Red dwarf stars are the most common in our galaxy, but they cannot be seen in the night sky without a telescope. The closest star to our solar system, Proxima Centauri, is also a red dwarf and is part of the Alpha Centauri system in the Centaurus constellation. Scientists are interested in finding exoplanets around red dwarfs because they think these stars are likely to have habitable planets. This article will explain why red dwarf stars are important in the search for exoplanets, particularly those that are like Earth. Let’s find out more!
Red dwarf star facts
Red dwarf or M-type red dwarf stars make up around 70-75% of the stars in the Milky Way. However, you can’t see them in the sky because of their low brightness. Red dwarfs are smaller, cooler, and less luminous than our sun (a G-type star), with a size of only 0.075 to 0.50 solar masses.
The brightness of the stars is presented in magnitude scale. Human eyes can only see night sky objects below the apparent magnitude of +6. Typically, a red dwarf’s apparent magnitude is in the range of +7 to +15. However, the magnitude of a star can vary significantly depending on size, temperature, and distance from Earth.
Red dwarf stars are not visible without the aid of telescopes. The closest star to the Sun – Proxima Centauri has an apparent magnitude of only about + 11.13. The next closest star to the Sun after the Alpha Centauri star system is also a red dwarf. Its name is Barnard’s Star- located 5.96 light-years (ly) away.
Nearby red dwarf stars
Most of the nearby stars to us are red dwarfs. Here is the list of the 20 closest stars to our solar systems together with their types and distances:
| # | Star/System Name | Star Type(s) | Distance (ly) |
| 1 | Proxima Centauri | Red Dwarf (M5.5Ve) | 4.24 |
| 2 | Alpha Centauri A | Yellow Dwarf (G2V) | 4.37 |
| 3 | Alpha Centauri B | Orange Dwarf (K1V) | 4.37 |
| 4 | Barnard’s Star | Red Dwarf (M4.0Ve) | 5.96 |
| 5 | Luhman 16A | Brown Dwarf (L7.5) | 6.59 |
| 6 | Luhman 16B | Brown Dwarf (T0.5) | 6.59 |
| 7 | WISE 0855−0714 | Sub-brown Dwarf | 7.2 |
| 8 | Wolf 359 | Red Dwarf (M6.0V) | 7.78 |
| 9 | Lalande 21185 | Red Dwarf (M2V) | 8.29 |
| 10 | Sirius A | Main Sequence Star (A1V) | 8.60 |
| 11 | Sirius B | White Dwarf (DA2) | 8.60 |
| 12 | Ross 154 | Red Dwarf (M3.5Ve) | 9.68 |
| 13 | Ross 248 | Red Dwarf (M5.5Ve) | 10.29 |
| 14 | Epsilon Eridani | Orange Dwarf (K2V) | 10.5 |
| 15 | Lacaille 9352 | Red Dwarf (M1.5Ve) | 10.72 |
| 16 | Ross 128 | Red Dwarf (M4V) | 11.01 |
| 17 | EZ Aquarii A | Red Dwarf (M5.0Ve) | 11.26 |
| 18 | EZ Aquarii B | Red Dwarf (M) | 11.26 |
| 19 | EZ Aquarii C | Red Dwarf (M) | 11.26 |
| 20 | Procyon A | Main Sequence Star (F5IV-V) | 11.46 |
| Procyon B | White Dwarf (DA) | 11.46 |
What makes red dwarf stars good candidates for hosting exoplanets?
Red dwarf stars have been expected by astronomers to have higher chances to host planetary systems compared to other types of stars. There are several reasons scientists focus on them in the search for extraterrestrial life and exoplanetary studies. Here are the key factors that contribute to their potential for hosting exoplanets:
- Abundance
Red dwarfs are the most common type of star in the Milky Way galaxy, comprising about 70-75% of all the stars. This sheer number increases the statistical likelihood of finding exoplanets orbiting these stars.
- Longevity
Red dwarfs have significantly long lifespans, often spanning trillions of years. They will outlast stars such as our Sun, which only has a lifespan of 10 billion years. The enduring nature of red dwarfs offers a potentially stable environment for the development and evolution of life on exoplanets orbiting them.
- Habitable Zone Proximity
The habitable zone, also called the Goldilocks zone, is the area around a star where conditions could support liquid water on a planet’s surface. Planets in this zone are easier to detect using telescopes, especially if the star is a red dwarf. This detection method involves the planet moving in front of its star as observed from Earth, leading to a small decrease in the star’s brightness.
- Frequent Planetary Systems
Observations and studies show that red dwarf stars frequently have planetary systems. Some of these systems have planets similar in size to Earth. An example of this is the TRAPPIST-1 system, which is a system with a red dwarf star and seven exoplanets that are similar in size to Earth. Three of these planets are located in the habitable zone.
- Observational Advantage
Astronomers find it easier to detect and study the exoplanets around red dwarfs. This is due to the lower in brightness and smaller size of those stars compared to stars like our Sun. In addition, the contrast between the light of the star and the shadow or signal from an orbiting planet is more favorable for observations.
How does the habitable zone of a red dwarf compare to that of our Sun?
