This ESO (European Southern Observatory) report has had probably the most violated embargo of any ESO news. Great to see the full report finally made available to everyone:

Planet Found in Habitable Zone Around Nearest Star
Pale Red Dot campaign reveals Earth-mass world in orbit around Proxima Centauri

Astronomers using ESO telescopes and other facilities have found clear evidence of a planet orbiting the closest star to Earth, Proxima Centauri. The long-sought world, designated Proxima b, orbits its cool red parent star every 11 days and has a temperature suitable for liquid water to exist on its surface. This rocky world is a little more massive than the Earth and is the closest exoplanet to us — and it may also be the closest possible abode for life outside the Solar System. A paper describing this milestone finding will be published in the journal Nature on 25 August 2016.

Just over four light-years from the Solar System lies a red dwarf star that has been named Proxima Centauri as it is the closest star to Earth apart from the Sun. This cool star in the constellation of Centaurus is too faint to be seen with the unaided eye and lies near to the much brighter pair of stars known as Alpha Centauri AB.

During the first half of 2016 Proxima Centauri was regularly observed with the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile and simultaneously monitored by other telescopes around the world [1]. This was the Pale Red Dot campaign, in which a team of astronomers led by Guillem Anglada-Escudé, from Queen Mary University of London, was looking for the tiny back and forth wobble of the star that would be caused by the gravitational pull of a possible orbiting planet [2].

As this was a topic with very wide public interest, the progress of the campaign between mid-January and April 2016 was shared publicly as it happened on the Pale Red Dot website and via social media. The reports were accompanied by numerous outreach articles written by specialists around the world.

Guillem Anglada-Escudé explains the background to this unique search:

“The first hints of a possible planet were spotted back in 2013, but the detection was not convincing. Since then we have worked hard to get further observations off the ground with help from ESO and others. The recent Pale Red Dot campaign has been about two years in the planning.”

The Pale Red Dot data, when combined with earlier observations made at ESO observatories and elsewhere, revealed the clear signal of a truly exciting result. At times Proxima Centauri is approaching Earth at about 5 kilometres per hour — normal human walking pace — and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri — only 5% of the Earth-Sun distance [3].

Guillem Anglada-Escudé comments on the excitement of the last few months:

“I kept checking the consistency of the signal every single day during the 60 nights of the Pale Red Dot campaign. The first 10 were promising, the first 20 were consistent with expectations, and at 30 days the result was pretty much definitive, so we started drafting the paper!”

Red dwarfs like Proxima Centauri are active stars and can vary in ways that would mimic the presence of a planet. To exclude this possibility the team also monitored the changing brightness of the star very carefully during the campaign using the ASH2 telescope at the San Pedro de Atacama Celestial Explorations Observatory in Chile and the Las Cumbres Observatory telescope network. Radial velocity data taken when the star was flaring were excluded from the final analysis.

Although Proxima b orbits much closer to its star than Mercury does to the Sun in the Solar System, the star itself is far fainter than the Sun. As a result Proxima b lies well within the habitable zone around the star and has an estimated surface temperature that would allow the presence of liquid water. Despite the temperate orbit of Proxima b, the conditions on the surface may be strongly affected by the ultraviolet and X-ray flares from the star — far more intense than the Earth experiences from the Sun [4].

Two separate papers discuss the habitability of Proxima b and its climate. They find that the existence of liquid water on the planet today cannot be ruled out and, in such case, it may be present over the surface of the planet only in the sunniest regions, either in an area in the hemisphere of the planet facing the star (synchronous rotation) or in a tropical belt (3:2 resonance rotation). Proxima b’s rotation, the strong radiation from its star and the formation history of the planet makes its climate quite different from that of the Earth, and it is unlikely that Proxima b has seasons.

This video takes the viewer from Earth to the closest star, Proxima Centauri. Here we can see the planet Proxima b, which orbits its red dwarf star every 11.2 days. This planet orbits within the habitable zone, shown in green, which means that liquid water could exist on its surface. Credit: PHL @ UPR Arecibo, ESO. Music by Lyford Rome

This discovery will be the beginning of extensive further observations, both with current instruments [5] and with the next generation of giant telescopes such as the European Extremely Large Telescope (E-ELT). Proxima b will be a prime target for the hunt for evidence of life elsewhere in the Universe. Indeed, the Alpha Centauri system is also the target of humankind’s first attempt to travel to another star system, the StarShot project.

A numerical simulation of possible surface temperatures on Proxima b performed with the Laboratoire de Météorologie Dynamique’s Planetary Global Climate Model. Here it is hypothesised that the planet possesses an Earth-like atmosphere and that it is covered by an ocean (the dashed line is the frontier between the liquid and icy oceanic surface). Two models exist for the planet’s rotation. Here the planet is in synchronous rotation (like the Moon around the Earth), and is seen as a distant observer would do during one full orbit. Another model is that it is trapped in a so-called 3:2 resonance (a natural frequency for the orbit).

