarf stars has just been announced by an international team using observations with the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile [1]. A recent announcement (eso1204), showing that planets are ubiquitous in our galaxy, used a different method that was not sensitive to the important class of exoplanets that lie in the habitable zones around red dwarfs.

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 [2]). 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 where liquid water can exist on the surface of the planet,” says Xavier Bonfils (IPAG, Observatoire des Sciences de l’Univers de Grenoble, France), the leader of the team. “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 Gliese 581 (eso0915) 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 around red dwarfs. They find that the frequency of occurrence of super-Earths [3] 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 around red dwarfs. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1000 times that of the Earth).

As there are many red dwarf stars close to the Sun the new estimate means that there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years [4].

“The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the 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 [5]. This is the second planet in this triple star system (see eso0939 for the first) and seems to be situated close to the centre of the habitable zone. 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. This 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. Some of these planets are expected to pass in front of their parent star as they orbit — this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life,” concludes Xavier Delfosse, another member of the team (eso1210).

Correction (added 30 March 2012):

Please note that the original version of this press release incorrectly implied that the microlensing method was not sensitive to all planets around red dwarfs. This has now been corrected to say that it is not sensitive to planets in the habitable zones around red dwarfs.

[1] HARPS measures the radial velocity of a star with extraordinary precision. A planet in orbit around a star causes the star to regularly move towards and away from a distant observer on Earth. Due to the Doppler effect, this radial velocity change induces a shift of the star’s spectrum towards longer wavelengths as it moves away (called a redshift) and a blueshift (towards shorter wavelengths) as it approaches. This tiny shift of the star’s spectrum can be measured with a high-precision spectrograph such as HARPS and used to infer the presence of a planet.

[2] These stars are called M dwarfs because they have the spectral class M. This is the coolest of the seven classes in the simplest scheme for classifying stars accordingly to decreasing temperature and the appearance of their spectra.

[3] Planets with a mass between one and ten times that of the Earth are called super-Earths. There are no such planets in our Solar System, but they appear to be very common around other stars. Discoveries of such planets in the habitable zones around their stars are very exciting because — if the planet were rocky and had water, like Earth — they could potentially be an abode of life.

[4] The astronomers used ten parsecs as their definition of “close”. This corresponds to about 32.6 light-years.

[5] The name means that the planet is the second discovered (c) orbiting the third component (C) of the triple star system called Gliese 667. The bright stellar companions Gliese 667 A and B would be prominent in the skies of Gliese 667 Cc. The discovery of Gliese 667 Cc was independently announced by Guillem Anglada-Escude and colleagues in February 2012, roughly two months after the electronic preprint of the Bonfils et al. paper went online. This confirmation of the planets Gliese 667 Cb and Cc by Anglada-Escude and collaborators was largely based on HARPS observations and data processing of the European team that were made publicly available through the ESO archive.

More information
This research was presented in a paper “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample”, by Bonfils et al. to appear in the journal Astronomy & Astrophysics.

The team is composed of X. Bonfils (UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, France [IPAG]; Geneva Observatory, Switzerland), X. Delfosse (IPAG), S. Udry (Geneva Observatory), T. Forveille (IPAG), M. Mayor (Geneva Observatory), C. Perrier (IPAG), F. Bouchy (Institut d’Astrophysique de Paris, CNRS, France; Observatoire de Haute-Provence, France), M. Gillon (Université de Liège, Belgium; Geneva Observatory), C. Lovis (Geneva Observatory), F. Pepe (Geneva Observatory), D. Queloz (Geneva Observatory), N. C. Santos (Centro de Astrofísica da Universidade do Porto, Portugal), D. Ségransan (Geneva Observatory), J.-L. Bertaux (Service d’Aéronomie du CNRS, Verrières-le-Buisson, France), and Vasco Neves (Centro de Astrofísica da Universidade do Porto, Portugal and UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, France [IPAG]).

ESO – eso1214 – Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way.

Jupiter’s melting heart sheds light on mysterious exoplanet

March 22, 2012 By Brian Jacobsmeyer

Credit: Forsetius via flickr

Scientists now have evidence that Jupiter’s core has been dissolving, and the implications stretch far outside of our solar system.