The habitable zone of a red dwarf star differs significantly from that of a star like our Sun. For red dwarf stars, which are smaller and cooler than the Sun, the habitable zone is much closer to the star. This means that a planet would need to orbit much closer to a red dwarf to be within the temperature range that could support liquid water.
The habitable zone around red dwarfs can be as close as 0.1 to 0.2 astronomical units (AU) from the star (1 AU is the average distance from the Earth to the Sun, about 93 million miles or 150 million kilometers). In contrast, the habitable zone of a Sun-like star (a G-type star) is much farther out, roughly from 0.95 to 1.67 AU. Earth is well-positioned within our solar system’s habitable zone at about 1 AU from the Sun.
What is the main challenge for life on planets orbiting a red dwarf star?
The habitability of exoplanets orbiting red dwarf stars has been questioned. These stars tend to emit powerful stellar flares. Astonishingly, it can be hundreds of times more powerful than those emitted by our Sun. Such flare activity raised concerns about the habitability of exoplanets orbiting these stars, primarily for two reasons:
- Atmospheric Erosion
Intense stellar flares can strip away a planet’s atmosphere over time, particularly if the planet lacks a protective magnetic field. An eroded atmosphere could lead to the loss of essential elements for life, such as water and nitrogen.
- Radiation Exposure
Stellar flares emit high levels of ultraviolet and X-ray radiation, which could be harmful to life forms and could potentially disrupt the physiological processes necessary for life.
Another big challenge is the tendency for the planets in the habitable zone to be tidally locked to the host star. Tidal locking can result in one side of the planet permanently facing the star. This situation caused extreme temperature differences between the day and night sides.
Can red dwarf stars support habitable exoplanets despite their flare activity?
Despite the challenges, scientists are still optimistic about finding habitable exoplanets around red dwarfs. Subsequent research has suggested scenarios in which habitable exoplanets could exist around red dwarf:
- Strong Magnetic Fields
Exoplanets with strong magnetic fields might be able to deflect some of the stellar wind and flare-induced radiation, protecting their atmospheres and surface conditions conducive to life.
- Atmospheric Composition and Density
A dense and robust atmosphere could offer protection against radiation and help distribute heat more evenly around the planet, especially important for planets that are tidally locked (where one side always faces the star).
- Subsurface Habitability
Even if the surface of a planet is exposed to harmful radiation, life could potentially exist below the surface. They would be shielded from direct flare activity.
- Early Stellar Activity
Some studies reported that the most intense period of flare activity occurs early in a red dwarf’s life. If a planet can retain its atmosphere through this turbulent time, or if it forms or gains a secondary atmosphere later, it could enjoy stable conditions for billions of years afterward, given the long lifespans of red dwarfs.
- Adaptation
Life, if it develops, might adapt to the conditions of exoplanets orbiting red dwarf stars. This could include the evolution of protective mechanisms against radiation, similar to how some organisms on Earth have adapted to extreme environments.
10 closest habitable exoplanets
Below is a list of 10 exoplanets that are considered potentially habitable and are among the closest known to Earth. This list highlights the diversity of potentially habitable exoplanets discovered in proximity to our solar system.
The majority of those planets orbit red dwarf stars. This is consistent with red dwarfs being the most common type of star in the Milky Way, offering numerous opportunities for the presence of exoplanets in habitable zones.
| # | Exoplanet Name | Host Star Type | Distance (Light-Years) | Remarks |
| 1 | Proxima Centauri b | M5.5Ve (Red Dwarf) | 4.24 | Orbiting the closest star to the Sun. |
| 2 | Ross 128 b | M4V (Red Dwarf) | 11.01 | Considered to have a temperate climate. |
| 3 | LHS 1140 b | M4.5 (Red Dwarf) | 40.7 | Known for its high mass, suggesting a dense atmosphere. |
| 4 | Teegarden’s Star b | M7 (Red Dwarf) | 12.5 | One of the best candidates for habitability. |
| 5 | TRAPPIST-1 e | M8V (Red Dwarf) | 39.5 | Part of a system with seven Earth-sized planets. |
| 6 | TRAPPIST-1 f | M8V (Red Dwarf) | 39.5 | Another potentially habitable planet in the TRAPPIST-1 system. |
| 7 | TRAPPIST-1 g | M8V (Red Dwarf) | 39.5 | The largest of the TRAPPIST-1 planets in the habitable zone. |
| 8 | Kepler-442b | K-type (Orange Dwarf) | 1,206 | Larger than Earth, but within the habitable zone. |
| 9 | Kepler-22b | G5V (Sun-like) | 620 | Was the first known exoplanet to orbit within the habitable zone of a Sun-like star. |
| 10 | Gliese 667 Cc | M1.5 (Red Dwarf) | 23.6 | Orbits within the habitable zone of a star in a triple system. |
Summary
While red dwarfs have habitable zones closer to the star and face challenges like tidal locking and strong stellar activity, their long lifespans and abundance in the galaxy make them attractive for finding habitable exoplanets. The varying conditions in these habitable zones compared to those around Sun-like stars show the wide range of possible environments where life could exist in the universe. As our methods for observation and modeling get better, we will probably learn more about these unique and fascinating environments.
Disclaimer:
While we strive to provide accurate and reliable information, please be aware that the content of this blog post is subject to a margin of error. The probability of absolute accuracy is not guaranteed.
