Proxima b, the exoplanet closest to the Solar System, may be covered by an ocean of liquid water and have a thin atmosphere, according to researchers at France's CNRS research institute.
The European Southern Observatory (ESO) launched its Pale Red Dot project to search for an exoplanet around the red dwarf star Proxima Centauri in January 2016. Discovery of Proxima Centauri b was announced on August 24, 2016. Astronomers have been observing Proxima b, essentially non-stop ever since then.
And now they have found some evidence of water and oceans on the exoplanet.
Exoplanet Promixa b
Proxima b is an exoplanet, 4.25 light-years from Earth and is located in the habitable zone of the small red dwarf star Proxima Centauri, one of three stars that make up the Alpha Centauri system.
Proxima b orbits its star at a distance of 7.4 million km, just one-tenth of the distance at which Mercury orbits the Sun. This means that like Mercury and other planets in very close orbits around their stars, Proxima b is likely tidally locked, with one side always facing the star and the other always facing away from it.
PARAMETER |
VALUE |
Mass |
~1.27 times that of Earth |
Radius |
0.94 to 1.4 times that of Earth |
Planetary Equilibrium Temperature
en.wikipedia.org/...
|
-39 oC |
Distance |
4.25 light years |
Orbital Period |
11.2 days |
Orbit radius |
7.4 million km
(Mercury is ~69 million km from the Sun)
|
Water on Promixa b
A group of researchers, led by scientists at Marseille Astrophysics Laboratory in France, calculated the size and surface properties for Proxima b. Their results indicate that in spite of being tidally locked, the planet could be completely covered in liquid water capable of hosting microbial life, much like the icy moons orbiting Jupiter and Saturn in our own solar system.
Two separate computer simulations carried out by the research team show Proxima b having a thin atmosphere along with temperatures that stay cool enough for liquid water to exist on the surface. Proxima b’s radius is estimated to range between 0.94 to 1.4 that of Earth.
An atmosphere can circulate warmth from the hot side of the planet to the cold side as well as prevent water from boiling away into space.
A radius 0.94 times that of Earth would result in the planet being very dense, with a metallic core comprising two-thirds of its mass, surrounded by a rocky mantle. Under those circumstances, any water on the surface would make up just 0.05% of the planet’s total mass, a situation similar to that of Earth, whose surface water comprises 0.02% of its mass.
In contrast, a radius 1.4 times that of Earth would result in Proxima b having a rocky core that comprises 50% of its mass surrounded by an ocean that makes up the other 50% . In this case, Proxima b would be covered by a single ocean 200 km deep.
Tidal Locking and Habitability
For planets such as Proxima b, that orbit close to their stars, tidal locking to the host star is likely, causing the planet to rotate around its axis once for every revolution around the star; as a result, one side of the planet would eternally face the star and another side would perpetually face away, creating extremes of temperature. For many years, it was believed that life on such planets would be limited to a ring-like region known as the terminator, where the star would always appear on the horizon.
Tidal forces created by the gravity of the star also cause a planet to flex and stretch when its orbit is an ellipse, rather than a circle. This continuous flexing produces heat within the planet’s core.
Tidal heating can play a major role in preventing or supporting life on planets — it can cook some planets, thaw others, and drive plate tectonics needed for life. uanews.arizona.edu/… and arxiv.org/...
Tidal heating may create habitable conditions on planets that otherwise are too small or too cold to support life. Tidal heating can enhance outgassing of volatiles that contribute or replenish a planet's atmosphere through volcanism. Tidal heating also can generate sub-surface liquid oceans on water-rich rocky planets that would otherwise be frozen, just as tidal heating is believed to warm a sub-surface liquid water ocean on Jupiter's moon Europa.
Also, tidal heating can drive plate tectonics, a mechanism that checks excessive carbon dioxide from accumulating in the atmosphere, which can produce the kind of run-away greenhouse atmosphere found on Venus.
Future Observations
The best way to learn more about an exoplanet is to watch it transit in front of its star, which can yield a wealth of knowledge about the planet’s size, atmosphere and even its surface conditions.
Before Proxima b was spotted, David Kipping at Columbia University in New York and his colleagues had spent over 40 days in 2014 and 2015 hunting for planets orbiting Proxima Centauri using the MOST Space Telescope. Once the discovery was announced, they narrowed their search, looking for signals that could match a planetary transit. Combined with newer observations, there’s no evidence that Proxima b passes between its star and Earth.
The powerful James Webb Space Telescope, set to launch in 2018, will be able to supply some details about Proxima b. Future telescopes — the European Extremely Large Telescope, the Giant Magellan Telescope, and the Thirty Meter Telescope — will also have the capability to better characterize Proxima b. A team of scientists think they can image Proxima b and probe the planet's atmosphere for signs of oxygen, water vapor and methane, combining data from the ESPRESSO and SPHERE instruments on ESO’s Very Large Telescope.