Two additional papers about Proxima b’s possibility for habitability are described at proximacentauri.info. Credit: M. Turbet/I. Ribas/ESO

Guillem Anglada-Escudé concludes:

“Many exoplanets have been found and many more will be found, but searching for the closest potential Earth-analogue and succeeding has been the experience of a lifetime for all of us. Many people’s stories and efforts have converged on this discovery. The result is also a tribute to all of them. The search for life on Proxima b comes next…”

Notes

[1] Besides data from the recent Pale Red Dot campaign, the paper incorporates contributions from scientists who have been observing Proxima Centauri for many years. These include members of the original UVES/ESO M-dwarf programme (Martin Kürster and Michael Endl), and exoplanet search pioneers such as R. Paul Butler. Public observations from the HARPS/Geneva team obtained over many years were also included.

[2] The name Pale Red Dot reflects Carl Sagan’s famous reference to the Earth as a pale blue dot. As Proxima Centauri is a red dwarf star it will bathe its orbiting planet in a pale red glow.

[3] The detection reported today has been technically possible for the last 10 years. In fact, signals with smaller amplitudes have been detected previously. However, stars are not smooth balls of gas and Proxima Centauri is an active star. The robust detection of Proxima b has only been possible after reaching a detailed understanding of how the star changes on timescales from minutes to a decade, and monitoring its brightness with photometric telescopes.

[4] The actual suitability of this kind of planet to support water and Earth-like life is a matter of intense but mostly theoretical debate. Major concerns that count against the presence of life are related to the closeness of the star. For example gravitational forces probably lock the same side of the planet in perpetual daylight, while the other side is in perpetual night. The planet’s atmosphere might also slowly be evaporating or have more complex chemistry than Earth’s due to stronger ultraviolet and X-ray radiation, especially during the first billion years of the star’s life. However, none of the arguments has been proven conclusively and they are unlikely to be settled without direct observational evidence and characterisation of the planet’s atmosphere. Similar factors apply to the planets recently found around TRAPPIST-1.

[5] Some methods to study a planet’s atmosphere depend on it passing in front of its star and the starlight passing through the atmosphere on its way to Earth. Currently there is no evidence that Proxima b transits across the disc of its parent star, and the chances of this happening seem small, but further observations to check this possibility are in progress.

A star displaying peculiar light patterns was first noticed in Kepler space telescope data by a group of citizen scientists working in the Planet Hunters program. Their work convinced astronomer Tabetha Boyajian to investigate star KIC 8462852, which is also known as Tabby’s Star or the WTF star (‘Where’s the Flux?’), in more detail. The investigation by her group found the star to be quite an oddity. Boyajian is interviewed about it in this article: How Astronomers Plan to Solve the Mystery of the “Alien Megastructure Star” – Out There/Discover Magazine

Tabby’s Star is so unusual that a few scientists, including Boyajian’s colleague Jason Wright, raised the possibility that its flickering is not natural but is due to the presence of an enormous artificial construct. That speculation quickly lent KIC 8462852 another nickname, the “alien megastructure star,” and prompted a flood of breathless news stories; it even got a shout out on Saturday Night Live. Boyajian’s subsequent TED lecture drew even more attention to her star.

To obtain telescope time to study the star continuously for at least a year, a successful Kickstarter campaign raised $107,421: The most mysterious star in the Galaxy by Tabetha Boyajian — Kickstarter.

From the interview:

The plan is to observe the star through a full calendar year at the [private] Las Cumbres Observatory Global Telescope Network (LCOGT). We  have the funds to cover that, and a little bit more. We’re observing now, running off time LCOGT has gifted us, 200 hours there. At the end of the summer, when the Kickstarter funds get transferred, we’ll be able to set up the process through August and probably through December of 2017.

We want to see the star’s brightness dip again—it’s as simple as that. When it dips, how long the dips are, if there are many dips, all of the stuff relevant to any theory that’s on the table. Also, we’ll be able to get more detailed observations of whatever stuff is passing in front of the star, because we have a system to notify us when it’s not at its normal brightness. LCOGT is set up so we can get a spectrum as soon as that trigger happens, and also more intense observations.

