Is there life around the nearest stars? – by Avi Loeb

by Prof. Avi Loeb of  Harvard University. 

Edited by Zaira M. Berdiñas

Is there extra-terrestrial life just outside the solar system? The recent discovery of Proxima b1, a habitable Earth-mass planet next to the nearest star, opened a unique opportunity in the search for extra-terrestrial life. It is much easier to study nearby habitats for life, either by remote sensing of the feeble radiation signals from biologically-produced molecules (e.g. oxygen) or by sending spacecrafts that will image the planet’s surface or collect samples from its atmosphere through a close encounter. The Breakthrough Starshot initiative, announced in April 2016 (and whose advisory committee I chair) aims to send lightweight (gram scale) probes to the nearest stars at a fifth of the speed of light, so as to inform us of nearby life-hosting environments within our generation. To properly select the Starshot targets, we would like to know which nearby stars host habitable planets like Proxima b. The treasure of data expected from the Red Dots campaign will be invaluable for guiding and motivating the Starshot project.

Artistic’s conception showing the Starshot project concept.  A laser beam propels a light sail towards a nearby exoplanet such as Proxima b. The sail carries on its center a lightweight probe with several measuring instruments. Starshot will start soon the first five-year phase of technology demonstration at a funding level of $100M, provided by the entrepreneur and physicist Yuri Milner (Credit: Breakthrough Starshot).

The chemistry of life as we know it requires liquid water, but being at the right distance from the host star for a comfortable temperature on the planet’s surface, is not a sufficient condition. The planet also needs to have an atmosphere. In the absence of an external atmospheric pressure, the warming of water ice transforms it into directly into gas phase rather than liquid. The warning sign is just next door: Mars has a tenth of the Earth’s mass and lost its atmosphere. Does Proxima b have an atmosphere? If so, the atmosphere and any surface ocean it sustains, will moderate the temperature contrast between its permanent day and night sides. In collaboration with Laura Kreidberg, we showed2 that the James Webb Space Telescope, scheduled for launch in October 2018, will be able to distinguish between the temperature contrast expected if Proxima b is bare rock compared to the case where its climate is moderated by an atmosphere.

Some people part of the James Webb Space Telescope team are shown here in front of the JWST full-scale model at the Goddard Space Flight Center in Greenbelt. (Credits: NASA).

Proxima Centauri is a red dwarf star with 12% of the mass of the Sun. Another dwarf star, TRAPPIST-1, with 8% of the solar mass, was discovered recently3,4 to host 3 habitable planets out of a total of 7 and if life forms in one of the three it will likely spread to the others5. Such dwarf stars have a lifetime that is up to a thousand times longer than the Sun. Hence, they provide excellent prospects for life in the distant future, a trillion years from now, long after the Sun will die and turn into an Earth-size cold remnant, known as a white dwarf. I therefore advise my wealthy friends to buy real estate on Proxima b, since its value is likely to go up dramatically in the future. But this also raises an important scientific question: is life most likely to emerge at the present cosmic time near a star like the Sun? By studying the habitability of the Universe throughout cosmic history from the birth of the first stars 30 million years after the Big Bang to the death of the last stars in ten trillion years, I concluded6,7 that unless habitability around low mass stars is suppressed, life is most likely to exist near dwarf stars like Proxima or TRAPPIST-1 ten trillion years from now. This highlights the importance of searching for life around these nearby red dwarf stars, namely the Red Dots campaign. Finding bio-signatures in the atmospheres of transiting Earth-mass planets around such stars will determine whether present-day life is indeed premature or typical from a cosmic perspective.


  1. Anglada-Escudé G. et al. “A Terrestrial Candidate in a Temperate Orbit Around Proxima Centauri”, Nature536, 437 (2016).
  2. Kreidberg, L. & Loeb, A. “Prospects for Characterising the Atmosphere of Proxima b”, ApJ832, L12 (2016).
  3. Gillon, M. et al. “Temperate Earth-Sized Planets Transiting a Nearby Ultracool Dwarf Star”, Nature533, 221 (2016).
  4. Gillon, M, et al. “Seven temperate terretrial planets around the nearby ultracool dwarf star TRAPPIST-1”, Nature542, 456–460
  5. Lingam, M., & Loeb, A. “Enhanced Interplanetary Panspermia in the TRAPPIST-1 System”, PNAS, in press (2017); arXiv: 1703.00878.
  6. Loeb, A., Batista, R. A., & Sloan, D. “Relative Likelihood for Life as a Function of Cosmic Time”, JCAP8, 40 (2016). “
  7. Loeb, A. “On the Habitability of Our Universe”, chapter for the book “Consolidation of Fine Tuning”, edited by R. Davies, J. Silk and D. Sloan (Oxford University, 2017); arXiv:1606.0892

About the author

Prof. Avi Loeb

Avi Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He serves as Chair of Harvard’s Department of Astronomy, Founding Director of Harvard’s Black Hole Initiative and Director of the Institute for Theory and Computation (ITC). He also chairs the Advisory Committee for the Breakthrough Starshot Initiative, and holds the Sackler Senior professorship by Special Appointment at Tel Aviv University. He is an elected fellow of the American Academy of Arts & Sciences, the American Physical Society, and the International Academy of Astronautics, as well as Vice Chair of the Board on Physics and Astronomy of the National Academies. In 2012, TIME magazine selected Loeb as one of the 25 most influential people in space. Loeb published over 550 scientific papers and 3 books.

1 Comment

  1. Wow that was unusual. I just wrote an very long comment but after
    I clicked submit my comment didn’t show up. Grrrr…
    well I’m not writing all that over again. Anyway, just wanted to say wonderful blog!

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