Jupiter might be having a change of heart. Literally.

New simulations suggest that Jupiter’s rocky core has been liquefying and mixing with the rest of the planet’s innards. With this new data, astronomers hope to better explain a recent puzzling discovery of a strange planet outside of our solar system.

“It’s a really important piece of the puzzle of trying to figure out what’s going on inside giant planets,” said Jonathan Fortney, a planetary scientist at the University of California Santa Cruz who was not affiliated with the research.

Conventional planetary formation theory has modeled Jupiter as a set of neat layers with a gassy outer envelope surrounding a rocky core consisting of heavier elements. But increasing evidence has indicated that the insides of gas giants like Jupiter are a messy mixture of elements without strictly defined borders.

This new research on a melting Jovian core bolsters a mixing model of gas giant planets and would provide another avenue for heavier elements to flow throughout the planet.

“People have been working on the assumption that these planets are layered because it’s easier to work on this assumption,” said Hugh Wilson, a planetary scientist at the University of California Berkeley and a coauthor of the new research appearing in Physical Review Letters.

Although scientists had previously toyed with the idea of melting cores in large planets, nobody sat down and did the necessary calculations, said Wilson.

Scientists have to rely on calculations of Jupiter’s core environment because the conditions there are far too extreme to recreate on Earth. Wilson and his UC-Berkeley colleague Burkhard Militzer used a computer program to simulate temperatures exceeding 7,000 degrees Celsius and pressures reaching 40 million times the air pressure found on Earth at sea level.

Those conditions are thought to be underestimates of the actual conditions inside Jupiter’s core.

Nonetheless, the authors found that magnesium oxide — an important compound likely found in Jupiter’s core — would liquefy and begin drifting into Jupiter’s fluid upper envelope under these relatively tame conditions.

Researchers believe that similarly-sized gas giant exoplanets — planets found outside of our solar system — probably have similar internal structures to Jupiter. Consequently, scientists were baffled earlier this year when they found a planet with approximately the same volume as Jupiter yet four to five times more mass.

Called CoRoT-20b, the new planet was announced in February, and its discoverers searched for a suitable explanation for its unusual density. Using conventional models, the astronomers calculated that the core would have to make up over half of the planet. For comparison, Jupiter’s core only represents about between 3-15 percent of the planet’s total mass.

With a core that large, CoRoT-20b presented a huge problem for traditional assumptions surrounding planet formation.

“It’s much easier to explain the composition of this planet under a model where you have a mixed interior,” said Wilson.

Even the team that discovered the planet noted that a mixing model could allow for a more palatable planet density. Wilson’s simulations not only add credence to the mixing model of giant planets but also suggest that this specific exoplanet’s core is probably melting just like Jupiter’s.

This melting may help explain why the exoplanet’s heavy elements are likely stirred up and distributed throughout its volume, said Wilson.

Santa Cruz’s Fortney agrees that most of the exoplanet’s heavy elements likely reside in the outer envelope. Nonetheless, he expects other factors played a larger role in how the planet’s interior became mixed: “It’s more of a planet formation issue.”

Several other events, such as two gas giants colliding together, might explain the ultra-high density of this new planet, Wilson admits. Certain processes may also limit the effectiveness of the melting and mixing process.

Liquefied parts of a gas giant’s core may have trouble reaching the outer envelope due to double diffusive convection — a process commonly found in Earth’s oceans. When salty water accumulates at the bottom of the ocean, its density keeps it from mixing thoroughly with the upper layers. In a similar fashion, the heavy elements in Jupiter’s core may have trouble gaining enough energy to move upward and outward.

Scientists don’t know how much this hindrance will affect potential mixing inside Jupiter, and many other questions remain to be answered about the melting process.

“The next question is, ‘How efficient is this process?'” said Fortney.

Researchers will have more tools to answer this question once NASA’s Juno probe reaches Jupiter in 2016. With the spacecraft’s instruments carefully analyzing Jupiter’s composition, Wilson believes that there will be signatures of mixing and core erosion.

Source: Inside Science News Service

Jupiter’s melting heart sheds light on mysterious exoplanet.