These two videos from the Kickstarter campaign describe the star and their research plan:

 

Below is a video of Sarah Ballard of MIT giving a recent SETI Institute talk titled, Exoplanets: Under a Microscope, and Through a Wide-field Lens –

The Solar System furnishes the most familiar planetary architecture: many planets, orbiting nearly coplanar to one another. We can examine the composition and atmospheres of the Solar System planets in detail, even occasionally in situ. Studies of planets orbiting other stars (exoplanets), in contrast, only begin to approach the precision of humanity’s knowledge of Earth five hundred years ago. I will describe a two-pronged approach to the study of exoplanets.

One approach involves time-intensive investigations of individual planets to eke out bulk density or single molecules in the planetary atmosphere.

Another involves studies of the ensemble properties of planetary systems, and addresses the question of a “typical” planetary system in the Milky Way. In an era with thousands of exoplanet discoveries in hand and thousands more to follow in short order, a judicious combination of these approaches is emerging.

I’ll showcase some of my own detailed findings of other worlds (placing Earth in context), in addition to wider-field studies of typical planet occurrence and formation.

I’ll close with an opportunity, using an existing data set, to make inroads into the singular question driving much of exoplanetary science: the detectability of signatures of life.

The Kepler space telescope continues to find more planets around other stars:

NASA’s Kepler Confirms 100+ Exoplanets During Its K2 Mission

An international team of astronomers has discovered and confirmed a treasure trove of new worlds using NASA’s Kepler spacecraft on its K2 mission. Among the findings tallying 197 initial planet candidates, scientists have confirmed 104 planets outside our solar system. Among the confirmed is a planetary system comprising four promising planets that could be rocky.

The planets, all between 20 and 50 percent larger than Earth by diameter, are orbiting the M dwarf star K2-72, found 181 light years away in the direction of the Aquarius constellation. The host star is less than half the size of the sun and less bright. The planets’ orbital periods range from five and a half to 24 days, and two of them may experience irradiation levels from their star comparable to those on Earth. Despite their tight orbits — closer than Mercury’s orbit around the sun — the possibility that life could arise on a planet around such a star cannot be ruled out, according to lead author Crossfield, a Sagan Fellow at the University of Arizona’s Lunar and Planetary Laboratory.

The researchers achieved this extraordinary “roundup” of exoplanets by combining data with follow-up observations by earth-based telescopes including the North Gemini telescope and the W. M. Keck Observatory in Hawaii, the Automated Planet Finder of the University of California Observatories, and the Large Binocular Telescope operated by the University of Arizona. The discoveries are published online in the Astrophysical Journal Supplement Series.

Both Kepler and its K2 mission discover new planets by measuring the subtle dip in a star’s brightness caused by a planet passing in front of its star.  In its initial mission, Kepler surveyed just one patch of sky in the northern hemisphere, determining the frequency of planets whose size and temperature might be similar to Earth orbiting stars similar to our sun. In the spacecraft’s extended mission in 2013, it lost its ability to precisely stare at its original target area, but a brilliant fix created a second life for the telescope that is proving scientifically fruitful.

After the fix, Kepler started its K2 mission, which has provided an ecliptic field of view with greater opportunities for Earth-based observatories in both the northern and southern hemispheres. Additionally, the K2 mission is entirely community-driven with all targets proposed by the scientific community.

Because it covers more of the sky, the K2 mission is capable of observing a larger fraction of cooler, smaller, red-dwarf type stars, and because such stars are much more common in the Milky Way than sun-like stars, nearby stars will predominantly be red dwarfs.

“An analogy would be to say that Kepler performed a demographic study, while the K2 mission focuses on the bright and nearby stars with different types of planets,” said Ian Crossfield. “The K2 mission allows us to increase the number of small, red stars by a factor of 20, significantly increasing the number of astronomical ‘movie stars’ that make the best systems for further study.”

To validate candidate planets identified by K2, the researchers obtained high-resolution images of the planet-hosting stars as well as high-resolution optical spectroscopy. By dispersing the starlight as through a prism, the spectrographs allowed the researchers to infer the physical properties of a star — such as mass, radius and temperature — from which the properties of any planets orbiting it can be inferred.

These observations represent a natural stepping stone from the K2 mission to NASA’s other upcoming exoplanet missions such as the Transiting Exoplanet Survey Satellite and James Webb Space Telescope.

“This bountiful list of validated exoplanets from the K2 mission highlights the fact that the targeted examination of bright stars and nearby stars along the ecliptic is providing many interesting new planets,” said Steve Howell, project scientist for the K2 mission at NASA’s Ames Research Center in Moffett Field, California. “These targets allow the astronomical community ease of follow-up and characterization, providing a few gems for first study by the James Webb Space Telescope, which could perhaps tell us about the planets’ atmospheres.”

This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute.

NASA Ames manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA’s Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder.

For more information on the Kepler and the K2 mission, visit: www.nasa.gov/kepler

For more information about exoplanets, visit: exoplanets.nasa.gov/