Every Thursday, Laboratory Equipment features a Scientist of the Week, chosen from the science industry’s latest headlines. This week’s scientist is Avi Loeb from Harvard Univ. He and a team are looking to dying stars in a search for evidence of alien life.
Q: What made you interested in looking to stars for alien life?
A: The most exciting question that astronomers can answer is "are we alone?"
A positive answer would have dramatic implications on our worldview beyond science, including religion and philosophy. So far, astrophysicists focused on the physical universe, but life may be flourishing outside the solar system. If intelligent civilizations like our own are common, then the recognition that they exist and pose a potential threat may bring people together here on Earth.
It is very likely, in my opinion, that primitive forms of life exist on planets around other stars. How can we find evidence for life?
The simplest method would be to search for molecules that are generically produced by the chemistry of life. The most ubiquitous among them is molecular oxygen, O2. If all forms of life were eliminated from Earth, then oxygen would have been depleted from our atmosphere within a million years. The reason for that is that oxygen is so reactive that it gets consumed very quickly by chemical reactions that are energetically favorable.
If we imagine an Earth twin orbiting a star like the Sun, then a distant observer could find the spectral signature of absorption by oxygen in the planet's atmosphere whenever the planet crosses the face of the star. As the Earth moves around the Sun, the transit would happen once per year and only for observers with the proper alignment. The problem is that the area of the Earth is ten thousand times smaller than that of the Sun. Therefore most of the light that the observer sees is not occulted by the planet.
This makes the detection of oxygen absorption very challenging since it amount to a tiny feature in the spectrum of the background star.
A couple of months ago, I realized in a conversation with my colleague Dani Maoz (Tel Aviv Univ.) that the situation is much more favorable for an Earth-like planet orbiting a white dwarf, the end state of stars like the Sun once they consume their nuclear fuel and cool off.
A white dwarf with an age of about a billion years has the same surface temperature of the Sun, but it is much fainter because of its smaller size. A planet can still be warm for life to exist if it is closer to the "furnace" to heats it. An Earth orbiting a hundred times closer to a white dwarf than the Earth-Sun distance would have the same surface temperature as our planet and could sustain liquid water and the chemistry of life as we know it. The sky of the dayside of such a planet would look similar to our sky. The white dwarf will have the same color as the Sun and will occupy the same angle on the sky as the Sun.
In our paper with Dani Maoz we demonstrated that the absorption signature of oxygen can be detected in the next decade with the James Webb Space Telescope, the successor to the Hubble Space Telescope, scheduled for launch in 2018.
Q: What are the future implications of your research and findings?
A: We can now design an observing program that could detect bio-markers in the atmosphere of an Earth twin orbiting in the habitable zone around a white dwarf, if such a planet exists. The first component of this program would be to identify a thousand nearby white dwarf and check whether any of them shows evidence for a transiting Earth at the appropriate distance from its parent star.
Q: What was the most surprising thing you found in your research?
A: I was surprised to find that the above program is feasible within my lifetime. It is exciting to live at a time when we could answer the question "are we alone" scientifically, using state-of-the-art technology.
Q: What is the take home message of your research and results?
A: Most of my papers are in other areas of astrophysics. Not being immersed in active research programs on extra-solar planets enabled me to think creatively about the topic of detecting bio-markers on exo-planets.
It always pays to step back and view an active field of research from a distance.
My main field of research involves the first stars and galaxies in the Universe. In fact, I just published a textbook on this topic, http://press.princeton.edu/titles/9914.html following on a more popular book from 2010, http://press.princeton.edu/titles/9373.html
Q: What is next for you and your research?
A: I am working on a new theory for the formation of the moon and a lot of other fun projects.