# A terrestrial, temperate planet around Proxima Centauri

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Published today in Nature:
At a distance of 1.295 parsecs1, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun’s closest stellar neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun2 and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days (ref. 3) and its quiescent activity levels and X-ray luminosity4 are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface5.
Anglada-Escudé et al. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature: 536: 437. doi:10.1038/nature19106

Popular press summary: http://www.nature.com/news/earth-si...rby-star-is-astronomy-dream-come-true-1.20445

Proxima Centauri, the star closest to the Sun, has an Earth-sized planet orbiting it at the right distance for liquid water to exist. The discovery, reported today in Nature1, fulfils a longstanding dream of science-fiction writers — a potentially habitable world that is close enough for humans to send their first interstellar spacecraft.

“The search for life starts now,” says Guillem Anglada-Escudé, an astronomer at Queen Mary University of London and leader of the team that made the discovery.

I like Serena, Monsterboy, Orodruin and 4 others

Borg
Gold Member
The main problem is the 11 day orbit which means that it's probably tidally locked and the tendancy of red dwarfs to flare which would mean lots of radiation. Probably not a nice place to visit.

Cozma Alex, 1oldman2 and Ygggdrasil
The main problem is the 11 day orbit which means that it's probably tidally locked and the tendancy of red dwarfs to flare which would mean lots of radiation. Probably not a nice place to visit.
For us, yes, but I don't think this damages the prospect of life-- a quality of all living species is the ability to adapt.

1oldman2
Gold Member
According to the paper, x-ray fluxes on Proxima b are 400 times as high as they are on Earth. However, organisms have been found near Chernobyl that thrive in environments with radiation levels 500 times higher than normal (https://en.wikipedia.org/wiki/Radiotrophic_fungus), so radiation levels on Proxima b would not necessarily preclude the possibility of life.

Steelwolf, stefan r, Monsterboy and 1 other person
For us, yes, but I don't think this damages the prospect of life-- a quality of all living species is the ability to adapt.
However, being tidally locked means that only a small part of the planet would have conditions in which life might conceivably get started.
About 80% of the surface it would be either permanently arid desert on the sun side or permanent ice desert on the far side.

Heat distribution to the dark side is possible if the planet still has an atmosphere capable of resisting the flares.

Protective magnetic field is also a possibility.

Related question-- will we have to wait another 8.4 years to find out if there is life, and to find out the answers to the other questions we have, such as if it has an atmosphere?

Related question-- will we have to wait another 8.4 years to find out if there is life, and to find out the answers to the other questions we have, such as if it has an atmosphere?
That would only be applicable of we intended to send a probe there, (and at lightspeed).
It's possible that we could get signatures of molecules and other phenomena indicating life just from next generation telescopes and other technologies.
In astromomical terms it's so close it like being just outside the front door of our house.

Steelwolf and 1oldman2
That would only be applicable of we intended to send a probe there, (and at lightspeed).
Or sent some form of light that would give us a better look, like a radar.

I guess in principle something similar to radar could be used to do terrain mapping, and from this we might be able to draw a few indirect conclusions regarding life.
I don't think there is any such technology with that sort of resolution anywhere near available in the near future.
If it was available though, then yes we wouldn't get any return signal for over eight years.

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No space imaging technology is based on the premise of sending a signal to a target and analyze it's bounce back. It's unnecessary and not really practical for objects that are light-years away.

We can simply analyze the light received from an object. The problem is that imaging exoplanets is very tricky given their dim light compared to the host stars. There are techniques to derive chemical signatures from the light radiated (or reflected) by an atmosphere. If Proxima b has an atmosphere we might be able to detect it through infrared spectroscopy if it transits the red dwarf. With UV or X-ray imaging, we may be able to determine whether there's a magnetic field or not (by tracking potential deflections of the red dwarf's 'solar storms', I think).

Direct imaging is a pain with exoplanets, especially in visible light. James Web will scan the infrared spectrum within two years, but with an angular resolution practically identical to the Hubble.

EDIT: We'd probably have to send probes to find evidence of life. Project Starshot kind of thing.

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Monsterboy, nikkkom, Cozma Alex and 2 others
gmax137
Just curious, this planet is called "proxima b." Is there a "proxima a"? Is that another planet? Or is "proxima a" the star itself?

Just curious, this planet is called "proxima b." Is there a "proxima a"? Is that another planet? Or is "proxima a" the star itself?
It's the star itself.

gmax137
EDIT: We'd probably have to send probes to find evidence of life. Project Starshot kind of thing.

We've identified water, methane, oxygen and other molecules in atmospheres of hot Jupiters, but not (as far as I know) for smaller planets. It is determined from the starlight passing through the planetary atmospheres during transits.

