Is Proxima Centauri b Basically Kevin Costner’s Waterworld?

Artist's depiction of an waterworld. New research suggest that Proxima b might be just such an environment. Credit: David A. Aguilar (CfA)

The discovery of an exoplanet candidate orbiting around nearby Proxima Centauri has certainly been exciting news. In addition to being the closest exoplanet to our Solar System yet discovered, all indications point to it being terrestrial and located within the stars’ circumstellar habitable zone. However, this announcement contained its share of bad news as well.

For one, the team behind the discovery indicated that given the nature of its orbit around Proxima Centauri, the planet likely in terms of how much water it actually had on its surface. But a recent research study by scientists from the University of Marseilles and the Carl Sagan Institute may contradict this assessment. According to their study, the exoplanet’s mass may consist of up to 50% water – making it an “ocean planet”.

According to the findings of the Pale Red Dot team, Proxima Centauri b orbits its star at an estimated distance of 7 million km (4.35 million mi) – only 5% of the Earth’s distance from the Sun. It also orbits Proxima Centauri with an orbital period of 11 days, and either has a synchronous rotation, or a 3:2 orbital resonance (i.e. three rotations for every two orbits).

Because of this, liquid water is likely to be confined to either the sun-facing side of the planet (in the case of a synchronous rotation), or in its tropical zone (in the case of a 3:2 resonance). In addition, the radiation Proxima b receives from its red dwarf star would be significantly higher than what we are used to here on Earth.

However, according to a study led by Bastien Brugger of the Astrophysics Laboratory at the University of Marseilles, Proxima b may be wetter than we previously thought. For the sake of their study, titled “Possible Internal Structures and Compositions of Proxima Centauri b” (which was accepted for publication in The Astrophysical Journal Letters), the research team used internal structure models to compute the radius and mass of Proxima b.

Their models were based on the assumptions that Proxima b is both a terrestrial planet (i.e. composed of rocky material and minerals) and did not have a massive atmosphere. Based on these assumptions, they concluded that Proxima b has a radius that is between 0.94 and 1.4 times that of Earth, and a mass that is roughly 1.1 to 1.46 times that of Earth.

This range in size and mass allows for some very different planetary compositions. At the lower end, being slightly smaller but a bit more massive than Earth, Proxima b would likely be a Mercury-like planet with a 65% core mass fraction – i.e. an oversized core, presumably composed of iron and nickel. However, at the higher end of the radii and mass estimates, Proxima b would likely be half water by mass.

In other words, Proxima b could be an “eyeball planet”, where the sun-facing side has a liquid ocean surface, while the dark side is covered in frozen ice. Recent studies have suggested that this may be the case with planet’s that orbit within the habitable zones of red dwarf stars, where tidal-locking ensures that only one side gets the heat necessary to maintain liquid water on the surface.

On the other hand, if it has an orbital resonance of 3:2, its likely to have a double-eyeball pattern – with liquid oceans in both the eastern and western hemispheres – while remaining frozen at the terminators and poles. Both of these scenarios create some very interesting possibilities as extra-terrestrial life goes. Perhaps Proxima b could give rise to a rich aquatic ecosystem (if it hasn’t already), with creatures adapted to both warm and frigid water conditions.

But of course, both of these scenarios are based on Proxima b having a radius and mass greater than that of Earth. If the lower estimates should be true, then Proxima b is likely to be a rocky, dense planet with warmer conditions and liquid water on the sun-facing side, or warm spots in the eastern and western hemisphere and frigid regions everywhere else.

Naturally, the team expressed that these are just estimates based on Solar System values. As they wrote in the paper:

“Although this range of radii still allows very different planet compositions, it helps characterizing many aspects of Proxima Centauri b, such as the formation conditions of the system or the current amount of water on the planet. This work can also help ruling out future measurements of the planet’s radius that would be physically incompatible with a solid planetary body.”

Ultimately, we’re still a long ways away from determining Proxima b’s exact size, composition, and surface features – to say nothing about whether or not it can actually support life. Nevertheless, research like this is beneficial in that it helps us to come up with constrains on what kind of planetary conditions could exist there.

And who knows? Someday, we may be able to send probes or crewed missions to the planet, and perhaps they will beam back images of sentient beings navigating vast oceans, looking for some fabled parcel of land they heard about? God I hope not! Once was more than enough!

Further Reading: arXiv

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Earth-Like Planet Around Proxima Centauri Discovered

Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada

The hunt for exoplanets has been heating up in recent years. Since it began its mission in 2009, over four thousand exoplanet candidates discovered by the Kepler mission, several hundred of which have been confirmed to be “Earth-like” (i.e. terrestrial). And of these, some 216 planets have been shown to be both terrestrial and located within their parent star’s habitable zone (aka. “Goldilocks zone”).

But in what may prove to be the most exciting find to date, the German weekly Der Spiegel announced recently that astronomers have discovered an Earth like planet orbiting Proxima Centauri, just 4.25 light-years away. Yes, in what is an apparent trifecta, this newly-discovered exoplanet is Earth-like, orbits within it’s sun’s habitable zone, and is within our reach. But is this too good to be true?

