Exo Planets

In celebration of International SPACE Week, I wanted to share a few interesting facts about exo-planets and put our planet in perspective. The first planet that does not orbit our Sun was only discovered in 1992. While there are probably billions of them, there are now over 4,000 confirmed planets in our Galaxy alone and hundreds more have already been discovered this year. The first planet beyond our Galaxy, however, was only discovered last month, and orbits a star in the Whirlpool Galaxy some 23 million light-years away.

There are only about 100 confirmed planets within 30 light-years of Earth, and the closest is still over 4 light-years away. The furthest man-made object — the Voyager 1 Spacecraft — is still less than a light-day away after travelling for almost ½ a century. While a tiny virus, a Gigafire and biblical flooding has rendered Planet Earth less habitable than usual, it is still prime real estate in the scheme of the Universe. The closest even tolerable planet is probably a long, long way away.

SPACE is a BIG place!

An Excerpt from my upcoming book:

CHAPTER 9: Life Beyond

Exoplanets

We have a remarkable home planet. But how does it compare to other planets orbiting other stars? We call these Exoplanets. It is difficult to see exoplanets because the closest one is over 4 light-years away, and, unlike stars, they do not shine light. It is also difficult to see a tiny ball of dull rock next to a giant ball of bright nuclear fire. While a few exoplanets have been discovered by the light reflected off their surface, most have only been detected indirectly. The first exoplanet was only discovered in 1992, orbiting a pulsar neutron star that spins and pulses extremely fast — over 100 times per second! It was detected by gaps or blockages in the radio wave pulses when the planet passed between the star and the Earth. 

Wobble method

The first exoplanet orbiting a normal star was only discovered in 1995. The gravity of a massive planet causes the star to wobble around a shared centre of gravity. Similarly, our sun wobbles due to the gravity from Jupiter. This wobble causes the light coming from the star to be stretched or compressed by the doppler effect. This first discovered exoplanet orbiting a normal star could be detected from 50 light-years away because it was over 150 times the mass of Earth and orbited closer to its star than Mercury is from our sun. Earlier this year, this discovery was finally awarded the Nobel Prize in Physics. The bigger the planet and the closer it is to the star, the easier it is to spot. The first multiple-planet system orbiting the same star was discovered in 1999, and the first exoplanet found at a distance from its star where liquid water could exist was discovered in 2001. Although the reason it could be spotted farther from the star was its massive size — it is 6 times bigger than Jupiter!

Transit method

The Kepler Space Telescope, launched by NASA in 2009, was a complete game-changer. Instead of the wobble method, it detected exoplanets by what is called the Transit method. When a planet passes in front of a star, as the moon does during a solar eclipse, the brightness of the star momentarily drops. The Kepler telescope was put in orbit around the sun just behind the Earth. From there, it continually looked towards the same tiny patch of stars in our galaxy, watching for any changes in brightness. It would detect exoplanets only if they passed exactly between the star and the telescope, and only if they were big enough and close enough to the star to be measurable by the instrument. Think about it: how often do even our neighbouring planets come between us and the sun? While Mercury transited the sun only a couple of weeks ago, the next time will not be until 2032. The last Venus transit was in 2012, and the next one will be in 2117. Furthermore, unlike exoplanets, all the planets within our solar system orbit within 7° of Earth’s orbit, making the transits much more likely. It is difficult to see planet transits in our own solar system, let alone in others. So there are likely many, many more exoplanets yet to be discovered. Just over a year ago, after 9 years of operation, the spacecraft ran out of fuel and was shut down. It had discovered 2,662 confirmed exoplanets, and thousands more yet to be confirmed. This one space telescope has discovered almost ¾ of all confirmed exoplanets to date. In the same year, Kepler was replaced by TESS — The Transiting Exoplanet Survey Satellite. As the name suggests, TESS also uses the transit method of detection but has a much wider field of view - 400 times bigger than Kepler. It was expected to discover many thousands more exoplanets, but is yet to succeed in the ground-breaking way that Kepler did. Still, it has discovered over 1,400 potential exoplanets and confirmed 34. And this is set to continue, with the mission recently being extended until at least 2022.

Missing exoplanets?

There are presently 4,099 [4,284 as of September 2020] confirmed exoplanets in our galaxy in 3,063 different solar systems, with 669 stars being orbited by more than one planet. Most of the confirmed exoplanets are as big or bigger than Jupiter, which is more than 300 times as massive as Earth. The biggest known planet is 8 times as massive as Jupiter, which in theory should have turned into a star. Many of these huge exoplanets orbit very close to their host star and are called Hot Jupiters. About ½ to a ¼ of the hot Jupiters discovered have a retrograde orbit, meaning that, unlike any planet in our solar system, they do not orbit in the same direction that the star spins. This suggests they may have migrated closer to the star, and that their orbit was thrown out of whack in the process. Migration and unstable planetary orbits might be more common than we expected. I’m glad that our Jupiter is relatively balanced where it is. There are also many confirmed exoplanets around 10 times the mass of Earth, dubbed Super-Earths. But what about all the exoplanets the size of Earth or smaller? 95% of confirmed exoplanets are significantly larger than Earth. There are exceptions, such as an exoplanet smaller than our moon, but they make up a small fraction of the total number. In 2015, Kepler discovered the first Earth-sized planet with an Earth-sized orbit (385 Earth-days). Earlier this year, 18 new Earth-sized exoplanets were discovered by re-analyzing old data from Kepler while specifically looking for smaller planets. Why are there so few confirmed Earth-sized exoplanets? One explanation is that our solar system has an unusual abundance of small rocky planets. Another explanation is that our detection methods, both wobbles and transits, favour detecting exoplanets that are huge and close to the sun. The first exoplanet discovered orbiting a star was 150 times bigger than Earth and orbits its star closer than Mercury. Therefore, I believe it is likely that there are many smaller exoplanets yet to be discovered.

