Habitability of alien worlds: hype or reality?

Figure 1. Artist's conception of Kepler-62f. Birght star on the right would be Kepler-62e seen as the morning star from the other potentially habitable world. | Credit: NASA Ames/JPL-Caltech
Figure 1. Artist’s conception of Kepler-62f. Birght star on the right would be Kepler-62e seen as the morning star from the other potentially habitable world. | Credit: NASA Ames/JPL-Caltech

There are some science news that appear recursively in the media. Just to stick to my field of knowledge we could point to Mars’ water and habitability of extra solar planets. It is certainly great to have people reading about those topic, although some confusion seems to have settled down among public opinion. I am speaking, for example, of most people not knowing if we have already found a habitable world or not, because of inexact information or directly because of dramatic promotion of a scientifically valuable discovery. This is our fault, scientists and journalist fault. We have been as successful in promoting some research lines as unsuccessful in making clear how far we can draw our conclusions.

Having said that, let’s discuss a recent discovery published in Science magazine by Borucki et al. 1. In a previous entry I had already commented on the statistical difficulties faced by the scientists dealing with data from the Kepler mission and how they try to quantify the chance of getting a false positive by using BLENDER to mimic the observed signal with other possible natural phenomena. Using the transit technique, Borucki and collaborators have shown the existence of two small planets (Kepler-62e and Kepler-62f) orbiting around a star every 122 and 267 days, respectively. Such planets are in a system composed by five planets. Their radius is about 40% and 60% greater than that of the Earth. These are really the facts: a periodic modulation of the light coming from a K2V star has been detected, such a modulation it is very unlikely (1 chance out 5,000, in the worst case) to be produced by anything different from planets orbiting the star, whose radius and period follow unambiguously from the photometric data and the planetary assumption.

Now the big thing is that if you compute the stellar radiation intercepted by such planets, taking into account both stellar temperature (somewhat cooler than our Sun) and distance, you find that it is roughly the same as in the case of Earth (Kepler-62e) or just the half (Kepler-62f). Following a kind of Goldilock’s principle, this falls somewhere in between too hot and too cold. And this might mean warm enough. Too hot, too cold, warm enough… for what? The answer is water. It is commonly accepted that a planet able to have liquid water on its surface would be similar enough to our own planet that life may rise as a logical consequence.

Figure 2. Comparative plot of the Habitable Zone location as a function of stellar luminosity. Solar System in high in the graph, above the previously reported Kepler-22b habitable world. Kepler-62 system is around a much cooler star and therefore in the lower part of the graph. | Credit: Borucki et al. (2013).
Figure 2. Comparative plot of the Habitable Zone location as a function of stellar luminosity. Solar System in high in the graph, above the previously reported Kepler-22b habitable world. Kepler-62 system is around a much cooler star and therefore in the lower part of the graph. | Credit: Borucki et al. (2013).

The determination of the Habitable Zones (HZ) around main sequence stars was developed in the early 90s by a paper published by Kasting et al. 2 in the Icarus magazine. Shortly, they used a relatively simple one-dimensional climate model in order to determine the location of such a ‘water-friendly’ region depending on stellar temperature. The inner edge of the HZ would be determined by the loss of water because of photolysis and hydrogen escape. The outer edge, instead, would depend on the formation of CO2 clouds, which very effectively reduce planetary temperature by means of reflection. In between those edges, climate is governed by a feedback mechanism lead by CO2. This basic idea can be refined by taking into account further effects, even resulting in narrower HZ for more restricting conditions. Of course, the HZ is an evolving feature around a given star as long as the stellar flux also changes during its main sequence stage.

One of the authors of the Science paper I have previously mentioned, L. Kaltenegger, has extended the basic idea in Kasting’s paper to the peculiar geophysical properties of the so-called ‘water planets’ 3. These planets, whose existence is still to be confirmed, would be completely cover by oceans. There is a good chance that Kepler-62e and -62f are one of those. They show that it is possible to distinguish a water habitable planet by means of detecting its transmission spectrum in the infrared wavelengths. Oxygen, ozone, carbon dioxide, methane and other spectral features will tell us some day that a planet is habitable indeed.

