A planet orbiting in the "habitable zone" of its parent star has the potential to host liquid water on its surface — a critical ingredient for life as we know it. But what if a planet is in the habitable zone only some of the time? Can life still thrive there?

It's lucky for life on Earth that the sun's habitable zone is effectively stationary and unmoving, providing life with a steady source of radiation. But that isn't the case in every star system. Physicist Tobias Müller and astrophysicist Nader Haghighipour wrote a computer program that demonstrates how the position and shape of habitable zones can change rapidly in double- and triple-star systems, which are thought to be extremely common in the universe.

The program — which they call the "HZ calculator" — creates animations that illustrate how the habitable zone warps and evolves for a star system. [How Habitable Zones for Alien Planets and Stars Work (Infographic)]

On a website the two scientists created to host the program (and make it available to other researchers) is an animation of the star system KIC 4150611.

The three-star system KIC 4150611 has a peculiar orbit that creates a rapidly changing "habitable zone" (in dark green). The black dots are stars. (Image credit: Tobias Müller/Nader Haghighipour /HZ Calculator

The animation shows three stars engaged in a complicated orbit — two of them (K1 and K2) orbit close together, completing a single orbit in less than two Earth days. The third star (A) orbits farther away, looping around the close-knit pair on the order of months. But star A's orbit is not circular, so its distance to K1 and K2 changes. When the three stars come close together, they form a single habitable zone. But as they move apart that zone separates into two separate habitable zones. (In the video above, the dark green area is the habitable zone; the light green areas show places where scientists think habitability might be possible, but that would depend on other factors including the nature of the planet's atmosphere.)

The oddly-evolving habitable zone of the three-star system KID 5653126. The black dots are stars and the dark green region is the habitable zone. (Image credit: Tobias Müller/Nader Haghighipour /HZ Calculator

In another extreme scenario, in the star system KID 5653126, a stellar pair orbit closely around each other and create a mostly stable habitable zone. But a third star orbits the pair and wanders erratically through the habitable zone — a potentially disastrous event for any planets that might reside there.

These sample animations can be found here: http://astro.twam.info/hz/. A complete description of the HZ calculator can be found here: http://iopscience.iop.org/article/10.1088/0004-637X/782/1/26/meta. There are no known planets around the two star systems mentioned above, but there are star systems with unstable habitable zones that are known to host planets. If researchers are going to go hunting for life on those worlds, it would be good to know ahead of time how a changing habitable zone affects a planet's habitability.

At home on Tatooine

The fictional planet of Tatooine from the "Star Wars" universe orbits two suns. The planet is a harsh desert, but was supposedly temperate enough for life to evolve. Scientists have shown that planets around double-star systems exist in the universe, and that they could even support life. But how would the orbits of the two parent stars affect temperatures on the planet?

The HZ calculator provides some insight to this question. The real-world double-star system Kepler 453 is home to two stars, one of which is about five times larger than the other. That means the smaller star effectively orbits the larger one (as opposed to the two stars both orbiting a point in space between them). At least one planet is known to orbit both stars, but the movement of that smaller star means the amount of radiation reaching the planet changes regularly.

The orbit of two stars in the system Kepler 453 causes the surrounding habitable zone to shift. The white dot shows a potential planet in the system that might be affected by the changing levels of radiation. (Image credit: Tobias Müller/Nader Haghighipour /HZ Calculator

In the animation produced by the HZ calculator, the planet's position in the habitable zone is an illustration of how much radiation the planet is receiving from its parent stars. Over the course of a year, the planet drifts from the middle of the habitable zone (the dark-green region) to the very inner edge of that zone (the light-green region), where temperatures might be too hot to support liquid water on the planet's surface.

This would result in large seasonal swings on the planet's surface, according to Elizabeth Tasker, an associate professor in solar system science at the Japanese Aerospace Agency (JAXA). She addressed this issue during a recent talk on planet habitability at a meeting of the committee on Astrobiology Science Strategy for the Search for Life in the Universe for the National Academy of Sciences. The meeting was held at the University of California, Irvine.