I've heard the argument that life is the only mechanism that will produce a significant component in a planetary atmosphere, so if we find someplace that is more than about 15% oxygen, it is a strong indicator of life.

Finding free Oxygen would certainly be a primary indicator of life, but life can also exist in a low oxygen atmosphere.
It's thought that the earliest life on Earth existed in a low oxygen environment, then later some simple lifeform hit upon photosynthesis.
(and that resulting free O2 inserted to the atmosphere was actually poison as far as a lot of the other early microbes were concerned - apparently most of them went extinct within a geologically fairly short time )
https://en.wikipedia.org/wiki/Great_Oxygenation_Event

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Irfan Nafi
I've found: X-Ray attenuation & absorption calculator, something that uses NIST: X-Ray Mass Attenuation Coefficients. In particular, I've found NIST: X-Ray Mass Attenuation Coefficients - Air, Dry. That attenuation coefficient is given as cm^2/g, and one multiplies it by the material's column density in g/cm^2 to give the optical depth. Our atmosphere has a column density of about 1034 g/cm^2.

The smallest mass attenuation coefficient in that table is 0.018 cm^2/g for 100 MeV, giving an optical depth of 18. At 1 MeV, it is about 0.1 cm^2/g, giving an optical depth of 100, and for lower energies, it is even higher.

For N2, O2, and CO2, it is not much different.

So Proxima b's atmosphere will easily absorb Proxima's X-rays unless it is as thin as Mars's atmosphere or thinner.

How old is Proxima Centauri?

Wikipedia has a good article on Proxima Centauri. Almost anything you'd like to know about it is there.

James Web will scan the infrared spectrum within two years

This is wrong, by the way. Launch is scheduled for October 2018, so more like three years.

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rootone, Garth and 1oldman2
I'd heard the argument from a geochemist a few years back that once a planet is tidally locked, the dark side cools so much that the atmosphere will solidify and be lost over a shorter time span that it would likely take like to start.

Just looking it up know I find some reports of modelling indicate this will not happen in all cases, and others show that even water might not entirely freeze. This search also led me to a paper that concludes the presence of an atmosphere can prevent tidal locking, although the reasoning for that is not entirely clear to me...I think I need to review the specifics of becoming locked in the first place.

Garth
Gold Member
I'd heard the argument from a geochemist a few years back that once a planet is tidally locked, the dark side cools so much that the atmosphere will solidify and be lost over a shorter time span that it would likely take like to start.

Just looking it up know I find some reports of modelling indicate this will not happen in all cases, and others show that even water might not entirely freeze.
This eprint today suggests even a thin atmosphere is sufficient to prevent this happening to Proxima b: Tutorial models of the climate and habitability of Proxima Centauri b: a thin atmosphere is sufficient to distribute heat given low stellar flux.
A theoretical framework of synchronously rotating planets, in which the risk of a runaway greenhouse on the sunlight side and atmospheric collapse on the reverse side are mutually ameliorated via heat transport is discussed. This is developed via simple (tutorial) models of the climate. These show that lower incident stellar ux means that less heat transport, so less atmospheric mass, is required. The incident stellar ux at Proxima Centauri b is indeed low, which may help enhance habitability if it has suffered some atmospheric loss or began with a low volatile inventory.

Garth

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Garth
Gold Member
More on the James Webb telescope and observing the planet: Prospects for Characterizing the Atmosphere of Proxima Centauri b.

The newly detected Earth-mass planet in the habitable zone of Proxima Centauri could potentially host life - if it has an atmosphere that supports surface liquid water. We show that thermal phase curve observations with the James Webb Space Telescope (JWST) from 5-12 microns can be used to test the existence of such an atmosphere. We predict the thermal variation for a bare rock versus a planet with 35% heat redistribution to the nightside and show that a JWST phase curve measurement can distinguish between these cases at 5 $\sigma$ confidence. We also consider the case of an Earth-like atmosphere, and find that the ozone 9.8 micron band could be detected with longer integration times (a few months). We conclude that JWST observations have the potential to put the first constraints on the possibility of life around the nearest star to the Solar System.

Garth

I wanted to know why this planet had not been found before. Kepler and other telescopes have found planets on stars much further away, and giving Proxima's status as the nearest star, I imagine it was the first one they pointed the telescopes towards. What stopped this planet from having been found on previous passes?

Most of planets discovered by Kepler and other telescopes in the earliest stages were very big and obvious, Jupiter sized and bigger.
The smaller planets in most cases started to reveal themselves only after several times of reprocessing the data.
Quite a lot of the early results turned out to be false positives when rechecked
The teams working on this would not want to make a press release until they had a high level of confidence.

Remember this is not normal optical telescope photography, exoplanets are detected by statistically analysing variations in the light received from the parent star.

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