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How Far Away Are They

In the search for extraterrestrial life, we are of course interested in all the habitable planets where living organisms might emerge. However, the closer the planet, the more interesting it will be, because if there is intelligent life there, communication is going to be possible within reasonable time frames. For example, if a planet is four light-years away, signals sent at the speed of light will arrive in four years and a response would be received in four years, for a total of eight years. By contrast, if a habitable planet is 100 light-years away, any two-way conversation will outlast the lives of everyone on Earth.

How, then, can we estimate the distance to the nearest habitable planet? In the previous unit, we were able to estimate the number of stars in our galaxy that are hosts to planetary systems. Now, we'll use the planet occurrence rates we calculated to estimate how far away the nearest habitable planet is. At the end of this section, we'll also discuss the candidate planet orbiting Alpha Centauri B, which made headlines in 2012.

How far away is the nearest habitable planet?

We don't know of any planets that are definitely habitable, but we can make an estimate of how far away such a planet is likely to be.

To begin, let's consider a single star. Assuming an occurrence rate of habitable planets of 20%, the chance that this star has a habitable planet around it is one in five, or expressing the probability as a fraction, 0.2. Those odds are not bad, but let's improve our chances by looking around more stars.

If we consider two or more stars, what are the chances that at least one of them has a habitable planet? To calculate this probability, we need to consider the opposite scenario: that none of them has a habitable planet. Then, P(at least one) = 1 – P(none). For one star, the probability that it does not have a planet is P1(none) = 0.8. For two stars, we multiply the probabilities together: P2(none) = 0.8 x 0.8. For any number of stars (N), PN(none) = 0.8N. Putting everything together,

To be 95% sure we find a planet, we'd have to look around at least 14 stars.

How far away is the nearest habitable, transiting planet?

We already calculated how many stars we'd have to observe before we could be reasonably confident that one of them hosted a habitable planet. As we've discussed (or will discuss), transiting planets provide opportunities to study the atmospheres of planets. We might therefore want to know how far away the nearest transiting habitable planet is.

We've already discussed how the geometric transit probability means that most planets won't transit. Therefore, we will have to look around more stars to find a transiting habitable planet than we would if we didn't care whether the planet transited. Let's assume that the geometric transit probability for a habitable planet is 2% transit probability.* For every 100 planets, we would expect only 2 of them to transit. That's 1 out of of every 50 planets. We therefore need to look at 50 times as many stars as when we didn’t care whether the planet transited. To be 95% sure of finding our transiting habitable planet, we’d have to look at 14*50 stars, or 700 stars.

What type of star does this planet orbit?

We mentioned briefly in the previous unit that 80% of the stars in our galaxy are red dwarf stars. In fact, the nearest star to the Earth (Proxima Centauri) is one of this type of star, as are 248 of 376 of the stars within 32 light years. Given the sheer numbers of these stars and the high likelihood that any given red dwarf hosts a planet, the nearest planet – and the nearest habitable planet – likely orbits a red dwarf.

Red dwarfs offer both opportunities and challenges to those studying exoplanets. Their small size and intrinsic faintness make planet hunting easier. Both the transit and the radial velocity signal of a planet orbiting an M dwarf will be larger than if that same planet orbited a star like the Sun, which makes the planet easier to detect. Moreover, studying the atmosphere of the planet through transit methods is easier when the planet blocks out relatively more of its star’s light. The fact that the star is dimmer means that the habitable zone for a red dwarf is closer than it is for a Sun-like star: a habitable planet would have an orbital period measured in weeks, rather than being about a year. This makes the geometric transit probability higher – so the planet is more likely to transit – and means we don't have to wait as long for the planet to complete one orbit. As you might imagine, it's easier to study a planet when you do not have to wait an entire year for it to come back around!

One difficulty with studying planets orbiting red dwarfs is that the typical red dwarf isn't as well characterized as the typical Sun-like star can be. Because planet parameters can only be measured relative to the stellar parameters, not knowing the mass or radius of your star can pose serious problems! Red dwarfs also experience strong flares and because a habitable planet around a red dwarf is on a close orbit, they will experience intense magnetic activity, which could be problematic for life on the planet's surface.

The intriguing case of Alpha Centauri Bb

It's possible that we've already found a very nearby (non-habitable) planet - it might orbit a star just 4 lightyears away. Alpha Cen Bb was detected by Xavier Dumusque, an astronomer at the Harvard-Smithsonian Center for Astrophysics, and the HARPS (High Accuracy Radial velocity Planet Search) collaboration using the radial velocity technique. It proved to be an extremely difficult measurement: the size of the signal was just half a meter per second. To find the signal, they had to correct for a large variety of other radial velocity variations; only then did the planet make itself known.

Because the signal is so small, the methods used to extract the signal are important. Dumusque made public all of their data, which allowed other researchers to try their hands at identifying the signal. According to reports published as of August 2014, the follow-up studies failed to recover the signal. These results don't mean the planet isn't there, but that the current data aren't solid enough to demonstrate the planet's existence.

Another group, led by Debra Fisher of Yale University, also observed Alpha Centauri B, and has an independent data set. Their group did not detect the planet, but noted that they expected to be able to only marginally detect such a small signal.

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