"If you have very extreme seasons brought on by an eccentric orbit, can you still discuss habitability? Can life survive this?" Tasker said. "Well, of course, we don't really know, but the outlook isn't completely bleak."

Tasker said exoplanet scientists have theorized that planets that drift across the inner edge of the habitable zone during an orbit might experience extreme seasons, but could potentially retain liquid water during those swings. Perhaps on the fictional planet of Tatooine, Luke Skywalker's aunt and uncle harvest moisture during the cooler seasons, and live off their harvest through the harsher periods brought on by the movement of the two suns.

It could also be that life-forms on the planet go underground or hibernate during the hot periods; but if that's the case, it could be difficult for Earth-bound scientists to detect those life-forms.

This kind of information is becoming relevant as scientists get closer to being able to look for signs of habitability on alien worlds. With thousands of planets to choose from, where will scientists go looking? The HZ calculator is one tool researchers can use to help narrow the list of planets to target for follow-up study, according to Tasker.

Müller, a professor of mathematics and computer science at the University of Groningen in Germany, told Space.com in an email that the HZ calculator has been helpful in illustrating that habitable zones are not static, something that can be difficult to understand without a visual aid, he said. He and Haghighipour, a researcher at the Institute for Astronomy and the NASA Astrobiology Institute at the University of Hawaii-Manoa, have collaborated on scientific papers that made use of the HZ calculator.

No guarantees

This illustration shows the "habitable zone" of Earth's sun. Earth lies inside the habitable zone, while Venus and Mars lie just outside it, in a region where certain conditions could make liquid water possible. (Image credit: NASA)

A planet that lies in the "habitable zone" of a star has no guarantee of being "habitable." But a planet in the habitable zone receives enough radiation from its parent star (or stars) that the temperature on its surface could be right for hosting liquid water on its surface, an essential ingredient for life as we know it. Too much heat and liquid water evaporate; too little heat and the water would freeze.

The habitable zone is a good starting place to search for habitable worlds, but planets in this region could very easily be unfit for life.

For a planet to have liquid water on its surface, it has to have a surface, which means it has to be rocky, like Earth, not a gas giant like Jupiter or Saturn. The planet also has to have an atmosphere, but that atmosphere has to be moderate — Venus' atmospheremakes the planet's surface far too hot for water to remain a liquid, but that may not have always been the case. And scientists think it's likely that many planets migrate through their solar system, so they could move out of the habitable zone, killing off any life that might have emerged there. Most stars experience a change in their radiation output near the beginning of their lifetimes, which could have the same effect.

"When we talk about 'the habitable zone,' we're really just talking about a crude sample selection technique," Tasker said. "It is not a quantitative measurement of a planet's habitability."

Tasker also mentioned that there are multiple scenarios in which life might survive outside the habitable zone. Take, for example, a moon orbiting a very large, hot planet around a star. The moon not only receives heat from the star, but may also receive heat from the planet. In addition, if the moon has an eccentric orbit (meaning not circular), it could experience extreme tidal heating (wherein the gravity of a planet pulls on the interior of a moon, creating a potential internal heat source for the moon). In that case, the moon might be too hot if it lies in the habitable zone of the parent star, but could be perfectly habitable if it lies just outside the habitable zone.

In total, researchers have identified thousands of alien planets, and there are new instruments coming online that will identify even more. Advanced instruments, like the James Webb Space Telescope (JWST), set to launch in 2019, will allow scientists to study those known planets in more detail, and search for complex "biosignatures" in the exoplanet atmospheres, or chemical combinations that are typically produced by life-forms. The upcoming Wide-Field Infrared Survey Explorer (WFIRST), set to launch in the mid-2020s, and the proposed Large UV/ Optical/Infrared Survey (LUVOIR), could also search for signs of habitability on exoplanets.

"I personally think this is perhaps the most exciting time for our field," Tasker said during her presentation. "I think, perhaps, in the next 10 years we're going to be able to really start talking about habitability on another planet.

"Now whether we will discover — conclusively — life, I think that's more questionable, but the amount of information we're going to get from things like JWST or WFIRST or LUVOIR is going to be game changing," she said. "It's going to be amazing."

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