which exoplanet is discovered by an astronomer who has an academic degree of doctorate degrees? 
Entity
m.01nc77
PSR B1620-26 b
class class class entity
base.exoplanetology.exoplanet astronomy.astronomer education.education m.02_xgp2
base.exoplanetology.exoplanet astronomy.astronomer education.education education.educational_degree
Exoplanet Astronomer Education doctorate degrees
none none none none
astronomy.astronomical_discovery.discoverer education.education.student education.education.degree
Discoverer Student Degree
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> PREFIX : <http://rdf.freebase.com/ns/> 
SELECT (?x0 AS ?value) WHERE {
SELECT DISTINCT ?x0  WHERE { 
?x0 :type.object.type :base.exoplanetology.exoplanet . 
?x1 :type.object.type :astronomy.astronomer . 
?x2 :type.object.type :education.education . 
VALUES ?x3 { :m.02_xgp2 } 
?x0 :astronomy.astronomical_discovery.discoverer ?x1 . 
?x2 :education.education.student ?x1 . 
?x2 :education.education.degree ?x3 . 
FILTER ( ?x0 != ?x1 && ?x0 != ?x2 && ?x0 != ?x3 && ?x1 != ?x2 && ?x1 != ?x3 && ?x2 != ?x3  )
}
}
(AND base.exoplanetology.exoplanet (JOIN astronomy.astronomical_discovery.discoverer (JOIN (R education.education.student) (JOIN education.education.degree m.02_xgp2))))