A slightly serious habitability ranking of TRAPPIST-1's planets

The discovery of seven Earth-sized planets orbiting a (relatively) nearby dwarf star raises plenty of questions. Could they possibly harbor liquid water? What are the conditions like and what kind of life, if any, could exist there? While there is much to learn about these distant, potentially habitable worlds, scientists do already have a handle on a few of the key features. So here's an early snapshot of each and which we'd lean towards in the event we ever needed to, you know, to relocate to another solar system.

Forty light-years away, the TRAPPIST-1 system hosts seven planets in tight orbit around an ultra-cool red dwarf star. Red dwarfs are a favored hunting ground for exoplanet researchers, because they last a lot longer, are much cooler and their planets are proportionally larger than those of yellow stars like our own.

Every one of planets in the TRAPPIST-1 system circles its parent star in much closer orbit than Mercury does to our Sun. These vary from 1.51 days to 20 days, but what the planets do have in common is a rocky composition and mass and diameter similar to Earth. Of the seven, three are in the habitable zone where scientists calculate that conditions could support liquid water.

The possibilities for these planets include barren rock stripped bare by radiation and super-hot, cloud-covered worlds like Venus. Much of what we find there might depend on the age of the parent star. Red dwarfs are very active during their early years, unleashing flares and bursts of damaging radiation that spell trouble for planets in the surrounding area. Scientists believe TRAPPIST-1's parent star to be at least 500 million years old, but it is possible that red dwarfs can take their first billion years to calm down enough for the nearby planets to become habitable (they can go on to live for trillions of years!).

On the one hand, the host star's high-energy x-rays and ultraviolet emissions could kick off runaway greenhouse gas effects that make the climate too hot, strip the atmospheres of oxygen, or eliminate the atmospheres altogether. On the other hand, this stellar radiation could be something of a blessing, stripping away inhospitable gases like hydrogen and creating a habitable planet in the process.

Without yet knowing what these atmospheres look like, it makes calculating the planet's temperatures pretty much impossible as Amaury Triad, an exoplanet hunter at the University of Cambridge and co-author on the groundbreaking TRAPPIST-1 paper explains.

"We have an idea on the amount of energy the planets receive, but the actual temperature is highly dependent on the atmospheric and geologic conditions," he explains to New Atlas. "What we measure at the moment is an equilibrium temperature, which assumes that there is no atmosphere and that all the starlight reaches the surface, warms it and nothing else happens. Those values are indicative."

All of that is to say, there are all kinds of factors at play in terms of what conditions might possibly support life, and a lot of further studies will be carried out before scientists can answer those kinds of questions with any certainty. Nonetheless, based on what we know so far (and the great deal that we don't), here's a brief look at each of the new exoplanets in the order that we'd like to pay them a visit.

1. TRAPPIST-1f

• Orbit: 9.21 days
• Distance from star: 0.037 AU
• Radius: 1.04 that of Earth
• Mass: 0.68 that of Earth

What we know: The fifth closest to the TRAPPIST-1 star, smack bang in the middle of the habitable zone and our first stop in the TRAPPIST-1 system. This planet receives about the same amount of light that Mars does from our star and is believed to be a good candidate for hosting liquid water.
But the real reason we're making a bee-line for TRAPPIST-1f is that according to current estimates of its radius and mass, scientists calculate its surface gravity to be 32 percent lower than ours, while the other planets in the system are around the same as on Earth. Can you imagine the slam dunk contests on all-star basketball weekend?

2. TRAPPIST-1e

• Orbit: 6.10 days
• Distance from star: 0.028 AU (times Earth's distance to the Sun)
• Radius: 0.92 that of Earth
• Mass: 0.62 that of Earth

What we know: Of the three planets within the habitable zone, TRAPPIST-1e is the closest to the parent star so we're leaping here inwards from the neighboring TRAPPIST-1f. It is probably tidally locked, which means one side is constantly facing the star and would be living out an endless summer, while the other side of the planet would be permanently icy and cold.
Researchers believe that if conditions are right, the two regions on a tidally locked planet in TRAPPIST-1 could be separated by a so-called Terminator zone, where the solid ice could give way to liquid water. Check out the artist's concept of this below.

Scientists actually say that all of the planets in the system are probably tidally locked, but TRAPPIST-1e has the smallest radius of those in the habitable zone. This means fuel savings when vacationing to the other side, and considering the likely current state of the job market in the TRAPPIST-1 system, this could be rather handy indeed.

3. TRAPPIST-1g

• Orbit: 12.35 days
• Distance from star: 0.045 AU
• Radius: 1.13 that of Earth
• Mass: 1.34 that of Earth

What we know: TRAPPIST-1g just scrapes into the outer edge of the system's habitable zone, and we're headed there next. It is the biggest of the planets in the TRAPPIST-1 system and is thought to be a good candidate for hosting liquid water. But the real reason it is so high on our list is that it boasts rather long years for a TRAPPIST-1 planet, and the less we have to celebrate Valentine's Day, the better.

4. TRAPPIST-1d

• Orbit: 4.05 days
• Distance from star: 0.021 AU
• Radius: 0.77 that of Earth
• Mass: 0.41 that of Earth

What we know: The third in line from the parent star, TRAPPIST-1d and the system's other two most inner planets were actually found last year by the same researchers behind last week's discovery. It is likely too warm to sustain water, but its mass makes it the lightest of the planets in the system (the mass of the outermost TRAPPIST-1h is still unknown). More importantly its orbit of 4 Earth days make for neatly organized school semesters, a great place to raise children.

5. TRAPPIST-1c

• Orbit: 2.42 days
• Distance from star: 0.015 AU
• Radius: 1.06 that of Earth
• Mass: 1.38 that of Earth

What we know: This planet is the second closest to TRAPPIST-1's parent star. Scientists say there is a low chance of water and that it receives the same amount of energy as Venus. Conveniently, Venus reflects around 70 percent of the sunlight it receives back out into space thanks to its thick, mostly carbon dioxide gas atmosphere. Could TRAPPIST-1c be benefitting from the same phenomenon? We'd rather not risk it.

6. TRAPPIST-1b

• Orbit: 1.51 days
• Distance from star: 0.011 AU
• Radius: 1.09 that of Earth
• Mass: 0.85 that of Earth

What we know: While the system's host star is a lot cooler than our Sun, its closest planetary neighbor is likely still too hot to hold liquid water without it boiling off. Plus, with the years consisting of just 1.51 Earth days, you'd be celebrating birthdays almost every day and nobody's got enough room for that much cosmic cake.

7. TRAPPIST-1h

• Orbit: Approximately 20 days
• Distance from star: 0.045 AU (times Earth's distance to the Sun)
• Radius: 1.13 that of Earth
• Mass: Uknown

What we know: Cold and lonely, is this TRAPPIST-1's Pluto? The exact orbit of the system's outermost planet is yet to be confirmed but scientists say it is likely too distant to harbor liquid water. Unless there is some kind of alternative heating process at play, that is.
Furthermore, scientists say that most of the planets in the TRAPPIST-1 system are close enough that on a clear day, you could stand on the surface of TRAPPIST-1d (the fourth from the star), for example, and see its sister planets with the same visibility as we see the Moon from Earth. Standing way out there on the surface of TRAPPIST-1h though, your gaze might be met only by darkness, and how lonely would that be? It'd be like looking at the night sky on Earth, that place you travelled 40 light years to get away from in the first place.

Sources: NASA 1, 2,