Our sun is orbited by a number of planets, which all have similar, yet different, compositions: rocky, gaseous, airless, hot, cold… but are planets outside our solar system similar to those of our own solar system?

In short...

We always believed there were other planets around other stars. Today, we know there are! In our solar system, all of the planets orbit around the Sun.

Extrasolar planets – exoplanets – on the other hand, are planets that orbit around a star which is found outside of our solar system. There are so many stars out there, many of which are able to host orbiting planets, as our Sun does!

What do exoplanets look like?

In fact, there is a much greater diversity of extrasolar planets compared to the planets in our solar system. Exoplanets are made up of elements similar to that of the planets in our solar system, such as iron and carbon, but their mixes of those elements may differ. Hot Jupiters (hot planets with a large mass and low density similar to Jupiter) are the easiest to find, as they are massive and fast, making them easier to detect. They are hot because they have low orbital periods (they orbit closer to their star) which is what makes them also orbit faster.
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But trying to find other planets is no easy task…

This is why POET is on a mission!

Understanding the formation, internal structure, atmosphere and evolution of extrasolar planets is important, not only to investigate distant exoplanetary systems, but also to understand the formation and evolution of our own Solar System!

What is a star?

We generally associate stars to the nighttime sky. Yet the fact that we have daylight here on Earth is due to a star that shines bright, day in and day out - our Sun!

From dust to star

All stars are formed from collapsing clouds of gas and dust, known as giant molecular clouds. Giant molecular clouds can become gravitationally unstable and begin to fragment and collapse. This process can take millions of years, with the end result: a new cluster of young stars. These young stars will form protoplanetary disks which will coalesce into planetary systems. Eventually, the cluster will dissipate, populating the galaxy with new stars and planets.
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A young cluster will be composed of stars with a variety of masses, with low-mass stars being the most common. Low-mass stars, also known as M-dwarfs, are also the least luminous and are relatively cool. The average M-dwarf only puts out 10% the total energy compared to the Sun!
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Some stars in the cluster will be of such low-mass that the temperature and pressure at their centre will be insufficient for nuclear fusion to occur. The fusion of hydrogen into helium is what makes a star like the Sun shine. The stars that do not have sufficient mass to initiate nuclear fusion are known as Brown Dwarfs. Such objects have masses 10-100x more than Jupiter, which is less than 10% the mass of the Sun.

POET’s Target Stars

As explained above, brown dwarfs are protostars that have not gained enough mass during their formation to reach temperatures high enough for nuclear fusion of hydrogen to begin.

Therefore, both brown dwarfs and very low-mass stars are of interest for the POET mission: their temperatures are relatively cool.

Searching for the habitable zone around cooler stars

POET will be searching for habitable zone planets around cool stars. More precisely, for potentially rocky and habitable exoplanets via the transit method around very low-mass stars and Brown Dwarfs.

What makes a planet potentially habitable?

The standard definition for a habitable planet is one that can sustain life for a significant period of time. But for life to be sustained, certain conditions need to be available!

The Perfect Distance from it's star

Not too hot, not too cold - such is the sweet spot for life.
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The region around a star where liquid surface water can exist on a planet’s surface such as Earth is called the habitable zone. When a planet, such as the Earth, orbits around a star, the temperature of its surface is influenced by the heat generated by the star. Because Brown Dwarfs and very low-mass stars are cool, liquid water could exist on planets orbiting very close to them without evaporating - closer than is possible on planets in our solar system!

How do cool stars generate heat?

As we saw earlier, high-mass stars burn hydrogen, a process that generates a lot of heat, and visible light. But what about low-mass stars, which are way cooler than the Sun?
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Stars that are much cooler than the Sun emit most of their light in the infrared. Many objects - even cooler ones such as ice - emit infrared light. Think of a hot piece of metal; you can feel the heat radiating, but metal does not generate visible light. That’s infrared light!
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To detect infrared light, our microsatellite will be equipped with an infrared filter, able to detect invisible wavelengths that radiate from cooler stars. Such a filter enables transit searches of cool stars for potentially habitable zone planets.

Short orbitals for “easier” detection

The orbital periods of planets within the habitable zone around very low-mass stars are short (10 days or less), and so observing multiple transits requires little time. For brown dwarfs and very low-mass stars, the habitable zone offers higher probability of transits compared to higher-mass stars. Therefore, if one could host life, it makes it easier to detect.

Think about it: if a planet has a 3-day orbital, POET can observe it 10 times or more, and therefore gain a lot of information within 30 days of observation. But for a habitable zone planet orbiting around a high-mass star, such as Earth, recording a 365-day orbital 10 times would require... 10 years of non-stop observation!

The search for Earth-like conditions

By targeting the habitable zones of low-mass stars, over a two-year period, POET will be able to produce an outsized yield of rocky habitable-zone exoplanets.

Discovering Exoplanets

How do we find planets orbiting stars other than our Sun?