For over a century, astronomers have known about Proxima Centauri and believed that it is likely to be part of a trinary star system (along with Alpha Centauri A and B). Located just 0.237 ± 0.011 light years from the binary pair, this low-mass red dwarf star is also 0.12 light years (~7590 AUs) closer to Earth, making it the closest star system to our own.

In the past, the Kepler mission has revealed several Earth-like exoplanets that were deemed to be likely habitable. And recently, an international team of researchers narrowed the number of potentially-habitable exoplanets in the Kepler catalog down to the 20 that are most likely to support life. However, in just about all cases, these planets are hundreds (if not thousands) of light years away from Earth.

Knowing that there is a habitable planet that a mission from Earth could reach within our own lifetimes is nothing short of amazing! But of course, there is reason to be skeptical. Citing anomalous sources, the magazine stated:

“The still nameless planet is believed to be Earth-like and orbits at a distance to Proxima Centauri that could allow it to have liquid water on its surface — an important requirement for the emergence of life. Never before have scientists discovered a second Earth that is so close by.”

In addition, they claim that the discovery was made by the European Southern Observatory (ESO) using the La Silla Observatory‘s reflecting telescope. Coincidentally, it was this same observatory that announced the discovery of Alpha Centauri Bb back in 2012, which was also declared to be “the closest exoplanet to Earth”. Unfortunately, subsequent analysis cast doubt on its existence, claiming it was a spurious artifact of the data analysis.

However, according to Der Spiegel’s unnamed source – whom they claim was involved with the La Silla team that made the find – this latest discovery is the real deal, and was the result of intensive work. “Finding small celestial bodies is a lot of hard work,” the source was quoted as saying. “We were moving at the technically feasible limit of measurement.”

The article goes on to state that the European Southern Observatory (ESO) will be announcing the finding at the end of August. But according to numerous sources, in response to a request for comment by AFP, ESO spokesman Richard Hook refused to confirm or deny the discovery of an exoplanet around Proxima Centauri. “We are not making any comment,” he is reported as saying.

Naturally, there is the desire (especially amongst exoplanet enthusiasts) to intepret the ESO’s refusal to comment either way as a sort of tacit confirmation. But of course, that would be entirely premature. If the statements made by the unnamed source, and quoted by Der Speigel, are to be taken at face value, then confirmation (or denial) will be coming shortly. In the meantime, we’ll all just need to be patient.

Still, you have to admit, it’s an exciting prospect: an Earth-like planet that’s actually within reach! Should it prove to be true, I imagine Project Starshot will be reassessing its proposed mission to Alpha Centauri. Instead of sending their nanocraft to this system to have a look around, perhaps it should be going to Proxima Centauri to look specifically at this maybe-planet. Would certainly make for some interesting findings!

Further Reading: Der Speigel

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Focusing On ‘Second-Earth’ Candidates In The Kepler Catalog

Artist’s impression of how an infant earth might look. Credit: ESO.

The ongoing hunt for exoplanets has yielded some very interesting returns in recent years. All told, the Kepler mission has discovered more than 4000 candidates since it began its mission in March of 2009. Amidst the many “Super-Jupiters” and assorted gas giants (which account for the majority of Kepler’s discoveries) astronomers have been particularly interested in those exoplanets which resemble Earth.

And now, an international team of scientists has finished perusing the Kepler catalog in an effort to determine just how many of these planets are in fact “Earth-like”. Their study, titled “A Catalog of Kepler Habitable Zone Exoplanet Candidates” (which will be published soon in the Astrophysical Journal), explains how the team discovered 216 planets that are both terrestrial and located within their parent star’s “habitable zone” (HZ).

The international team was made up of researchers from NASA, San Francisco State University, Arizona State University, Caltech, University of Hawaii-Manoa, the University of Bordeaux, Cornell University and the Harvard-Smithsonian Center for Astrophysics. Having spent the past three years looking over the more than 4000 entries, they have determined that 20 of the candidates are most like Earth (i.e. likely habitable).

As Stephen Kane, an associate professor of physics and astronomy at San Fransisco University and lead author of the study, explained in a recent statement:

“This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars. That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable.”

In addition to isolating 216 terrestrial planets from the Kepler catalog, they also devised a system of four categories to determine which of these were most like Earth. These included “Recent Venus”, where conditions are like that of Venus (i.e. extremely hot); “Runaway Greenhouse”, where planets are undergoing serious heating; “Maximum Greenhouse”, where planets are within their star’s HZ; and “Recent Mars”, where conditions approximate those of Mars.

From this, they determined that of the Kepler candidates, 20 had radii less than twice that of Earth (i.e. on the smaller end of the Super-Earth category) and existed within their star’s HZ. In other words, of all the planets discovered in our local Universe, they were able to isolate those where liquid water can exist on the surface, and the gravity would likely be comparable to Earth’s and not crushing!

