Earth-Like Planet Around Proxima Centauri Discovered

The hunt for exoplanets has been heating up in recent years. Since it began its mission in 2009, over four thousand exoplanet candidates have been 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 its 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 cautiously optimistic. Citing anonymous 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.

Artist's impression of the Earth-like exoplanet discovered orbiting Alpha Centauri B iby the European Southern Observatory on October 17, 2012. Credit: ESO
Artist’s impression of the Earth-like exoplanet discovered orbiting Alpha Centauri B by the European Southern Observatory on October 17, 2012. Credit: ESO

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.

What’s more, the folks at Project Starshot are certainly excited by the news. As part of Breakthrough Initiatives – a program founded by Russian billionaire Yuri Milner to search for intelligent life (with backing from Stephen Hawking and Mark Zuckerberg) – Starshot intends to send a laser-sail driven-nanocraft to Alpha Centauri in the coming years.

This craft, they claim, will be able to reach speeds of up to 20% the speed of light. At this speed, it will able to traverse the 4.37 light years that lie between Earth and Alpha Centauri in just 20 years. But with the possible discovery of an Earth-like planet orbiting Proxima Centauri, which lies even closer, they may want to rethink that objective.

Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity's first interstellar voyage. Credit: breakthroughinitiatives.org
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org

As Professor Phillip Lubin – a professor at the University of California, Santa Barbara, the brains behind Project Starshot, and a key advisor to NASA’s DEEP-IN program – told Universe Today via email:

“The discovery of possible planet around Proxima Centauri is very exciting. It makes the case of visiting nearby stellar systems even more compelling, though we know there are many exoplanets around other nearby stars and it is very likely that the Alpha Centauri system will also have planets.”

Naturally, there is the desire (especially amongst exoplanet enthusiasts) to interpret the ESO’s refusal to comment either way as a sort of tacit confirmation. And knowing that industry professionals are excited it about it does lend an air of legitimacy. But of course, assuming anything at this point would be 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! And with a mission that could make it there within our own lifetimes. This is the stuff good science fiction is made of, you know.

Further Reading: Der Speigel

HARPS Tunes In On Habitable Planet

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Using the High Accuracy Radial velocity Planet Searcher (HARPS), a team of scientists at University of Geneva, Switzerland, led by the Swiss astronomer Stephane Udry made a sound discovery… an Earth-like planet orbiting star HD 85512. Located about 36 light years away in the constellation of Vela, this extrasolar planet is one of the smallest to be documented in the “habitable zone” and could very well be a potential home to living organisms.

Circling its parent star every 54 days at about the quarter of the distance which Earth orbits the Sun, the newly discovered planet shows every sign of a temperate climate and a possibility of water. However, the rocky little world would need to exhibit some very cloudy skies to make the grade.

“We model rocky planets with H2O/CO2/N2 atmospheres, representative of geological active planets like Earth, to calculate the maximum Bond albedo as a function of irradiation and atmosphere composition and the edges of the HZ for HD 85512 b. These models represent rocky geological active planets and produce a dense CO2 atmosphere at the outer edge, an Earth-like atmosphere in the middle, and a dense H2O atmospheres at the inner edge of the HZ.” says the team. “The inner limit for the 50% cloud case corresponds to the “Venus water loss limit”, a limit that was empirically derived from Venus position in our Solar System (0.72 AU).”

But there’s always from one extreme to another when it comes to a planet being in just the right place. “The inner edge of the (Habitable zone) denotes the location where the entire water reservoir can be vaporized by runaway greenhouse conditions, followed by the photo-dissociation of water vapor and subsequent escape of free hydrogen into space. The outer boundary denotes the distance from the star where the maximum greenhouse effect fails to keep CO2 from condensing permanently, leading to runaway glaciation,” says the Kaltenegger/Udry/Pepe study.

While the whole scenario might not be exciting to some, the study is helping to lay a very solid foundation for evaluating current and future planet candidates for life supporting conditions. “A larger sample will improve our understanding of this field and promises to explore a very interesting parameter space that indicates the potential coexistence of extended H/He and H2O dominated atmospheres as well as rocky planet atmospheres in the same mass and temperature range.” says Kaltenegger. “HD 85512 b is, with Gl 581 d, the best candidate for exploring habitability to date, a planet on the edge of habitability.”

And one step closer to better understanding what’s out there…

For further reading: A Habitable Planet around HD 85512?.

Update on Gliese 581d’s Habitability

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When last we checked in on Gliese 581d, a team from the University of Paris had suggested that the popular exoplanet, Gliese 581d may be habitable. This super-Earth found itself just on the edge of the Goldilocks zone which could make liquid water present on the surface under the right atmospheric conditions. However, the team’s work was based on one dimensional simulations of a column of hypothetical atmospheres on the day side of the planet. To have a better understanding of what Gliese 581d might be like, a three dimensional simulation was in order. Fortunately, a new study from the same team has investigated the possibility with just such an investigation.

