For many decades we thought that our planet is unique. Actually, we thought the same for our Solar System. Of course, we were always wondering if other stars host planets around them, but we had no observational evidence. This changed during the 90s, when we detected the first exoplanets (i.e., planets outside our Solar System). So far, 4,200 exoplanets have been discovered, and there are two questions. The first is, if the Earth is unique. The second is, if there is life in any of these planets. So, let’s see which of these exoplanets are similar to Earth. In other words time to overview the goldilocks zone planets.
How is an earth like planet?
I am sure you may wonder what is an Earth-like planet or what makes a planet an Earth-like planet. Most simply rock, water, and air. In reality, there is a bit more into this, since Venus, for instance, is a terrestrial planet with an atmosphere. So let’s review the parameters that make an Earth-like planet or an Earth-analog. Thus the recipe for an Earth-analog is:
Size – Size is an important factor, since planets with similar size to Earth’s are more likely to be terrestrial in nature. Additionally, a planet should be able to retain an atmosphere, thus it needs significant surface gravity.
Terrestrial – For classifying a planet as an Earth-analog, it must be terrestrial. This means the planet should have a similar surface, and interior in terms of composition with Earth.
Temperature – An Earth-like planet should have a temperature that is friendly for life to exist.
Solar analog – A planet that matches well with our Earth should orbit a star like our Sun. But the reality perhaps is different, as we will see below.
Water – The concept of the habitable zone is defined as a region where water may exist in liquid form. This model was made from the properties of the Earth and Sun. Based on this model, the Earth orbits the Sun at the ideal distance, allowing for the presence of oceans, rivers, and lakes. So far, the only planet with confirmed large bodies of water is our Earth. Note that an Earth analog will need a combination of both landmass and water bodies.
Atmosphere – An atmosphere is crucial for many reasons. First, an atmosphere with a similar mixture of gases like our own is important for life. Additionally, the presence of an atmosphere acts as protection for both the maintenance of the liquid water masses and water-cycle (i.e., clouds, hydrological cycle).
#Earth-like planets facts
- Planets with mass between 0.8–1.9 times Earth’s mass, classify as sub-Earth planets while those with higher mass as super-Earths.
- Before the arrival of the first probes in Venus, there was the belief that Venus would be an endless tropical paradise. The reality is way different.
- If a planet’s mass is lower than 2.7% of Earth’s, its atmosphere will escape before developing surface liquid water.
- The only body other than Earth, with liquid masses on its surface, is Saturn’s satellite Titan. The distinguishing feature is that instead of water, the lakes and rivers on Titan are from liquid methane.
- The Earth Similarity Index (ESI) is a suggested term of how similar is a planet or a satellite to Earth. It scales from zero to one, with Earth having an index of one. Now that this term has no quantitative meaning for habitability (for example, Mars has an ESI of 0.7, while Mercury 0.6).
- The highest ESI has been granted to KOI-4878.01. The planet is on the confirmation phase, but it can be an excellent candidate for being Earth 2.0, since it got an ESI of 0.98. This is due to its estimated mean surface temperature of ~19°C (66°F), the possibility of an atmosphere (10 Atm, which is quite high), and its host star, which is similar to the Sun (with a bit lower surface temperature).
- The second highest ESI has been granted to TRAPPIST-1e (0.95). This planet has been confirmed, and it is one of the most potentially habitable planets discovered so far. The estimated mean temperature for this planet is ~12°C (53°F), and its atmospheric pressure of is 6 Atm. The downturns on this planet are that the host star is a red dwarf, and also, there is a chance that it is tidally locked (i.e., like the Moon-Earth system).
For over the 20 years, astrophysicists and biologists, have formed a new section of science known as Astrobiology. The quest is one. Find extraterrestrial life, or at least find a planet that is friendly to it.
Thus, astrobiologists focus on exoplanets that orbit their host stars on what is called the habitable zone or circumstellar habitable zone (CHZ). This is the range of orbits, where a planet can support the presence of liquid water, providing the existence of an atmosphere. Thus, when a new planetary system is discovered, a model for a CHZ is created to check whether there are terrestrial planets in the habitable zone. The limits of a CHZ are modeled based on the amount of radiant energy our Earth receives from the Sun and the distance between the two.
