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The European Southern Observatory presents the latest findings from the TRAPPIST-1 planetary system, roughly 40 light years away.
OUT OF THIS WORLD | Earth, Space and Everything In-Between - a daily journey through weather, space and science with meteorologist/science writer Scott Sutherland

Seven alien planets around one star; three could have water!


Scott Sutherland
Meteorologist/Science Writer

Monday, February 27, 2017, 1:10 PM - Astronomers have found an amazing alien star system, with an incredible seven Earth-sized planets orbiting around one tiny, ultra-cool star. Even better? Three of these worlds may have the potential to harbor oceans of liquid water!

Roughly 40 light years away is an extremely compact, but quite amazing planetary system who's central star goes by the rather cumbersome name of 2MASS J23062928-0502285. Most know it more simply as TRAPPIST-1, as it is named after the telescopes that have made some incredible discoveries there.

Fast Facts

• No, aliens have not been found (I know, we were disappointed, too), but this is still exciting news!
• 7 rocky, Earth-sized exoplanets orbit the star TRAPPIST-1.
• This is the greatest number of Earth-sized worlds found orbiting around one star!
• 3 of these planets could have liquid water on their surface.
• This is the greatest number of potentially-habitable worlds that have been found around one star!
• Telescopes on the ground and in space will be watching these alien worlds, to determine if they have atmospheres.
• Read on for more on this incredible story!

TRAPPIST-1 itself is a tiny star, located just over 39 light years away from Earth, in the constellation Aquarius. At only around 10 per cent the size of the Sun, it is much cooler and much redder than our home star, and based on this, astronomers informally refer to it as an ultra-cool dwarf star. That alone makes this celestial body somewhat noteworthy, but what's truly remarkable is what's orbiting around it.

In May of 2016, astronomers reported finding three rocky planets circling very close to this star, each roughly the same size as Earth. A campaign of observations since then, however, has turned up much, much more.

As of today, February 22, 2017, we now know that TRAPPIST-1 is the home of at least seven different exoplanets: six rocky Earth-sized worlds called TRAPPIST-1b, -1c, -1d, -1e, -1f, and -1h, and a seventh, TRAPPIST-1g, that is probably a water-covered (or ice-covered) "super-Earth" planet. According to the astronomers who discovered them, this is the greatest number of Earth-sized planets found in a single star system.

Even more amazing, though, is the fact that three of these worlds - TRAPPIST-1e, -1f, and -1g - are in orbits where they receive just the right amount of heat and light from their star, so that it's not too hot or too cold to support oceans of liquid water on their surface.


Artist's impressions of TRAPPIST-1 and the seven known planets that orbit it. Credit: NASA/R. Hurt/T. Pyle

All of this was revealed in a NASA press conference, on Wednesday, February 22, which discussed the results of a new research paper, now published in the journal Nature.

Present at this press conference were:

Thomas Zurbuchen, the associate administrator of the Science Mission Directorate at NASA Headquarters in Washington,
Michael Gillon, an astronomer at the University of Liege, in Belgium, who is one of the lead researchers with the TRAPPIST (TRAnsiting Planet and PlanetesImals Small Telescopes) project, and the leader of the SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) project, and whom was already involved in the discovery of three exoplanets orbiting a nearby ultra-cool dwarf star, named TRAPPIST-1,
Sean Carey, the senior astronomer for NASA's Spitzer Science Center, who heads up the team working with the Spitzer Space Telescope's Infrared Array Camera (IRAC), which has already been used by astronomers to image exoplanets around cool red dwarf stars and investigate their atmospheres,
Nikole Lewis, an astronomer at the Space Telescope Science Institute (which manages science from the Hubble Space Telescope), who is an expert in characterizing exoplanet atmospheres, and whom has used observations with the Spitzer Space Telescope to map out the atmospheres of several gas giant exoplanets, and
Sara Seager, a professor of planetary science and physics at Massachusetts Institute of Technology (MIT), who has literally written the textbook on exoplanet atmospheres (twice, in fact!), and is one of the leading experts on the search for other Earths beyond our solar system.

Watch a replay of the press conference, below:

Say Hello to the TRAPPIST-1 planetary system

The exoplanets of the TRAPPIST-1 system, as we know of them now, are as follows:


Artist's impressions of the seven known planets in the TRAPPIST-1 star system, with the planets of our inner solar system for comparison. TRAPPIST-1a is the ultra-cool dwarf star at the centre of the system. Credit: NASA

As the image shows, this star system is so compact, the innermost planet - TRAPPIST-1b - completes one orbit (what we on Earth call a year) in just over 1.5 Earth days, and the outermost planet (so far) - TRAPPIST-1h - goes around once every 20 days or so. According to the research, there is still some uncertainty about the exact orbit of TRAPPIST-1h. The distances between these planets are so close, for example, that the distance between TRAPPIST-1e and TRAPPIST-1f is only 3.5 times farther than the distance between the Earth and the Moon. If someone stood on the surface of one of these worlds, under a clear sky, they would be able to pick out features on the other planet's surface as easily as we see craters and maria on the surface of the Moon.

Last May, when the news first broke about TRAPPIST-1, the only members of the system known were the innermost three, but there was a slight quirk. The orbits of TRAPPIST-1b and TRAPPIST-1c were well known, but TRAPPIST-1d was being difficult. The closest the researchers had come up with was a range of orbits where it could be, some of which were in the habitable zone of the star - that band of space around the star where it could be the right temperature for liquid water to exist on a planet's surface.

