Let`s start exploring the world around us and how it works! Remember, this is your book! You need to use it to explore the world around you and ask questions, and also to learn more about yourself and who you are. Don`t be afraid to take notes in the margins of this book – make your own doodles and notes on the points you need to remember or the questions you want to ask. Be curious! Discover and imagine the possibilities of what you can do with science! There are some life forms on Earth that can`t get their energy from the sun, so instead they get it from deep hydrothermal vents that pump hot, mineral-rich water. In fact, the researchers published a paper in the journal Nature Microbiology in 2016 suggesting that hydrothermal vents were the energy source of Earth`s last common ancestor. The list of exoplanets is growing rapidly with a variety of masses, orbital distances, and star types. The long list motivates us to consider which of these worlds could support life and what kind of life could live there. The only approach to answer these questions is based on observing life on Earth. Compared to astronomical goals, life on Earth is easy to study and our knowledge of it is vast – but it is not complete. The most important area in which we lack knowledge about life on Earth is its origin.

We don`t have a consensus theory about the origin of life, nor do we know the time or place (1). What we know about life on Earth is what it is made of, and we know its ecological requirements and limitations. Therefore, it is not surprising that most discussions of life on exoplanets focus on the demands of life rather than its origin. In this article, we take the same approach, but return briefly later to the question of the origin of life. Complex life forms (such as humans) are sensitive to radiation, but the dose that can be tolerated by many microorganisms is surprisingly high given the natural radiation in the environment. Table 3 lists the tolerances and acute dose survival for Deinococcus radiodurans, a well-studied heterotrophic soil with high radiation tolerance (42). It has been suggested that the high radiation tolerance of D. radiodurans is due to adaptation to dehydration stress (43). Desert cyanobacteria of the genus Chroococcidiopsis (shown in their characteristic hypolithic growth form in Figure 3B) are extremely resistant to dehydration, ionizing radiation and UV radiation (44, 45). An exoplanet would not need a magnetic field to be habitable. Any plausible field would not deflect galactic cosmic rays because these particles are far too energetic.

These particles are stopped mainly by the mass of the atmosphere or surface materials. The mass of the column of the Earth`s atmosphere corresponds to 1 kg/cm2. Earth`s magnetic field deflects solar protons, which direct these particles to the polar regions that create the Northern Lights. But even without the magnetic field, these particles would not enter the Earth`s atmosphere and would not reach the surface. The Earth sometimes loses its strong dipole field during field reversals. These events are not correlated with extinction in the fossil record. If learners have included a certain number of seeds in each test group/condition, they may want to express the result as the number or percentage of seeds that have germinated. The water cycle shows different sources of fresh and salt water. Many plants, animals and microorganisms have adapted to live in an aquatic habitat.

A very small percentage of the world`s water sources are fresh water and the rest are salt water. Note as many different types of aquatic habitats as you can imagine where different organisms exist. When the groups have discussed their most important requirements, ask them to share their lists with the other groups and have a class discussion. List their answers on the board and check a box for each repeated answer – for example, food/oxygen/water could be repeated so that whenever it repeats, check a box next to it. This allows them to quickly see which requirements are repeated most often in the classroom. Learners could identify “food” rather than energy. Remind them that they should also think about plants that don`t need food. So ask learners what term they could use as a more general term for food? This is related to nutrition in the seven processes of life.