This is certainly exciting news, since one of the most important aspects of exoplanet hunting has been finding worlds that could support life. Naturally, it might sound a bit anthropocentric or naive to assume that planets which have similar conditions to our own would be the most likely places for it to emerge. But this is what is known as the “low-hanging fruit” approach, where scientists seek out conditions which they know can lead to life.

“There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them,” said Kane. “This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are planets like the Earth.”

Professor Kane is renowned for being one of the world’s leading “planet-hunters”. In addition to discovering several hundred exoplanets (using data obtained by the Kepler mission) he is also a contributor to two upcoming satellite missions – the NASA Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s Characterizing ExOPLanet Satellite (CHEOPS).

These next-generation exoplanet hunters will pick up where Kepler left off, and are likely to benefit greatly from this recent study.

Further Reading: arXiv

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Starshade Prepares To Image New Earths

Artist's concept of the prototype starshade, a giant structure designed to block the glare of stars so that future space telescopes can take pictures of planets. Credit: NASA/JPL

For countless generations, people have looked up at the stars and wondered if life exists somewhere out there, perhaps on planets much like ours. But it has only been in recent decades that we have been able to confirm the existence of extrasolar planets (aka. exoplanets) in other star systems. In fact, between 1988 and April 20th of 2016, astronomers have been able to account for the existence of 2108 planets in 1350 different star systems, including 511 multiple planetary systems.

Most of these discoveries have taken place within just the past three years, thanks to improvements in our detection methods, and the deployment of the Kepler space observatory in 2009. Looking ahead, astronomers hope to improve on these methods even further with the introduction of the Starshade, a giant space structure designed to block the glare of stars, thus making it easier to find planets – and perhaps another Earth!

While some planets have been observed directly with telescopes (a process known as “Direct Imaging”), the vast majority have been detected through indirect methods such as the Transit Method. This method attempts to spot planets as they cross in front of the parent star’s disk – during which time there will be a temporary drop in observed brightness – and can also reveal the radius of a planet and sometimes yield information on its atmosphere (with the help of spectrometers).

This method remains the most widely-used means of detection and is responsible for more exoplanet discoveries than all other methods combined. However, due to interference from other light sources, it also suffers from a substantial rate of false positives, and generally requires that part of the planet’s orbit intersect a line-of-sight between the host star and Earth.

To address this, NASA is developing some key technologies that will help block out light interference so that future astronomers can detect exoplanets more easily. For instruments here on Earth, they are developing chronographs, single instruments that fit inside telescopes to block light. But looking to space, NASA’s Jet Propulsion Laboratory is also developing the Starshade.

This concept calls for a giant, flower-shaped spacecraft that would be launched with one of NASA’s next-generation space telescopes. Once deployed, it would fly around in front of the telescope in order to obscure the light of distant stars. This way, the light being reflected off of planets in orbit around them will be detectable, thus making it that much easier to confirm the presence of exoplanets.

The project is led by Prof. Jeremy Kasdin of Princeton University, in conjunction with the JPL and support from Northrop Grumman (which leads the mission and system design for Starshade). As Kasdin explained to Universe Today via email:

“The starshade works just like your thumb when trying to block the Sun; it blocks the starlight from entering the telescope but allows light from he planet close by to pass unimpeded.  Since planets are so much dimmer than their host stars, this technology eliminates the problem of glare from the star swamping light from the planet.  And because the starlight never enters the telescope, any conventional telescope can be used; no special attention needs to be paid to stabilities and precision in the telescope.”

The shade, which is about the  size of a baseball diamond, would be deployed as part a single mission. As the video above shows, the large shade would be mounted at the end of a space telescope – in this case, NASA’s upcoming Wide Field Infrared Survey Telescope (WFIRST) – and then detaches and deploys to a distance of several thousands kilometers in front of it.

Such a large shade operating at such a long distance from of its paired telescope is essential when dealing with distant stars.”Because stars are so far away the angular distance between the planet and star is quite small,” said Kasdin, “requiring a very large starshade (20 to 50 meters in diameter) flying very far from the telescope (up to 50,000 km). Nevertheless, many astronomers believe this is the best technology to detect an Earthlike planet in the near future, a belief aided by the fact that few special requirements are placed on the telescope.”

Paired with other instruments, like spectrometers, devices like the Starshade will not only allow astronomers to be able to spot planets more easily, but also obtain information about their atmospheres. By studying their chemical compositions – i.e. looking for the presence of oxygen/nitrogen, water vapor, etc. – we would be able to tell with a fair degree of certainty whether or not life exists on them.

The Starshade technology is one of the top candidates for a flagship-level mission in the next decade and a top Astro2010 priority for technology development. In addition to working with WFIRST, it is possible it will be paired with missions like the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope.

“We are hoping that a starshade capable of Earth detection will be recommended to fly with the upcoming WFIRST mission,” Kasdin added, “allowing the first image of an Earth in the next decade.”


Further Reading: JPL News

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