The index of habitability: a new way to classify exoplanets

The index of habitability: a new way to classify exoplanets

The NASA Kepler Space Telescope radically changed our view of the huge number of exoplanets orbiting other stars in our galaxy, but how many are suitable for life among them?

Today we are burning with an overwhelming desire to understand which of the more than 1000 planets discovered by the Kepler Space Telescope are habitable. Are they close enough to their star? How do their surfaces reflect light? How biased are their orbits?

The new document, which was recognized by the Astrophysical Journal and previously published on the site, proposes to take all these factors into account and classify the planets according to their “habitability index”.

This will help prioritize which planets should be considered more closely, says Rory Barnes, a research scientist and professor of astronomy at the University of Washington. He discussed with Discovery News how this index will work.

Key Features

The index of habitability: a new way to classify exoplanets

Diagram of what happens to the light curve of a star when the planet passes in front of it

When an exoplanet passes in front of a star and can be observed through a telescope, such as Kepler, then four main characteristics are visible, as Barnes says: the orbital period (how long it takes to go around the star), the duration of the transition (how long it takes to to make the transition), the transition depth (how much light is blocked) and the time between transitions.

“Only this data is not enough to draw conclusions about habitability,” says Barnes. "We are not talking about the mass, radius and brightness of the star - these are the things that we definitely need to know about the planets" in question "." This usually means that the conversion data is not enough. Astronomers get more detailed information by looking at the star’s “swinging” as the planet orbits around it. This method is known as the radial velocity method. This gives additional information, for example, on the mass of exoplanets and planetary orbit.

Step 1: Inspect the bright stars

The index of habitability: a new way to classify exoplanets

Artistic illustration of TESS (satellite, examining transitional exoplanets) and planets passing in front of a star

In 2017 or 2018, NASA’s satellite examining transient exoplanets (TESS) will begin to explore planets passing in front of bright stars. If the star is located close to the Earth, more information will be available to us simply because it is larger in the viewfinder than the size of the star, or because it is brighter, for example, through astroseismology - the science of “star earthquakes”.

“This is one of the biggest benefits of TESS,” said Barnes. "The TESS satellite looks at near stars that are much brighter, so that we can use more complex methods."

Step 2: Introduce Climate Models

The index of habitability: a new way to classify exoplanets

This art illustration depicts exoplanet OGLE-2005-BLG-390L b, which is estimated to have a surface temperature of just 50 Kelvin (-370 degrees Fahrenheit, or -223 degrees Celsius)

You can use several parameters that are available to us — the amount of light that is blocked, the size of the orbit — to see how much radiation hits the planet.

The problem, however, is that we also need to know how much light a planet can reflect. Ice reflects more light than water or land. This means that an ice planet will have a lower surface temperature than a planet with a lot of water. By the mass of the planet, it is not always clear whether it is gaseous (like Neptune) or rocky (like Earth), or something average. This makes the calculations even more complicated.

Step 3: Present eccentricity

The index of habitability: a new way to classify exoplanets

This scheme of the Solar System shows how much the orbits of the dwarf planets of Pluto and Eris are displaced compared to other planets.

But there is another big question: how much is the planet's orbit shifted? It is almost impossible to determine according to the conversion data without having other information.

If the planet is close to its star, more radiation will hit its surface. If the planet is far from its star, the radiation will be less. If a planet moves abruptly very close to a star, then moves away a long distance, the amount of radiation also changes dramatically.

"This is something like a dance between albedo and eccentricity, which largely determines how residential the planet is," said Barnes. "Eccentricity is easier to determine if there are several planets in the system, since you can see how their orbits interact with each other."

Index of habitability

The index of habitability: a new way to classify exoplanets

The habitability index will classify planets such as Kepler-62F, a small super-Earth, as shown in this artistic image.

Barnes and his colleagues derived a formula by which it is very likely to determine whether a particular planet is worth a more detailed study. Their habitability index is the product of certain planetary components that make it possible to create an inhabited world: stellar radiation falling on the surface, orbital displacement (eccentricity), albedo and rockiness.

He recommends using the formula as follows:

  1. The transition data may deceive you: it seems to you that there is a planet there, when in fact it is just a dimple of light that appears due to spots on the star. You must use an alternative method to confirm the presence of a planet. For example, the method of radial velocity, that is, to observe the oscillations of the star;
  2. As soon as you get more data after measurements using the radial velocity method, such as eccentricity, expose the planets according to a rating scheme. Those with the most favorable characteristics should be examined through a larger number of telescopes in order to obtain more reliable information;

He hopes that astronomers using TESS, the James Webb telescope (which is coming soon), and other tools will be able to make their searches for habitability more effective by applying an index.

"The resources needed to explore can be significant, especially for the smallest planets," said Barnes. "Our ranking scheme will help highlight more priority candidates to test their habitability."

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