What is a biomarker? Give some possible examples of biomarkers we might look for beyond the solar system.

A biomarker is a chemical substance or a structure, or a signal that could only have been formed by life. Beyond our solar system, we can only detect planet-scale biosignatures, biological impacts so great that they affect the way a planet appears in reflected or emitted electromagnetic radiation. An example of such an exoplanet biomarker would be unusual atmospheric composition, such as the mutual presence of methane and oxygen. While this would be a strong indication of life, it would not be unequivocal because methane and oxygen can be produced in the absence of life under special circumstances. Another possible example might be very short, very energetic pulses of visible light or infrared radiation or radio waves that are not just natural static, but are coded with information given off by huge structures in space built near or around stars. Both would be biomarkers of technologically advanced civilizations.