Proxima b Discovery
A seven-year campaign in the early 2000s, to look for planets in the so-called habitable zone around Proxima Centauri using the European Southern Observatory's (ESO) Very Large Telescope in Chile, came up empty.
Finding Proxima b proved to be difficult because it is a relatively small world that orbits a small star. Detection of such planets can be done by measuring the faint wobble of a star, which can be caused by orbiting planets.
A faint signal of a 11-day cycle was detected in 2013, but the data were not conclusive enough to say that Proxima Centauri had a planet.
The European Southern Observatory (ESO) launched its Pale Red Dot project in January 2016 to search for an exoplanet around Proxima Centauri. Discovery of Proxima b was announced on August 24, 2016.
The chart below shows the measured radial velocity of the Proxima Centauri star, which shows the wobble of the star around its center, which is caused by an orbiting planet or planets. Based on the measured wobble, the planet’s physical parameters were estimated.
Some new data hint at the presence of a second planet orbiting in the system with a period near 200 days, but its existence cannot be proven at this time.
Proxima Centauri
Proxima Centauri, the star around which Proxima b revolves, is a small, low-mass red dwarf star in the constellation of Centaurus. It was discovered in 1915 and is the nearest known star to the Sun.
Parameter |
Value |
Mass |
~12% of Sun |
Radius |
~14% of Sun |
Effective Temperature |
~3,042 K
Sun Photosphere (effective) temperature is 5,772 K
|
Luminosity |
0.17% of Sun |
Distance |
4.25 light years |
Age |
4.85 billion years (slightly older than our Sun) |
Proxima Centauri is part of the Alpha Centauri star system which contains two other stars — Alpha Centauri A and Alpha Centauri B.
At a distance to Alpha Centauri of just 0.21 ly, Proxima Centauri may be in orbit around Alpha Centauri, with an orbital period of the order of 500,000 years or more.
Proxima Centauri currently moves toward Earth at a rate of 22.4 km/s. Ηowever, after 26,700 years, when it will come as close as 3.11 light-years, it will begin to move farther away.
Six other single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point of origin, such as in a star cluster.
Computer models of the evolution of the Milky Way galaxy suggest Proxima Centauri has moved outward at least 10,000 light-years from where it formed, shown by the orange circle in the figure below. The Sun and Earth probably formed near where they orbit today (blue circle), which is where we find Proxima Centauri, too.
Proxima Centauri Star Spots
New research shows that Proxima Centauri is sun-like in one surprising way: it has a regular cycle of starspots. phys.org/...
Starspots (like sunspots) are dark blotches on a star's surface where the temperature is a little cooler than the surrounding area. They are driven by magnetic fields. A star is made of ionized gases called plasma. Magnetic fields can restrict the plasma's flow and create spots. Changes to a star's magnetic field can affect the number and distribution of starspots.
Our Sun experiences an 11-year activity cycle. At the solar minimum, the Sun is nearly spot-free. At solar maximum, typically more than 100 sunspots cover less than 1% of the Sun's surface on average.
The new study finds that Proxima Centauri undergoes a similar cycle lasting seven years from peak to peak. However, its cycle is much more dramatic. At least 20% of the star's surface is covered in spots at once. Also, some of those spots are much bigger relative to the star's size than the spots on our Sun.
Astronomers were surprised to detect a stellar activity cycle in Proxima Centauri because its interior is expected to be very different from the Sun's. The outer third of the Sun experiences a roiling motion called convection, similar to water boiling in a pot, while the Sun's interior remains relatively still and transfers heat through radiation. The difference in the speed of rotation between these two regions is believed to be responsible for generating the strong magnetic fields.
In contrast, the interior of a small red dwarf like Proxima Centauri should be convective all the way into the star's core. As a result, it shouldn't experience strong magnetic activity.
NASA researchers earlier studied other low-mass stars and found their X-ray emissions were similar to that of stars like the Sun, implying unexpectedly strong magnetic fields. If stars without such a boundary between the radiation and convection zones have relatively powerful magnetic fields, then this theory may need to be re-examined.
Implication of Star Spots on Proxima b
Theory suggests that flares or a stellar wind, both of which are driven by magnetic fields, could scour the planet and strip away any atmosphere. In that case, Proxima b might not be friendly to life on its surface. That does not preclude life in the depths of its oceans.
The Future of Promixa Centauri
Red dwarf stars are the smallest, coolest, and most common type of star — estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies. Red dwarfs, unlike our Sun, can last for trillions of years, because their nuclear reactions are far slower than those of larger stars, implying that life both would have longer to evolve and longer to survive.
Proxima Centauri is estimated to remain as a main sequence star for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming from red to blue. Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity and warming up any orbiting bodies for a period of several billion years. When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy.
