Our home star, the sun, is a middle-aged main sequence star. That is to say, it is roughly halfway through its lifecycle and is an average-sized star compared to the spectrum of stellar types that can be found elsewhere in our galaxy and beyond.
Life, in its 4.5 billion-year journey on Earth, has had an intimate relationship with the sun. It provides life with its main source of energy via photosynthesis, it keeps temperatures within a balmy range, and its energy drives many of Earth's cycles on which the rhythms of life are coupled to. Our planet's habitable conditions are in large part due to the type of star the sun is and its relative age.
There are, of course, a number of stars in the cosmos, each with different characteristics that will affect the conditions on any planets they may possess. Planetary scientists, as they search for habitable conditions on worlds other than our own, have to consider how different types of stars might alter the conditions for habitability on planets of interest.
So, let's explore why the relationship between a planet and its star is crucial to understanding whether a planet may have conditions that could support life like our own.
What is the Habitable Zone?
One of the main factors that planetary scientists consider when determining if a planet might have habitable conditions is whether it sits in the 'habitable zone'. This refers to a planet's orbital distance from a star so as to allow liquid water to exist on its surface - it's not too close so that water doesn't evaporate, and it's not too far so that water doesn't freeze.
Earth sits within the sun's habitable zone, which means exoplanets — planets that orbit other stars — that also sit in the habitable zone are of particular interest to astronomers. However, different stars, depending on their luminosity and effective temperature, will have different habitable zones.
"A star’s luminosity plays a dominant role in determining the position of the habitable zone. Smaller, dimmer stars, such as Trappist-1, have habitable zones that are much closer to the star compared to our sun’s habitable zone. In order for a planet around these types of stars to receive enough sunlight to maintain habitable conditions, it needs to be very close to its star as the star outputs much less energy," says Noah Tuchow, a planetary scientist at NASA's Goddard Space Flight Center.
Read More: 6 Exoplanets in our Universe That Could Support Life Other Than Earth
How Stars Affect the Habitable Zone
The opposite is true for more massive, brighter stars, where a planet has to be much farther away to receive the correct amount of starlight.
Tuchow explains that a star's luminosity will also evolve throughout its lifetime, altering the position of the habitable zone. A star's life begins during the 'pre-main sequence' when they are collapsing into a compact object capable of a thermonuclear reaction. As they contract, their luminosity decreases, causing their habitable zones to move inwards. However, this is pretty short-lived and may only last tens of million years in stars like our sun. Planets are also likely to still be forming during this time.
The majority of a star's lifetime is spent in the main sequence phase, where its luminosity will gradually increase, causing the habitable zone to slowly recede away from the star.
"For example, the sun has become around 30 percent brighter over the course of its lifetime, causing Earth to move from the center of the habitable zone towards the inner edge," says Tuchow.
And lastly, at the end of the star's lifetime, it will undergo rapid changes, expanding into a red giant phase where it is much more luminous and the habitable zone is much farther out. "For planets, we find around giant stars in these late phases of evolution, it is very likely that such planets only recently entered the habitable zone and spent most of their star’s lifetime outside of it," says Tuchow.
This will be the eventual fate of our own sun, and solar system. Frozen worlds and moons on the outskirts of the solar system may yet have their moment in the sun.
Read More: This Eyeball Planet Could Be The Best Place To Look For A Habitable World
Electromagnetic Radiation and Habitability
The electromagnetic radiation emitted from a host star will also play a key role in the habitability of a given planet. The sun's radiation bathes Earth in life-giving energy, and yet, radiation can also create havoc for life. Planets are largely at the mercy of their home star, and a star's high-energy radiation can cause a planet to lose its atmosphere to space. Atmospheres are thought to be a crucial component of the habitability question, as they shield planets from harmful radiation and can keep important ingredients for life, such as water, tied to a planet's surface.
"To mitigate the effects of high energy radiation, it helps to have a thick atmosphere. That way, even if some of the atmosphere is lost to escape, the planet will still be able to maintain surface conditions necessary for liquid water," explains Tuchow.
Planetary magnetic fields can help protect a planet against high-energy particles from a star's solar wind; however, it remains a topic of debate whether magnetic fields help a planet retain its atmosphere.
Read More: Though Rare, Exploding Stars Could Emit Radiation Harmful to Life On Earth
The Relationship Between Star Size and Exoplanets
A number of past studies have suggested that smaller stars typically host smaller planets and vice versa. This makes sense, as planets form from a disk of gas and dust around their forming host stars. More massive stars will have a larger disk around them and, therefore, more material to create planets.
Certain classes of stars then, such as red dwarfs, may be more likely to be home to small rocky planets like our own, planets that we know are capable of supporting life. The potential problem here is that red dwarfs spend a long time dimming during their early phases, which means planets in their habitable zone may have originally been too close to the star and, therefore, too hot.
Because of this, "if one is searching for signs of life on planets, they may want to focus on planets that spend a long enough duration in the habitable zone to allow for the emergence and evolution of detectable life," says Tuchow.
In this case, it would make sense to focus on stars whose habitable zones will remain in a relatively stable position for a long time. "K dwarf stars, slightly less massive than the sun, might meet this description, as they don’t spend too long of a duration dimming on the pre-main sequence, but also increase in luminosity very slowly on the main sequence," explains Tuchow.
Read More: 7 of the Brightest Stars You Can See with the Naked Eye on Earth
The Search for Planetary Habitability Continues
A number of factors come into play when planetary scientists consider whether a planet may have habitable conditions — its position in the habitable zone, the presence of an atmosphere, and a planet's composition (rocky vs. gassy), all of which are subject to the type and age of the star that the planet orbits.
Planetary scientists and astronomers interested in searching for habitable worlds are increasingly considering how stars contribute to the habitability of their planets — a guiding star that may one day help us answer whether we are alone in the cosmos.
Read More: Can Life Exist on a Rogue Planet?
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Planetary scientist at NASA's Goddard Space Flight Center. Noah Tuchow
Harvard.edu. The Occurrence Rate of Small Planets around Small Stars
Conor Feehly is a New Zealand-based science writer who covers a wide range of topics, including astronomy and neuroscience, with an eye for research at the intersection of science and philosophy. He received a master's in science communication degree from the University of Otago. Conor is a regular contributor to Discover Magazine, with his work also appearing in New Scientist, Nautilus Magazine, Live Science, and New Humanist among others.