Our Neighbourhood of exoplanets

There are about 100 confirmed exoplanets within 30 light-years from Earth, including at least one orbiting our closest star. Of the 316 known star systems within 30 light-years, around 60 are orbited by confirmed exoplanets. About 50 of these star systems are visible in the night sky, and 9 of them are orbited by more than one exoplanet. One is orbited by a solar system of 6 confirmed exoplanets.

The most interesting solar system in our neighbourhood is Trappist-1. A pair of ground-based telescopes were built to study exoplanets by the transit method, one in the northern hemisphere and one in the south. The system was called the TRAnsiting Planets and PlanetesImals Small Telescopes, or TRAPPIST. In 2015, the first discovery was a system of 3 planets orbiting a small star 40 light-years away that would come to be known as Trappist-1. Soon the Spitzer Space Telescope joined in the search, using its infrared light sensors to look at the planets and their atmospheres. Incidentally, the Spitzer space telescope will be shut down in a month after 16 years of service. Then the Hubble telescope and the Very Large Telescope both jumped in on the action. 

As it turns out, Trappist-1 is a small star only 9% the mass of our sun and is orbited by 7 Earth-sized terrestrial planets. 5 are about the same mass as Earth, and 2 are about ⅓. This is the most known planets in a solar system other than our own, and the most Earth-sized planets together in any solar system, including our own. Due to its low mass and temperature, Trappist-1 is expected to continue burning slowly for several trillion years. Our sun is only expected to last for another 5 billion years. The planets are packed tightly together, all 7 orbiting closer than Mercury orbits our sun. The shortest orbit (aka year) is 1.5 Earth-days, and the longest is 19 Earth-days. The distance between the orbits of the 2 innermost planets is only 1 ½ times the distance between the Earth and the moon. So on one of these planets, the other planets would be visible like moons in the sky, in some cases appearing several times larger than the moon appears from Earth. And there are 7 of them packed together! 

All 7 planets are thought to contain significant amounts of water, in the form of a thick atmosphere of steam in the closest, and as solid ice in the farthest. But at least 3 of the middle planets are at a distance from the star where liquid water could exist — known as the Habitable Zone. The 4th planet out is thought to contain many times more liquid water than the Earth and is currently the most likely exoplanet to support life. A bigger exoplanet with 7 times the mass of Earth (Gliese 1214-b) is also thought to contain significant amounts of liquid water and clouds in its atmosphere. So there is water out there in space.

Other exoplanets

While the Trappist-1 solar system is reminiscent of our own, there are many more unfamiliar exoplanets arrangements out there. One exoplanet the size of Saturn orbits a binary of stars every 230 Earth-days, while another orbits a trinary of stars every 550 Earth-years. During ¼ of this orbit — over 100 years — all 3 stars are visible during a single day, and the planet is in perpetual sunlight. One sun will rise before the last one sets! According to the Star Trek world, the character Spock’s home planet Vulcan orbits the main star of a triple star system called 40 Eridani, located 5 parsecs or 16 light-years from our home planet. Just over a year ago, an exoplanet twice the size of Earth was discovered in this exact location. However, it is so close to the star that it completes an orbit every 42 Earth-days, and is considered unlikely to support life.

While some exoplanets orbit 2 or more stars, some do not orbit any. Such Rogue Planets likely fell or were knocked out of orbit around their parent star, and flew off into interstellar space. While only a dozen or so rogue planets have been confirmed to date, computer modelling done within the last year suggests that there are likely billions of rogue planets within our galaxy alone. In the computer simulation, almost 1 in 10 newly formed planets did not achieve a stable orbit and floated off into space. Many of these lost planets may end up orbiting the supermassive black holes at the centre of their galaxy.

Future exoplanets

By 2030, we expect to discover many thousands more exoplanets and to learn much more about their characteristics. The GAIA space telescope, launched in 2013, measures the positions, distances and motions of stars and other bright objects. It does so by monitoring a billion different stars (about 1% of our galaxy) several times per year, and measuring the slight changes in their positions. The main purpose is to create an accurate 3D map of parts of our galaxy, especially the distances of stars from us. However, the wobbles it detects in those stars will also help to discover massive exoplanets, especially Hot-Jupiters. By 2024 when its power runs out, GAIA is expected to have discovered thousands more large exoplanets.

But the more exciting developments will come in discovering smaller exoplanets. PLATO, the PLAnetary Transits and Oscillations of stars, is a space telescope set to be launched in 2026. The telescope will be calibrated to detect the transit of Earth-sized rocky exoplanets in front of stars like our sun or smaller. There will be a particular emphasis on planets orbiting in the habitable zone where water could exist as a liquid. But the next game-changing development, the Webb Space Telescope, will be launched in 2021. The Webb telescope is designed to replace the infamous Hubble telescope with a lens 5 times larger — making it the biggest space telescope ever. Webb will be able to directly photograph exoplanets by mechanically blocking the light from the star, like putting your hand out in front of you to block the sun and looking for dim smaller planets around it. The spacecraft will orbit the sun hidden in the shadow of the Earth, and use its highly sensitive infrared sensors to look at even the atmospheric composition of distant exoplanets. Trappist-1 will be one of its first targets. This is very exciting! Furthermore, just over a week ago, the Characterising ExOPlanets space Satellite (CHEOPS) was launched into Earth’s orbit. From there it will look for new exoplanets orbiting bright stars already known to host planets and determine their mass and density. The satellite carried with it several drawings of space made by Earth Children.