By comparison, our Sun will exit the main sequence in approximately 5 billion years and start to turn into a red giant. As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth. en.wikipedia.org/...
So Near and Yet So Far
Using conventional rocket propulsion, the 4.25 light-year journey to Alpha Centauri would take tens of thousands of years. The Voyager 1 spacecraft, launched 39 years ago on Sep 5, 1977, is at a distance of 2 x1010 km from Earth, i.e, 0.0021 light years. At this rate, it will take another 78,367 years to get to Proxima Centauri.
How can we get there within human lifespan times?
Breakthrough Starshot is an ambitious project by Breakthrough Initiatives to develop a proof-of-concept fleet of tiny centimeter-scale light sail spacecraft, named StarChip, propelled by ground-based lasers, capable of making the journey to the Alpha Centauri star system, at speeds between 15% and 20% of the speed of light, taking between 20 and 30 years to get there, and about 4 years to notify Earth of a successful arrival.
The project was announced in April 2016 by physicist and venture capitalist Yuri Milner and cosmologist Stephen Hawking who is serving as board member of the initiatives. Other board members include Facebook CEO Mark Zuckerberg. The project has an initial funding of $100 million to start research. Milner places the final mission cost at $5–10 billion, and estimates the first craft could be launched around 2036.
Breakthrough Starshot Sail Design
One challenge for the Breakthrough Starshot project will be keeping the orientation of the sail just right in relation to the laser beam that propels it. Using on-board adjustment system that would constantly keep the sail facing the right way would add significant complexity and mass to the spacecraft.
Recently, researchers at Harvard, have suggested a spherical sail rather than the cone shape that others have previously suggested, helping to resolve orientation problems through a sphere’s symmetry. The beam would be multimodal, composed of the sum of multiple individual laser beams. The composite beam would be weakest at its center and strongest at its outer edges. That way, whenever the probe moved slightly off track, the increase in laser strength would stabilize its motion.
La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile, located 2400 m above sea-level, 150 km northeast of La Serena at the outskirts of the Chilean Atacama Desert, one of the driest and remotest areas of the world.
ESO operates three major optical and near infrared telescopes at the La Silla site - the New Technology Telescope (NTT), the 3.6-m ESO Telescope, and the 2.2-m Max-Planck-ESO Telescope.
The 3.6-m ESO Telescope hosts HARPS, the High Accuracy Radial velocity Planet Searcher, a spectograph, the world’s foremost exoplanet hunter.
Proxima b was discovered using the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile.
Here is an interesting set of time-lapse videos taken at each of ESO's three unique observatories in Chile —
- ALMA, the Atacama Large Millimeter/submillimeter Array — a huge new facility at 5000 metres above sea level on the Chajnantor Plateau
- La Silla, ESO’s first observatory — home to the ESO 3.6-metre telescope and the 3.58-metre New Technology Telescope and
- The Paranal Observatory in northern Chile, home to the Very Large Telescope array (VLT)
Is Proxima b habitable?
Here is how Prof. Rory Barnes, University of Washington, at palereddot.org/… answers the question -
The short answer is “It’s complicated.” Our observations are few, and what we do know allow for a dizzying array of possibilities. Did Proxima b move halfway across the galaxy? Did it endure a planetary-system-wide instability that launched its sibling planets into deep space and changed its orbit? How did it cope with the early high luminosity of its host star? What is it made of? Did it start out as a Neptune-like planet and then become Earth-like? Has it been relentlessly bombarded with flares and coronal mass ejections? Is it tidally heated into an Io-like (or worse) state?
Is the tidally heating mild enough to create a life-supporting heat source underneath its suspected oceans, especially on the dark side? How much water does it have? Has it retained an atmosphere? What is the strength of its magnetic field, which can deflect radiation from its star?
These questions are central to unlocking Proxima’s potential habitability and determining if our nearest galactic neighbor is an inhospitable wasteland, an inhabited planet or a future home for humanity.
References
- Possible Internal Structures and Compositions of Proxima Centauri b — arxiv.org/…
- ESA announcement of Proxima b discovery — www.eso.org/…
- Opportunities and Obstacles for Life on Proxima b — palereddot.org/...
- A terrestrial planet candidate in a temperate orbit around Proxima Centauri — www.eso.org/…
- MOST Observations of our Nearest Neighbor: Flares on Proxima Centauri — arxiv.org/…
- Proxima b info and bibliography — exoplanet.eu/...
- Breakthrough Starshot — breakthroughinitiatives.org
- Stability of a Light Sail Riding on a Laser Beam — arxiv.org/…
- The James Webb Space Telescope — www.dailykos.com/…
- FAST, the World's Largest Radio Telescope — www.dailykos.com/…
- Asteroids and Planetary Defense — www.dailykos.com/…
- Europa and water — www.dailykos.com/...