Evidence for Life Beyond Earth?

Question:  Hi! My name is Natasha and I live in New Jersey and am 17 years old. I am currently a Senior in high school and I’m planning on pursuing Astronomy and Physics in my future. Currently, I am in a Journalism class and I am writing a feature article on the existence of extraterrestrial life. I 100% believe in life other than ours, but I need an expert’s support in my stance. What blatant proof is there that life other than ours exists and any statistics of any kind are appreciated as well!  Thank you so much for your time!  – Natasha

Answer:  Well, the fact is that there is no evidence for the existence of living organisms such as those we find here on Earth on any other planet.  As you probably know, there have been many attempts to detect the signatures of life beyond Earth, all of which have resulted in what we call a “non-detection”.  These searches continue, though, through such measurements as studies of the properties of extrasolar planet, or “exoplanet”, atmospheres.  These exoplanetary atmosphere studies are attempting to detect the signatures of life, otherwise known as “biosignatures”.

Jeff Mangum

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How Many Planets Can Orbit a Star?

Question:  How many planets a stellar-planetary system can accommodate safely into a stable orbit around that star? What are the factors it depends on?

For Example: Currently our solar system has 8 planets in a stable orbit. I want to know the maximum possible number of planets our system can accommodate so that added planets(beyond the orbit of Neptune) also follow the orbital pattern of 8 existing planets?
Suppose all the planets in our solar system are exact replica like Earth, then how many earth-like planets that our current sun can get hold off?  – Vinod

Answer:  With no other constraints on the star or the planets that orbit the star, the only requirement for stable orbits of planets around the star is that the total mass of the planets be less than the mass of the star.  Therefore, one could in principal have a nearly infinite number of very small planets that orbit a star.  In reality there are other constraints, such as merging of small planets that are near each other in the early phases of the formation of a planetary system, that reduces the final configuration of a planetary system.  These additional constraints, though, conspire in a complex way to produce the final orbital configuration for an exoplanetary system.

Jeff Mangum

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How to Tell the Difference Between an Extrasolar Planet and a Star Spot

Question:  I am a retired PhD chemist..interested in amateur astronomy.  Recently a lot of exo planets have been discovered using a “transit” method of observing brightening and dimming of stars.  How do we know these are planet transits, and not the stellar equivalent of sun spots????  – T. Engle

Answer:  Let me first point you to a very nice description of the techniques used to detect extrasolar planets (called “exoplanets”) by my colleagues at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado.  There is also a very nice description of the transit method for exoplanet detection from the Las Cumbres Observatory Global Telescope (LCOGT) network.  As you pointed out, one of the methods used to detect exoplanets uses measurements of the dimming of a star’s light as a planet transits across the face of the star.  Stars also have “spots”, much like those found on our Sun (“sunspots”), which are cooler dark spots that could conceivably be mistaken for an exoplanet.  Starspots rotate with the star and cause relatively slow changes in the brightness of the star, while a transiting exoplanet crosses the star in a much shorter time, often as short as a few hours, and cause quick dips in the brightness of the star.  A much more in-depth answer to this question can be found at the Planet Hunter’s Blog.

Jeff Mangum

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How Does a Star’s Size and Luminosity Affect its Habitable Zone?

Question:  We are fortunate that the Earth orbits our Sun in the “Habitable Zone” where it’s not too hot, it’s not too cold and liquid water can exist giving life a chance to evolve. As stars vary so much in size and luminosity how do astronomers work out where the “Habitable Zone” is around any given star?  – David

Answer:  The definition as to what exactly is a “habitable zone” for planets orbiting a star is rather complex, involving many factors, including the type of star, size of the planet, and even the location of the planetary system in a galaxy.  A nice explanation of the constraints placed on the physical properties of a star which hosts potentially habitable planets can be found on the Penn State Astronomy Department e-Education description of the habitable zone.  Generally speaking, the larger or hotter a star is, the farther away its habitable zone will be (and vice-versa).

Jeff Mangum

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Can We Measure an Exoplanet’s Orbital Decay?

Question:  Is it possible for a planet of an established solar system orbiting a star to be in a decaying orbit and if so how do astronomers confirm this as the star and its solar system can be vast numbers of light years away?  – David

Answer:  Yes, it is possible to see an exoplanet’s orbit decaying, but this would require measurements of its orbit over a period long enough to measure the decay, or change, in its orbital parameters (i.e. the orbital period).  Also, this would be a measurement referenced to the state of this planet’s orbit when the light we receive was emitted by the planet, which might be many light years away.

Jeff Mangum

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Exoplanet Detection Techniques

Question: My question is two-fold and relates to exoplanet detection. Firstly, it seems like the two most successful detection methods, namely the transit and radial velocity methods, rely on exoplanet systems being edge-on relative to our vantage point. Is this correct? And if so, will we forever be limited to edge-on discoveries, or is there any hope for an alternative method that could be as successful as the aforementioned? – Eric

Answer: The answer to the first part of your question is yes.  The radial velocity and transit timing methods of exoplanet detection require that the planet pass along the line-of-sight between us and the star that the planet orbits.  As for the second part to your question, there are several alternate exoplanet detection methods that do not require the observational orientation needed for the radial velocity and transit methods:

  • Direct Imaging: In rare cases where the parent star is not very bright (like when it is a Brown Dwarf) and the exoplanet is very large (Jupiter sized or larger), one can directly detect an exoplanet in such a system.
  • Astrometry: By measuring the gravitational “wobble” of a star as a planet orbits around it we can infer the existence, and mass, of an exoplanet.
  • Polarimetry: Since light that passes through an exoplanet atmosphere gets polarized, one can infer the presence of an exoplanet by measuring the polarization of the star+exoplanet light.

Jeff Mangum

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