Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
The past year has been an exciting time for those engaged in the hunt for extra-solar planets and potentially habitable worlds. In August of 2016, researchers from the European Southern Observatory (ESO) confirmed the existence of the closest exoplanet to Earth (Proxima b) yet discovered. This was followed a few months later (February of 2017) with the announcement of a seven-planet system around TRAPPIST-1.
The discovery of these and other extra-solar planets (and their potential to host life) was an overarching theme at this year’s Breakthrough Discuss conference. Taking place between April 20th and 21st, the conference was hosted by Stanford University’s Department of Physics and sponsored by the Harvard-Smithsonian Center for Astrophysics and Breakthrough Initiatives.
Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. It's a very active flare star. Credits: NASA/JPL-Caltech
In what is surely the biggest news since the hunt for exoplanets began, NASA announced today the discovery of a system of seven exoplanets orbiting the nearby star of TRAPPIST-1. Discovered by a team of astronomers using data from the TRAPPIST telescope in Chile and the Spitzer Space Telescope, this find is especially exciting since all of these planets are believed to be Earth-sized and terrestrial (i.e. rocky).
But most exciting of all is the fact that three of these rocky exoplanets orbit within the star’s habitable zone (aka. “Goldilocks Zone”). This means, in effect, that these planets are capable of having liquid water on their surfaces and could therefore support life. As far as extra-solar planet discoveries go, this is without precedent, and the discovery heralds a new age in the search for life beyond our Solar System.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
Ever since the ESO announced the discovery of an extra-solar planet orbiting Proxima Centauri, scientists have been trying to determine what the conditions are like on this world. This has been especially important given the fact that while Proxima b orbits within the habitable zone of its sun, red dwarfs like Proxima Centauri are known to be somewhat inhospitable.
And while some research has cast doubt on the possibility that Proxima b could indeed support life, a new research study offers a more positive picture. The research comes from the Blue Marble Space Institute of Science (BMSIS) in Seattle, Washington, where astrobiologist Dimitra Atri has conducted simulations that show that Proxima b could indeed be habitable, assuming certain prerequisites were met.
Dr. Atri is a computational physicist whose work with the BMSIS includes the impacts of antiparticles and radiation on biological systems. For the sake of his study – “Modelling stellar proton event-induced particle radiation dose on close-in exoplanets“, which appeared recently in the Monthly Notices of the Royal Astronomical Society Letters – he conducted simulations to measure the impact stellar flares from its sun would have on Proxima b.
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
To put this perspective, it is important to note how the Kepler mission has found a plethora of planets orbiting red dwarf stars in recent years, many of which are believed to be “Earth-like” and close enough to their suns to have liquid water on their surfaces. However, red dwarfs have a number of issues that do not bode well for habitability, which include their variable nature and the fact they are cooler and fainter than other classes of stars.
This means that any planet close enough to orbit within a red dwarf’s habitable zone would be subject to powerful solar flares – aka. Stellar Proton Events (SPEs) – and would likely be tidally-locked with the star. In other words, only one side would be getting the light and heat necessary to support life, but it would be exposed to a lot of solar protons, which would interact with its atmosphere to create harmful radiation.
As such, the astronomical community is interested in what kinds of conditions are there for planets like Proxima b so they might know if life has (or had) a shot at evolving there. For the sake of his study, Dr. Atri conducted a series of probability (aka. Monte Carlo) simulations that took into account three factors – the type and size of stellar flares, various thicknesses of the planet’s atmosphere and the strength of its magnetic field.
As Dr. Atri explained to Universe Today via email, the results were encouraging – as far as the implications for extra-terrestrial life are concerned:
“I used Monte Carlo simulations to study the radiation dose on the surface of the planet for different types of atmospheres and magnetic field configurations. The results are optimistic. If the planet has both a good magnetic field and a sizable atmosphere, the effects of stellar flares are insignificant even if the star is in an active phase.”
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO
In other words, Atri found that the existence of a strong magnetic field, which would also ensure that the planet has a viable atmosphere, would lead to survivable conditions. While the planet would still experience a spike in radiation whenever a superflare took place, life could survive on a planet like Proxima b in the long run. On the other hand, a weak atmosphere or magnetic field would foretell doom.
“If the planet does not have a significant magnetic field, chances of having any atmosphere and moderate temperatures are negligible,” he said. “The planet would be bombarded with extinction level superflares. Although in case of Proxima b, the star is in a stable condition and does not have violent flaring activity any more – past activity in its history would make the planet a hostile place for a biosphere to originate/evolve.”
History is the key word here, since red dwarf stars like Proxima Centauri have incredible longevity (as noted, up to 10 trillion years). According to some research, this makes red dwarf stars good candidates for finding habitable exoplanets, since it takes billions of years for complex life to evolve. But in order for life to be able to achieve complexity, planets need to maintain their atmospheres over these long periods of time.
Naturally, Atri admits that his study cannot definitively answer whether our closest exoplanet-neighbor is habitable, and that the debate on this is likely to continue for some time. “It is premature to think that Proxima b is habitable or otherwise,” he says. “We need more data about its atmosphere and the strength of its magnetic field.”