The new investigation was called for because Gliese 581d is suspected to be tidally locked, much like Mercury is in our own solar system. If so, this would create a permanent night side on the planet. On this side, the temperatures would be significantly lower and gasses such as CO2 and H2O may find themselves in a region where they could no longer remain gaseous, freezing into ice crystals on the surface. Since that surface would never see the light of day, they could not be heated and released back into the atmosphere, thereby depleting the planet of greenhouse gasses necessary to warm the planet, causing what astronomers call an “atmospheric collapse.”

To conduct their simulation the team assumed that the climate was dominated by the greenhouse effects of CO2 and H2O since this is true for all rocky planets with significant atmospheres in our solar system. As with their previous study, they performed several iterations, each with varying atmospheric pressures and compositions. For atmospheres less than 10 bars, the simulations suggested that the atmosphere would collapse, either on the dark side of the planet, or near the poles. Past this, the effects of greenhouse gasses prevented the freezing of the atmosphere and it became stable. Some ice formation still occurred in the stable models where some of the CO2 would freeze in the upper atmosphere, forming clouds in much the same way it does on Mars. However, this had a net warming effect of ~12°C.

In other simulations, the team added in oceans of liquid water which would help to moderate the climate. Another effect of this was that the vaporization of water from these oceans also produced warming as it can serve as a greenhouse gas, but the formation of clouds could decrease the global temperature since water clouds increase the albedo of the planet, especially in the red region of the spectra which is the most prevalent form of light from the parent star, a red dwarf. However, as with models without oceans, the tipping point for stable atmospheres tended to be around 10 bars of pressure. Under that, “cooling effects dominated and runaway glaciation occurred, followed by atmospheric collapse.” Above 20 bars, the additional trapping of heat from the water vapor significantly increased temperatures compared to an entirely rocky planet.

The conclusion is that Gliese 581d is potentially habitable. The potential for surface water exists for a “wide range of plausible cases”. Ultimately, they all depend on the precise thickness and composition of any atmosphere. Since the planet does not transit the star, spectral analysis through transmission of starlight through the atmosphere will not be possible. Yet the team suggests that, since the Gliese 581 system is relatively close to Earth (only 20 lightyears), it may be possible to observe the spectra directly in the infrared portion of the spectra using future generations of instruments. Should the observations match the synthetic spectra predicted for the various habitable planets, this would be taken as strong evidence for the habitability of the planet.

Habitable Planet

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The term “habitable planet” seems rather broad. Does it mean that it is habitable for humans? Is it merely capable of supporting some other form of life? Quite simply, planetary habitability refers to a planet’s ability to both develop and sustain life.

Unfortunately, scientists have had to base their calculations for a habitable planet on Earth’s characteristics and do some guesswork. Some of the factors that astronomers look at when evaluating a planet’s habitability are mass, surface characteristics, orbit, rotation, and geochemistry.

One of the most basic assumptions that astronomers make when searching for a habitable planet is that it has to be terrestrial. This means that the planet is composed mostly of rock and metal and has a solid surface. A gas giant on the other hand has no solid surface, which makes it an unlikely candidate for supporting life. Mass is also an important factor, because low mass planets have too little gravity to keep their atmosphere. They also do not have live volcanoes and other geologic activity, which helps temper the surface to support life, because they lose energy as a result of a small diameter. Planets with high orbital eccentricity – the irregularity of the orbit – have a greater fluctuation in surface temperatures because they are closer to the Sun at some points and much further away at other points in the orbit. In order to be habitable, a planet has to have a moderate rotation. If there is no axial tilt then there are no change of seasons, and if the axial tilt is too severe than the planet will have a difficult time achieving homeostasis – balance. Another assumption astronomers make when determining planetary habitability is that life on other planets will also be carbon-based. The four elements most important for life are oxygen, nitrogen, carbon, and hydrogen. With so many considerations, it is not surprising that scientists have a difficult time determining whether a planet can sustain life.

Astronomers are searching for habitable planets in other solar systems too. They have started by searching in the habitable zones of other solar systems. A habitable zone is the region in space with conditions most favorable for supporting life. Astronomers are unsure exactly what the extent of the habitable zone of our Solar System is. Earth is located in the center of it, but it may even extend as far as Mars, and it almost reaches Venus. The habitable zone and planetary habitability focus on carbon-based life, so they do not help predict other forms of life.

Universe Today has a number of articles you should take a look at including the habitable zone and number of habitable planets.

You should also check out habitable planets and habitable planets are common.

Astronomy Cast has an episode on the search for water on Mars, which tells why finding water is a clue to finding life.