The initial speculation about this announcement, from this author, was that the astronomers had locked down TRAPPIST-1d's orbit, revealing that it was, in fact, in the star's habitable zone. That speculation turned out to be correct, but with an added bonus: the reason for the uncertainty in -1d's orbit was because the two transits the researchers had attributed to that one planet were actually from two planets! Further observations and analysis separated the two and went on to reveal three more, for the grand total of seven planets!


This orbital plot of the TRAPPIST-1 system shows the relative locations of the planets, along with the location of the star's habitable zone. The dashed and dotted lines indicate the limits of both the conservative and optimistic habitable zones. Credit: ESO/M. Gillon et al.

Who knows? The current uncertainty in TRAPPIST-1h's orbit may lead to finding even more planets in this system!

Habitable?

So, three of these newly-discovered worlds are in TRAPPIST-1's habitable zone. That means that these planets are at the right distance from the star, to receive enough heat and light so that the temperature there is cool enough where water could collect in liquid form on the surface (rather than being vapourized into steam), but also warm enough where that water wouldn't necessarily freeze solid over the entire surface.

While that doesn't mean that there definitely is life there, or even that we'd be able to live a comfortable life on the surface of one of these worlds, it does make them promising candidates to harbour some kind of alien life.

The Planetary Habitability Laboratory, at the University of Puerto Rico, catalogues exoplanet discoveries based on what they call the Earth Similarity Index (ESI). It's a simple number, between 0 and 1, that gauges how much the planet is like Earth. As of their latest update, TRAPPIST-1e and -1f now occupy spots in the top ten of their list of potentially habitable worlds. TRAPPIST-1e is now considered to be the second most Earth-like planet we know of (ESI=0.86), beat out only by Proxima Centauri b (ESI=0.87), and TRAPPIST-1f is seventh on the list (with ESI=0.68).


Credit: UPR/PHL


Credit: UPR/PHL

As to whether these planets can actually support life, that will come down to a few factors.

If they can generate protective magnetic fields, most likely due to tidal heating as they whip around their star once every few Earth-days, this will give them a chance to hold on to a thick atmosphere. With the combination of a magnetic field to deflect the stellar wind and any material from solar flares, and the atmosphere to filter out harmful radiation from the star, this could make the surface conditions more friendly to biological life.

Upcoming observations of these worlds will be able to tell us more about what kind of atmosphere they might have, which will provide more clues about just how habitable they might be.

Triple transit

Remember when we reported on the triple-moon transit for Jupiter roughly two years ago? Well, there are some very interesting parallels between the TRAPPIST-1 system and Jupiter and its system of moons. For one, TRAPPIST-1 is only slightly larger than Jupiter, but also, just based on size, the orbits of the planets around TRAPPIST-1 are actually more like the orbits of Jupiter's Galilean moons than they are the planets of our inner solar system.

So, on December 11, 2015, what did the European Southern Observatory's Very Large Telescope (VLT) managed to see? A similar triple transit across TRAPPIST-1!


A plot of the triple transit of TRAPPIST-1c, TRAPPIST-1e and TRAPPIST-1f, on December 11, 2015. The graph (top) shows how the multiple transit affected the dimming of the star, while the diagram (bottom) shows how the planets transited across the star's face. Credit: ESO/M. Gillon et al.

Why is this important?

Why all the excitement over discovering four more planets in this star system?

The simple fact that we've discovered more exoplanets is great. The more we discover, the more we learn about all the possibilities that exist, and the more we can potentially learn about our own solar system and how it formed. Also, given that some of these planets are potentially habitable, it's possible that we could even discover evidence of biological life there!

There's more to it, though. Basically, the TRAPPIST-1 system, even if it doesn't harbor alien life, is an amazing laboratory for astronomers, astrophysicists and other researchers, as it gives us a nearby, fairly easy to observe system of planets, and it has an abundance of rocky worlds to examine.

Furthermore, since TRAPPIST-1 is such a compact planetary system, astronomers will be able to get multiple planet transits in only a very short time on the telescope. This ability to get concentrated observations makes TRAPPIST-1 very valuable.

From here on, we can aim telescopes at this star system, like Spitzer and Hubble, and the James Webb Space Telescope when it is finally launched, and by watching these planets as they transit across the star's face, we can gather important information about them. They can refine their sizes and possibly determine their compositions, and detect which have atmospheres, and what might be contained within those atmospheres.

What is TRAPPIST?

TRAPPIST is an advanced telescope designed to detect planets as they pass between their star and Earth. This "transit" of an alien world causes a slight dip in the brightness of the star, as we watch it, and astronomers have figured out ways of separating out the signal for a planet transit from other phenomena that may cause a star to dim.

This is the same kind of work that NASA's Kepler Space Telescope performs, except that TRAPPIST works from Earth's surface - TRAPPIST-North from the Oukaïmeden Observatory in Morocco and TRAPPIST-South at the La Silla Observatory in Chile's Atacama Desert.

TRAPPIST-1, on the other hand, is the tiny red dwarf star, 39 light years away from us, that also goes by the name 2MASS J23062928-0502285 in stellar catalogues. Since planets tend to be named after the instrument that found them, just as we've seen with various Kepler Space Telescope discoveries, the star also takes on the instrument name, for simplicity's sake. Since this is the first system examined by the TRAPPIST telescopes, it takes on the designation TRAPPIST-1. Other systems it examines, in the future, will follow the same naming pattern.

Sources: NASA | University of Liege | European Southern Observatory | NASANature | Planetary Habitability Laboratory

Watch Below: Astronomers are very excited about TRAPPIST-1's seven Earth-sized exoplanets!

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