An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System. Credit: NASA/ESA/STScl
In the future, missions like the James Webb Space Telescope should tell us more about this system, its planet, and the kinds of conditions that are prevalent there. By aiming its extremely precise suite of instruments at this neighboring star, it is sure to detect transits of the planet around this faint sun. One can only hope that it finds evidence of a dense atmosphere, which will hint at the presence of a magnetic field and life-supporting conditions.
Hope is another key word here. Not only would a habitable Proxima b be good news for those of us hoping to find life beyond Earth, it would also be good news as far as the existence of life throughout the Universe is concerned. Red dwarf stars make up 70% of the stars in spiral galaxies and more than 90% of all stars in elliptical galaxies. Knowing that even a fraction of these could support life greatly increases the odds of finding intelligence out there!
Artist's impression of an "eyeball" planet, a water world where the sun-facing side is able to maintain a liquid-water ocean. Credit and Copyright: eburacum45/ DeviantArt
Proxima b is the subject of a lot interest these days. And why not? As the closest extrasolar planet to our Solar System, it is the best shot we have at studying exoplanets up close in the near future. However, a recent study from the University of Marseilles indicated that, contrary to what many hoped, the planet may be a “water world” – i.e. a planet where up to half of its mass consists of water.
And now, researchers from the University of Bern have taken this analysis a step further. Based on their study, which has been accepted for publication in the journal Astronomy and Astrophysics (A&A), they have determined that the majority planets that form within the habitable zones of a red dwarf star may be water worlds. These findings could have drastic implications for the search for habitable exoplanets around red dwarf stars.
The research was conducted by Dr. Yann Alibert from the National Centers for Competence in Research (NCCR) PlanetS center and Prof. Willy Benz from the Center of Space and Habitability (CSH). Both of these institutions, which are located at the University of Bern, are dedicated to understanding planetary formation and evolution, as well as fostering a dialogue with the public about exoplanet research.
An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System. Credit: NASA/ESA/STScl
For the sake of their study, titled “Formation and Composition of Planets Around Very Low Mass Stars“, Alibert and Benz carried out the first computer simulation designed to examine the formation of planets around stars that are ten times less massive than our Sun. This involved creating a model that included hundreds of thousands of identical low-mass stars, which were then given orbiting protoplanetary disks of dust and gas.
They then simulated what would happen if planets began to form from the accretion of these disks. For each, they assumed the existence of ten “planetary embryos” (equal to the mass of the Moon) which would grow and migrate over time, giving rise to a system of planets.
Ultimately, what they found was that the planets orbiting within the habitable zone of their parent star would likely to be comparable in size to Earth – ranging from 0.5 to 1.5 times the radius of Earth, with 1 Earth radii being the average. As Dr. Yann Alibert explained to Universe Today via email:
“In the simulations we have considered here, it appears that the majority of the mass (more than 99%) is in the solids. [W]e therefore start with a protoplanetary disk that is made of solids and gas and 10 planetary embryos. The solids in the disk are planetesimals (similar to present day asteriods, around 1 km in size), that can be dry (if they are located in the hot regions of the protoplanetary disk) or wet (around 50% per mass of water ice, if they are in the cold regions of the disk). The planetary embryos are small bodies, whose mass is similar to the moon mass. We then compute how much of the disk solids are capture by the planetary embryos.”
Artist’s impression of the view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser.
In addition, the simulations produced some interesting estimates on how much of the planets would consist of water. In 90% of cases, water would account for more than 10% of the planets’ mass. Compare that to Earth, where water covers over 70% of our surface, but makes up only about 0.02% of our planet’s total mass. This would mean that the exoplanets would have very deep oceans and a layer of ice at the bottom, owing to the extreme pressure.
Last, but not least, Alibert and Benze found that if the protoplanetary disks that these planets formed from lived longer than the models suggested, the situation would be even more extreme. All of this could be dire news for those hoping that we might find ET living next door, or that red dwarf stars are the best place to look for intelligent life.
“The fact that many planets are water rich could have potentially very strong (and negative) consequence on the habitability of such planets,” said Dr. Alibert. “In fact, we already showed in other articles (Alibert et al 2013, Kitzmann et al. 2015) that if there is too much water on a planet, this may lead to an unstable climate, and an atmosphere that could be very rich in CO2.”
However, Alibert indicates that these two studies were conducted based on planets that orbit stars similar to our Sun. Red dwarfs are different because they evolve much slower (i.e. the luminosity changes very slowly over time) and they are far more red than our Sun, meaning that the light coming from them has different wavelengths that will interact different with planetary atmospheres.
Artist’s impression of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser
“So, to summarize, it could be that the presence of large amounts of water is not so bad as in the case of solar type stars, but it could also well be that it is even worse for reasons that we do not know,” said Alibert. “Whatever the effect, it is something that is important to study, and we have started working on this subject.”
But regardless of whether or not planets that orbit red dwarf stars are habitable, simulations like this one are still exciting. Aside from offering data on what neighboring planets might look like, they also help us to understand the wide range of possibilities that await us out there. And last, they give us more incentive to actually get out there and explore these worlds up close.
Only be sending missions to other stars can we confirm or deny if they are capable of supporting life. And if in the end, we should find that the most common star in the Universe is unlikely to produce life-giving planets, it only serves to remind us how rare and precious “Earth-like” planets truly are.