#Goldilocks zone facts
- There are plenty of models that have been suggested for the CHZ of our Solar System.
- The tightest model for the habitable zone in our Solar System allows orbits between 0.99 – 1.01 AU (1 AU – Astronomical Unit, is the mean distance between the Earth and the Sun).
- In the most loose model, the habitable zone exists between 0.38 – 10 AU.
- An alternative term that scientists use for the CHZ is the “Goldilocks zone”.
Finding another Earth
So which type of stars can host planets where life may flourish (or it exists already)? The search focuses on average mass to low mass (i.e., from Sun-like to red dwarfs) stars. The reason is that these stars have long lifetimes, allowing for life to evolve.
What makes M dwarf stars excellent candidates is the fact that they have very long lives (~ 100 billion years). The downside for these stars is that they are not very friendly to life as we know it. The habitable zones on these stars are very narrow, and they have to be very close to their host star. This means that these planets will be exposed to tremendous levels of X-ray and ultraviolet radiation (UV), which can be thousands of times higher than the amount Earth receives from the Sun. But there is more. We are all familiar with the Solar activity. Similar activity exists amongst all-stars, including M dwarfs. Due to their proximity, planets around M-type stars can be baked from the activity of the host star. Finally, these stars, although they are small, they sim to be extremely active.
The situation is different when it comes to K-type stars. The lifetimes of these stars are between 15 – 45 billion years (i.e., higher than the Sun). These stars seem to be more friendly to life since they are more “quiet”. In these stars, planets in the habitable zone receive 5 – 25 times the amount of X-ray and UV that we receive on Earth, and their activity is not as high as in M-type stars.
Finally, Sun-like stars (G-type), have the shortest life expectancy amongst the three stellar types (around 10 billion years), but they tend to have broad habitable zones and they show the “best behavior”.
Abundance of stars in a galaxy
What is the most abundant stellar type in our Galaxy, thus the Universe? In terms of relative abundance, M-type stars make up 73% of the stellar population in our Galaxy. K-type stars are the second most abundant at 13%. Finally, Sun-like stars are not that frequent, and their frequency is only at 6%. Note that the Universe is dominated by low-mass stars. The sum of the abundance of all massive stars (O, B, A, and F) is at 8%.
When all three parameters (stellar activity, life expectancy, and frequency of occurrence) are balanced, then K-type stars (a.k.a. Goldilocks stars) are the most suitable candidates to look for Earth 2.0.
- The first-ever planets were discovered in 1992. Both are terrestrial planets around pulsar PSR B1257+12 (this is a neutron star!).
- The most distant planets have been discovered around SWEEPS J175902.67−291153.5 at a distance of 27,710 light-years.
- Proxima Centauri b is an exoplanet orbiting the star Proxima Centauri. Located at a distance of 4.2 light-years, is the closest planet to us, and the closest Earth-analog.
- The most distant habitable planet is Kepler-443b, located at a distance of 2.450 light-years.
- WD1145+017b is the least massive planet detected. Its mass is around 1,500 times lower than Earth’s.
- The upper limit for a planet (i.e., the most massive) is not clear, since the limits between gaseous-giants and brown dwarfs (failed stars) are not clear.
- K2-38b is the most dense planet detected (17.5 g/cm3). This is three times denser than Earth.
- The least dense planets are Kepler-51 b, c, and possibly d with a density of 0.03 g/cm3.
- The coldest planet is OGLE-2016-BLG-1195Lb, with a mean temperature of -242°C (-404°F).
- The hottest planet is Kepler-70b, with a mean temperature of 6725°C (12140°F).
- According to models, one out of five stars hosts planets. This means that only in our Galaxy we may have 100 billion planets. The number of terrestrial planets can be as high as 40 billion.
- So we may have 6 billion Earth-like planets only in our Galaxy.