The Search for Habitable Worlds

Are Habitable Worlds Common? We think so! Lots of rotating disks around newborn stars! This is where planets are born! We’ve detected lots of planets out there! If planets are common, why couldn’t HABITABLE planets be common? But the planets we have found are huge! (recently, 2 ~Neptune mass found) As of 10 Mar 2005 Global statistics:      134 planetary systems      152 planets      14 multiple planet systems

Distant Sun Candidates Main sequence star Long lifetime (low mass) Problem: Low mass stars are low temperature stars. Small habitable zone very close to the star. Advantage: 90% of the stars out there are lower mass than the Sun. Lots of possibilities to look at. Problem: Planets close to stars end up tidally locked. (one side hot, one frozen – may lose atmosphere) Problem: Small stars flare. (May cook anything on the close side.) Single star vs. Multiple star systems You need a Sun that can provide sufficient light/heat to support habitable worlds. Not all stars qualify. First, we look for stars on the main sequence (longest stage of their life). Before and after the star changes a lot. Long steady, low mass star. lowest mass stars (.1-.2 msun – Type M) can have habitable zones closer to them by ½ the distance of Mercury to the Sun. A moderate atmosphere could regulate the heat (moving it to the cold side). Venus is similar. So is Uranus. Flare radiation could be dealt with using an ozone layer. About ½ of all stars are in binary (or multiple) star systems. With MULTIPLE star systems Possible stability of planetary orbit if planet orbits both stars at a distance MORE THAN 5x THE SEPARATION OF THE STARS or if it orbits one star where the separation of the 2 stars is very great, MORE THAN 5x THE PLANET’S ORBITAL DISTANCE. Other than these cases, the planet gets ejected or hit. You could get habitable planets in a binary star system.

Habitable Zone (HZ)

habitable zones in other star systems HZ location determined by star's brightness, which depends on size, type, and age star 50% more massive than Sun enters red giant phase after only 2 billion years ---> outward migration of HZ is much faster and HZ has shorter duration stars more massive than Sun radiate more ultraviolet (UV) light, which breaks bonds of most biological molecules and heats atmosphere past escape velocity 95% of stars are less massive than Sun, for example, common M stars are 10% of Sun's mass planets of low-mass stars would need close orbits for liquid water to exist but, gravitational tidal effects would then introduce synchronous rotation (where planet spins only once on its axis each time it orbits star, like Earth tidally locks Moon) ---> atmospheric freeze-out due to cold dark side most stars are in binary or multiple star systems planets might not be able to form in first place if planets form, what happens to HZ's? orbits affected insolation (stellar energy received by planet) would vary evolution of two or more stars would cause HZ's to recede or even disappear

Finding Planets Direct vs. Indirect Methods Finding Planets Direct is beyond us at present Indirect: usually looking for a pull on a star by its planet(s), sometimes variations in the stars speed caused by its planet(s) Indirect: Astrometric Technique Doppler Technique Transits Gravitational Microlensing Finding Planets

Astrometric Technique Measure a star’s position to look for shifts around a center of mass. (side to side motion) Farther away is harder Larger shifts for larger planets that are farther from their star… easier to see. Far from star = long orbit. Takes a long time to find one.

Doppler (radial velocity) Study a star’s spectrum for Doppler shifting. Amount of shift gives radial velocity. The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars. Most extrasolar planets discovered this way.

Transits When the planet appears to move across the face of the star, it blocks some of the light. Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice-versa. A terrestrial planet in an Earth-like orbit, for example, would produce a minute dip in stellar brightness that would last just a few hours.

Gravitational Microlensing When a planet happens to pass in front of a host star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star. Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.

Direct Detection – Improvements? Use Infrared light (planets don’t give off visible light) Use an interferometer and nulling

The Nature of Extrasolar Planets Big and close to their star Those not close to their star are in highly elliptical orbits. Several systems have more than one planet. Problem: we think small rocky planets should be close. These are big Jovian type planets. Hmmmm. Are hot Jovian planets habitable? Their moons?

How do you get hot Jupiters? Maybe our Solar Nebula model is wrong. Maybe Jovian planets CAN form close to a star. Maybe they formed out farther and migrated inward. (dust drag, large collisions) Migration could cause disruption of inner, smaller planets and maybe life

Signature of Habitability and Life Is the planet in the habitable zone? A spectral analysis might tell us about its surface and atmosphere. If it looks like Earth, we might consider it habitable.

Earth-like planets… Rare? Should Earth-size planets be considered common? Are there enough heavy elements in enough places to make them common? What fraction of heavy elements is needed in the cloud to form an “Earth?” There should be enough. So maybe we should question the formation process. Oh, wait. We aren’t sure on that. So, the jury is still out on “Earths” being rare or common.

Impacts on “Earths” Jupiter probably deflected a lot of objects. Without a planet like it, the heavy bombardment might never have ended. That may have taken care of any potential life. Of course, we are finding a lot of Jovian type planets out there, so other Earths might have had a similar situation. But we don’t know if that is important or what other affects migration of Jovian planets would have had.

Are Stable Climates Rare? Earth is very stable compared to Venus and Mars (which presumably started in roughly the same place) Plate tectonics may be important in the carbon dioxide cycle. The moon helps stabilize our climate by stabilizing the tilt of our planet. However a radical tilt or the absence of plate tectonics won’t necessarily rule out habitability.

So are “Earths” rare or not?