This is the idea of the News & Analysis comment on Science magazine by Richard A. Kerr 4 based on the discovery of Kepler-62e and -62f. Sad but true, as Kerr points out, there is ‘no current, planned or even conceivable mission’ that could demonstrate we have really found a habitable planet. We simply don’t have enough information to say that. This will require more precise and sensitive instruments, maybe the James Webb Space Telescope that will hopefully fly at the end of this decade will be able to spectroscopically characterize the brightest systems detected by Kepler.

Figure 3. May we some day see something like that? Synthetic spectra from Kepler-62e (top), Kepler-62f (middle) and Earth (bottom). Should this spectra be detected we will be pretty close to confirm habitability. Credit: Borucki et al. (2013).
Figure 3. May we some day see something like that? Synthetic spectra from Kepler-62e (top), Kepler-62f (middle) and Earth (bottom). Should this spectra be detected we will be pretty close to confirm habitability. Credit: Borucki et al. (2013).

Even though, the discovery of Kepler-62e and -62f is an astonishing result, a new border in our exploration of the Universe. We should continue looking for habitable worlds, but we should not think that the path is already completed even before we have started walking. Kepler mission is great, it can tell us with high confidence how many Earth-like planets we can find in our neighbourhood. It can tell us how big they are. It can tell us how far they are from their host stars. But Kepler mission cannot measure planetary masses since for doing so you need radial velocity measurements. It can’t tell us either about atmospheric composition since this requires high-resolution spectra. So, let’s wait a little longer and enjoy the beautiful systems the Kepler mission is discovering for us.

References

  1. W.J. Borucki et al. (2013). Kepler-62: a five-planet system with planets of 1.4 and 1.6 Earth radii in the Habitable Zone. Science Express, d.o.i.: 10.1126/science.1234702
  2. J.F. Kasting, D.P. Whitmire and R.T. Reynolds (1993). Habitable zones around main sequence stars. Icarus 101, 108 – 128.
  3. L. Kaltenegger, D. Sasselov and S. Rugheimer (2013). Water planets in the Habitable Zone: atmospheric chemistry, observable features and the case of Kepler-62e and -62f. ArXiv: 1304.5058
  4. R.A. Kerr (2013). Kepler snags super-Earth-size planet squarely in a Habitable Zone. DOI: 10.1126/science.340.6130.262

2 Comments

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Torbjörn Larsson, OMTorbjörn Larsson, OM

I don’t know if I should be annoyed that immediately the goal is realized by finding terrestrials in the habitable zone, the goalposts shifts to establishing their habitability. It was always the strategy, so it can’t be the complaint.

As for Kepler-62e&f, bar an exotic mineral composition, they, like some other planets inhabiting the same radius/mass (composition) diagram, will be water planets. (Fig. 1. in Kaltenegger et al.) So it is nearly as certain an observation as their existence in the first place, unless I am mistaken.

I don’t think anyone has overplayed the potential habitability, but I do think there is a concerted rush to downplay it, this article is not alone in this. It is better to state the case as it is, IMO.

Nitpick: Earth similar surface conditions does not imply life may arise as a logical consequence.

But it is a logical consequence that such conditions may allow life to arise (as a consequence of similar chemical evolution to what likely happened here). Maybe that was what was intended.

Santiago Pérez-HoyosSantiago Pérez-Hoyos

Dear Torbjörn,

I agree with your comment on “life as a logical consequence”, you are absolutely right.

After giving many talks on astrobiology, exoplanets and similar stuff to a general public, I realized that it was usual to find people saying things like “But we ALREADY knew that!”, “Didn’t whatever-previous-mission prove that 10 years ago?”. I have some undergrads who even thought we know for sure that planet X is a comfortable place to live.

That motivated my post, since I felt that we were doing something wrong. Broadly speaking, I agree with Kerr’s comments on Science mag. I never intended to downplay potential habitability (to use your words) but to raise a word of caution about going too far with current data and models.

Needless to say, I am still to find a single scientific paper which hypes (to use my words) its results. It is probably our later comments on these kind of news which make the hype grow.

I am confident that we will go from Kepler results to detection of a “friendly” atmosphere some day. I would be surprised that our current expectations lie too far from reality. But I also wanted to highlight that we are still taking the first steps in this quest and that many ideas permeating the society are not necessarily wrong but still to be proved.

In any case, thanks for your comment. It certainly enriches the post and the discussion.

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